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74th EAGE Conference and Exhibition incorporating EUROPEC 2012
- Conference date: 04 Jun 2012 - 07 Jun 2012
- Location: Copenhagen, Denmark
- ISBN: 978-90-73834-27-9
- Published: 04 June 2012
401 - 500 of 948 results
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A Pre-stack Basis Pursuit Seismic Inversion
Authors R. Zhang, M. Sen and S. SrinivasanZhang and Castagna (2011) formulated basis pursuit inversion for post-stack seismic data. Here we extend it to the pre-stack angle gather domain. The major contribution of this work is the introduction and feasibility of application of basis pursuit inversion on pre-stack seismic data. The BPI inversion follows a L1 norm optimization framework. The resulting inversion enforces sharpness of the layer interfaces, thereby achieving better focus of layer boundaries. Unlike other AVA inversion methods which output smooth Vp, Vs and density, our method can generate spiky reflectivities of Rp, Rs and Rρ and blocky velocities and density, which can resolve structural boundaries and lithology information better for interpretation.
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Explore the Non-uniqueness on Interpretation of Amplitude Variation with Azimuth
More LessGenerally seismic amplitude varies with azimuth in a horizontal transverse isotropy medium (HTI). Uniform direction vertically fractured rock composes a HTI medium. The magnitude of seismic amplitude variation with azimuth (AVAZ) is determined by anisotropy parameters and mechanical property of the host rock. However the distribution of AVAZ result is usually interpreted as the representation of anisotropy variation caused by vertical fracturing change in the host rock. The change of mechanical property, such as Poisson ratio, is ignored. In this paper we demonstrate that host rock mechanical property change can complicate the correlation between fracture density and AVAZ amplitude. From a shally sand model we reveal that the AVAZ amplitude is not uniquely correlated to the fracture density change when the fracture density is small. Our results also indicate that host rock Poisson ratio change can manifest seismic data amplitude variation along azimuth direction. The complexity of AVAZ in response to the fracture density is illustrated from a fractured sand-shale mixing model. From the model study we find that anisotropy AVO intercept and gradient cross-plot technique is a more robust method for identification of fracture density change.
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Investigating Methods to Enforce the Stationary Zero Phase Assumption Prior to Inversion
By D. ClarkeThis study investigates the extent to which the assumption of constant phase data impact on the results of pre-stack simultaneous inversion. The well-tie procedure provides a means of estimating stationary phase wavelets. Statistical phase estimation techniques capable of estimating non-stationary phase can be used to phase-correct the seismic data to zero phase. Synthetic seismic volumes will be created with non-stationary phase. The results of pre-stack simultaneous inversion will be compared for data that account for non-stationary phase with data that rely on the stationary phase wavelets from the seismic-to-well tie. The implication of this study is that pre-stack simultaneous inversion can now be performed without the drilling of a well.
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Feasibility Assessment of Cavity Detection Using Geophysical Modelling
By P. M. JamesOld mine workings and natural cavities represent potential hazards before, during and after construction on such sites, and add further complication to subsurface parameters.Geophysical techniques can be used to detect cavities but the associated subtle signals mean that site conditions must be considered when choosing techniques and survey parameters. Here we propose the use of theoretical modelling to calculate the feasibility of cavity detection using a range of geophysical techniques. Typical cavity shapes are modelled and the use of site specific parameters increases reliability. We show examples of the limitations of cavity detection as a range of associated variables are altered. Three dimensional modelling allows assessment of optimum survey parameters in the survey design stage. This approach aids discriminate choice of technique and technique survey parameters at the desk study phase of survey design. The calculation of the minimum cavity detectable at any given depth gives precise information about the limitations of any geophysical survey undertaken. This analytical and accurate technique of survey design will increase openness about the limits of particular geophysical techniques in given site conditions and also optimise a geophysical survey to any specific site.
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Development of a Multistreamer 3D High Resolution System
Authors P. Sack and T. A. HauglandA high resolution system is proposed using the numerous acquisition tools available on the market. The focus of development was to use or adapting existing proven technologies rather than introduce unknowns through new tools or components. The system is developed within three aspects: towing, positioning and geophysical. These three areas of focus are the basis of the systematic development and testing of the system. Development spanned Q3 2010 through Q2 2011 and a production survey was acquired in June 2011. Results showed the method was effective and efficient.
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High Resolution Interpretation of Broad Bandwidth 3D Seismic Data for Shallow Geohazards
Authors J. I. Selvage, C. Jones, C. Scotellaro, J. A. Edgar and H. CrookSafe drilling operations require robust and timely identification of geohazards. Broad bandwidth 3D seismic data can help achieve this if appropriate interpretation is performed. By exploiting the recent developments in global interpretation algorithms and utilising quantitative interpretation techniques the benefits of broad bandwidth 3D seismic can be more fully leveraged. To this end the use of global interpretation algorithms is demonstrated on a broad bandwidth 3D seismic dataset and is used to perform stratigraphically consistent identification of geohazards. This analysis is complimented by a fault identification attribute that uses dip information and cross-plotting of AVA/AVO attributes to automatically map potential geohazards. Much of this analysis can be automated enabling more time to be spent analysing identified geohazards. Overall, our motivation for sharing this work is to keep shallow geohazard identification techniques up-to-date.
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Applying a New Full-waveform, Migration-based Deconvolution Approach to Locating Microseismic Events in Canada
Authors N. J. Brooks, J. B. U. Haldorsen, M. Milenkovic, M. B. Farmani and C. CrowellHere we present a new method of locating earthquake hypocenters that does not require the user to identify and associate P and S seismic arrivals, eliminates the use of hodogram plots and allows reliable and accurate real time processing with a hands-off approach. The whole event location process is fully or semi-automated. The new method is well suited to real time data processing as it has removed the need for manual picks and tricky association of P and S wave-modes. The method can be applied to temporary or permanent oilfield seismic sensor deployment. We apply this new deconvolution/migration approach to a real data set recorded in Canada. We compare this to the traditional method and compare the results and benefits of using the new approach. The new method yields accurate results that can be delivered in a timelier manner.
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Arabian Plate Earth Model in 3D - Application to Regional Play Evaluation of the Khuff
Authors N. Harvey, D. M. Casey, R. B. Davies, R. Martin and M. D. SimmonsWe present a 3D Earth Model of the Arabian Plate and demonstrate its use for the rapid assessment of play potential, with an example from the Khuff gas play in eastern Arabia. The model encompasses all strata from basement to surface, and covers an area of c. 1,200 by 1,200km with a grid size of 10km. Based on our robust sequence stratigraphic framework and a database of publically available information, the model is divided into 82 layers corresponding to key 1st, 2nd and 3rd order depositional sequences. Applied to the Khuff gas play, this enables us to rapidly produce (and update with new data if needed) a regional play risk map for the Khuff and identify areas of higher potential for this play.
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Modelling the Diagenetic Tipping Points in the Porosity-permeability Evolution of a Carbonate Reservoir Rock
Authors C. Van der Land, K. Wu, R. Wood, M. I. J. Van Dijke, Z. Jiang, D. Thorpe and P. CorbettCarbonate rock typing methods aim to associate texture with petrophysically similar rocks. Here, we use 2D thin sections of carbonate reservoir rocks to create 3D pore space models and extract pore networks from these to obtain their multiphase flow properties using pore network flow models. This novel rocktyping method is applied to a typical Middle East Cretaceous carbonate reservoir rock, where multiple 3D realisations were obtained. Based on knowledge of the major pore-occluding or pore enhancing diagenetic processes, we created synthetic images of the former states of the rock (diagenetic backstripping). For each stage in the paragenetic sequence, flow properties were calculated. When upscaled to flow units in reservoir models this method has the potential to identify (diagenetic) tipping points during the evolution of the carbonate reservoir during burial.
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Reservoir Impact of Small-scale Aeolian Dune Architecture - From Acquisition to Simulation in the Wahiba Sands, Oman
Authors D. I. Tatum, R. Nursaidova, C. Y. Hern, A. R. Westerman, J. Francke and A. R. GardinerSmall-scale dune heterogeneity has a significant impact upon recoverable reserves within aeolian hydrocarbon reservoirs. Complex geometries exist, with bounding surfaces and primary strata types often negatively impacting fluid flow. Incorporating the effects of such architectural elements into reservoir models is essential when accurately determining their effect on development strategies. In order to assess their impact, we acquired a small pseudo-3D dataset from the Wahiba Sands, Sultanate of Oman, using ground-penetrating radar (GPR). In this paper we discuss the acquisition, processing and modelling of this dataset. Data are interpreted to be of a small linear dune. Radar stratigraphic units have been interpreted and mapped in 3D; a small-scale analogue reservoir model has been produced. Simulation studies have been conducted to assess the impact of a range of sensitivities, including the affects of permeability contrast, flow direction and capillary pressure. Results indicate that permeability contrasts have a significant impact on recovery, whilst flow direction is the dominant factor. The resulting models may not be directly transferable to a specific subsurface scenario, but the generic spatial information can be a useful guide.
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An Integrated Approach for Building Geologic Models Consistent with Seismic Data
Authors M. Nasser, P. Walshe and M. ClevelandWe demonstrate how rock physics, when coupled with seismic interpretation, can lead to building geologic models that are consistent with seismic data. This approach was tested on the Bengo discovery, located offshore Angola. Two wells (discovery & appraisal) were used in this study to build a rock physics model which relates the seismic response to rock and fluid properties, which were later used for populating the geologic model. This study encompasses surface and subsurface elements including geology, geophysics, petrophysics, and reservoir engineering. However, in this abstract we will only focus on the geology and geophysics elements, which are critical for this study.
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Provenance, Sandstone Composition, and Seismic Facies of Paleocene Wilcox Sandstones, Gulf of Mexico Basin
Authors S. P. Dutton, A. McDonnell and R. G. LoucksUpper Paleocene sandstones of the lower Wilcox Group are deep (>4.5 km) to ultradeep (>6 km) exploration targets below the present-day shelf and in deep water of the Gulf of Mexico. These sandstones were sourced by continental-scale drainage systems that terminated in the Houston and Holly Springs deltas in Texas and Louisiana, respectively. We used seismic data to document the location of sandy fairways into the deep basin and petrographic data to document regional variation in detrital mineral composition of lower Wilcox sandstones. We identified three major sediment fairways that carried sandstone from the shelf into the deep basin in the northwestern Gulf by superimposing areas of moderate- to high-amplitude seismic reflections within the slope to basin-floor transition seismic facies onto the lower Wilcox isopach map. Sandstone composition was determined by point counts of 275 thin sections from 34 wells from onshore Texas and Louisiana. Provenance differences between the Houston and Holly Springs deltas resulted in differences in detrital mineral composition. Lower Wilcox sandstones in the Houston delta are mostly lithic arkoses, whereas those in the Holly Springs delta are feldspathic litharenites. Difference in feldspar content may distinguish reservoir sandstones in deep water derived from the two main deltaic sources.
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Thin Bed Reservoirs Formation Evaluation without Bimodal Sand-shale Assumption
Authors G. A. Bordakov and D. F. AllenThinly bedded reservoirs have been produced for many years, but their identification and quantitative evaluation remains a difficult challenge, since conventional deep induction resistivity lacks sensitivity to thin resistive pay sands. Introduction of vertical resistivity with triaxial induction tools overcame this, but analysis has been built on the simplification of a bimodal formation model because deep resistivity measurements have limited vertical resolution. This model assumes that formation consists of indiscernible thin sand and shale layers and all sand and all shale layers are identical. This assumption leads to significant overestimation of producible hydrocarbon volume if formation is not bimodal. Alternative model free of bimodal assumption is proposed which integrates textural measurements with resistivity via parameterization of the pore water volume. Flags to indicate applicability of bimodal approximation are constructed. Hydrocarbon volume estimates resolution matched to textural data are made. Proposed approach either used just as thin beds indicator in conjunction with known industry methods or used to incorporate higher resolution textural measurements is capable to deliver significantly better-quality estimates for producible hydrocarbons in thinly laminated siliciclastic reservoirs.
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Extrapolation of Water Saturation Using J-Leverett Functions and Bivariate Analysis
More LessOne of the most important elements in reserves calculation is the estimation of water saturation and its extrapolation away from the wells. Traditionally, workflows for water saturation interpretation for 3D geological models are based uniquely on water saturation measurements from log data using the Archie equation. These techniques although proven in several fields around the world, are heavily affected by the mineralogical content of the rock, porosity, permeability cementation, saturation exponents, etc. Two examples are shown in this paper. The methodology described in this paper uses capillary pressure data in conjunction with log data to reconcile the water saturation values both at the well locations and between them. The Leverett
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Pressure Sensitivity of the Joint Elastic-electrical Properties of Carbonate Reservoir Rocks
Authors L. J. North, A. I. Best and J. SothcottWe compare newly acquired laboratory data of P-wave velocity (Vp, 700 kHz) and electrical resistivity (expressed as apparent formation factor F, 80 Hz) on 24 carbonate samples with a range of porosities and permeabilities, to similar data for shaly sandstones. Crossplots of F - Vp, and the pressure sensitivity of the F - Vp relation, reveal similar trends for carbonates and sandstone, with the carbonates following closely, and extending, the clean sandstone trend. Unlike for shaly sandstones, the carbonate trends are only partly controlled by permeability. Other possible controls include heterogeneity, pore aspect ratio and size distribution.
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A New Approach to Reducing Multiple Leakage on Time Lapse Datasets
Authors E. Zabihi Naeini, H. Hoeber and S. CampbellThe standard processing solution in time-lapse analysis to the problem of non-repeatable noise sources, both coherent and random, is to perform the best denoise on all of the vintages independently. For this purpose, 4D QC measures and, of course, inspections of the seismic 4D differences are used to find the optimum parameter solution. This is currently also best practice in the removal of multiple energy, as there are many reasons for multiples to be non-repeatable, in particular changes in the source and receiver locations, changes in the water-layer and variations in the acquisition wavelet. Finding the optimal multiple subtraction operators on each vintage so as to obtain the least leakage of multiple energy on the 4D difference is notoriously difficult. In this paper we introduce a novel inversion scheme which designs shaping operators for the optimum adaptation of the multiple models to the individual vintages whilst simultaneously optimizing the subtractions for minimum leakage on the 4D difference.
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Suppressing 4D-noise by Weighted Stacking of Up-going and Down-going Wave-fields
Authors P. J. Hatchell, M. Tatanova and A. C. EvansIn deepwater environments where seismic recordings are made using ocean bottom sensors such as nodes (OBN) or cables (OBC) it is possible to produce two independent images of the subsurface using up- and down-going wave-fields. The 4D-signals should be nearly identical on these wave fields but the 4D noise differs greatly. A weighted stack of the up-going and down-going 4D differences based on their similarity is shown to suppress the 4D noises while preserving the signal. Tests of this method on deepwater OBN data acquired in the Mars field shows good results.
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4D Case Study at Ringhorne, Ringhorne East, Balder and Forseti - Integrating OBC with Streamer Data
Authors M. B. Helgerud, U. K. Tiwari, S. G. Woods, P. Homonko, A. K. Bucki, B. P. Laugier, E. Hicks, H. Hoeber and J. KhanTime-lapse (4D) seismic data at Ringhorne, Ringhorne East, Balder and Forseti in the Norwegian North Sea are used to monitor water and gas movement within the reservoirs and improve reservoir simulation models, enabling cost-effective field operations. The structural complexity of the reservoirs, their proximity to the high-impedance Cretaceous chalk, and a modest predicted 4D signal required significant effort in seismic acquisition and processing to achieve a successful final product. The 4D repeatability of the data was significantly improved at the processing stage through the use of robust quality control measures (QCs) analyzed in collaboration between CGGVeritas seismic processing specialists, ExxonMobil geophysicists and business unit geoscientists. Analysis was followed by targeted seismic processes which removed as much of the 4D noise as possible while retaining the true 4D signal amplitudes and the 4D resolution. The final result is a 4D dataset of outstanding quality that has had a major business impact.
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Evaluating Fidelity and Repeatability of Wavefields Reconstructed from Multicomponent Streamer Data
Authors K. Eggenberger, P. A. F. Christie, T. Curtis, M. Vassallo and D. J. van ManenIn marine time-lapse (4D) seismic data processing, each wavefield snapshot is usually interpolated onto a common grid to compensate for differences in acquisition, especially receiver, geometry. Because crossline sampling is usually sparse compared to inline sampling, and most mispositioning results from poor crossline repeatability from variable feathering, crossline interpolation is often aliased and causes 4D errors that increase with interpolation distance. Recent developments in multicomponent wavefield reconstruction hold promise for good amplitude fidelity, even for data that are spatially aliased to high order. In this paper we examine repeated data acquired by an experimental 3D-4C towed-cable array. Data from six crossline samples are reconstructed 1:12 and placed on a rectangular grid uniformly sampled at 6.25 m inline and crossline. Wavefields are interpolated using pressure alone and compared with wavefields reconstructed from pressure, crossline gradient and vertical gradient. We find that 3C reconstruction has excellent fidelity, recapturing highly aliased diffractions which are lost by pressure-only interpolation. 3C reconstruction is also more repeatable, outperforming 1C reconstruction, where error correlates with midpoint mispositioning. Although the experimental 3D data have limited inline aperture, the level of repeatability achieved is extremely encouraging compared to full-aperture, pressure-only, 4D datasets at an equivalent stage of processing.
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Frequency Decomposition Methods Applied to Synthetic Models of the Hermod Submarine Fan System in the North Sea
Authors N. J. McArdle, M. Ackers and B. K. BrynFrequency decomposition methods have been applied to a seismic dataset which images the late Palaeocene Hermod Fm. submarine fan system which occurs within the Viking Graben in the Northern North Sea. Conventional bandpass decomposition methods are compared to HD frequency decomposition – a technique based on matching pursuit of wavelets and the sensitivities of each method are discussed. Red-Green-Blue colour blending is shown to image in great detail channels, levees and splays. In order to understand the controlling factors determining the colour, contrast and amplitude shown in the RGB blends produced using each decomposition method, synthetic models of a Hermod splay has been produced. Within these models thickness and acoustic impedance are varied to investigate which has a larger effect. Frequency decomposition and blending of the synthetic models closely resembles blends created from the original data and it is likely that thickness changes, within the Hermod fan, which varies from above the tuning thickness in the channel core, to below tuning in the distal splays is mainly responsible for colour, amplitude and constrast changes within the blends.
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Automatic Identification of Seismic Anomalies
Authors J. A. Edgar and J. I. SelvageModern 3D seismic surveys can be so vast that expending equal interpretation effort on the entire volume of data may be impossible on any single project. Unfortunately, this may result in missed or delayed identification of opportunities. There is a need to develop techniques that help mitigate this. To this end we share the results of a promising anomaly highlighting methodology. The approach utilises seismic data and any seismically derived attributes. The seismic data is transformed into "Seabed" and "Wheeler" visualisation domains and statistical models are defined for each slice in each of these domains. Anomalies are identified based on their statistical deviation from these models. The result is a single anomaly highlighting volume. This approach enables large quantities of seismic data to be automatically scanned for anomalous zones. This ensures that more potential opportunities can be automatically catalogued and allows interpretation effort to be focussed on their possible causes.
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3D Inversion of Full Tensor Magnetic Gradiometry Data for Remanent Magnetization
Authors M. Cuma, M. S. Zhdanov, G. A. Wilson and L. PolomeFollowing recent advances in SQUID technology, airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical mineral exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes of remanent magnetization such as the amplitude and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dyke swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.
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Full Field Array Electromagnetics - Advanced EM from the Surface to the Borehole, Exploration to Reservoir Monitoring
Authors K. M. Strack and A. A. AzizIncreasing production efficiency and monitoring water/steam/CO2 movements are key issues for hydrocarbon production and geothermal reservoir monitoring. Similar technical issues exist for CO2 storage applications. They can be addressed with borehole and surface electromagnetic measurements, which are sensitive to fluid variations in the pore space. At the same time linking the electromagnetics (EM) information to 3D surface and borehole seismic data permits extrapolation to the inter-well space. Evaluating several reservoir dynamic monitoring methods and technologies leads to a practical concept of Full Field Fluid Monitoring with electromagnetics. Our implementation includes marine and land sources and receivers, surface-to-borehole arrays and a single well system that can look tens or even 100 m around the wellbore and ahead of the drill bit. On land we distinguish between exploration and production applications. For exploration it is essential to distinguish resistive and conductive targets equally well. To do this we can use natural field magnetotellurics (MT) for conductive targets like sediment thickness or geothermal targets. For resistive targets such as hydrocarbon reservoirs, we add Controlled Source ElectroMagnetics (CSEM) with a dipole transmitter. For ease of operation it is thus easiest to measure all EM components.
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Geologic Model and Fluid Flow Simulation of Woodbine Aquifer CO2 Sequestration
Authors O. Akinnikawe and C. E. Ehlig-EconomidesCO2 sequestration is one of the proposed methods for reducing anthropogenic CO2 emissions. Few studies on storing CO2 in an aquifer have been conducted on a regional scale. This case study offers a full field simulation of CO2 injection in a deep saline aquifer. A geologic model of the Woodbine aquifer was created using contour maps of the formation top, formation thickness, net sand thickness, porosity, and permeability, as well as a fault map. The woodbine aquifer has three regions; the outcrop, the fresh water region and the saline region. The simulation study considered three aquifer management strategies for injecting 63 MMT per year of CO2 from plants generating capacity of a total of 10.4GW in the vicinity of the aquifer. The aquifer management strategies used were bulk CO2 injection and two CO2-brine displacement strategies. Bulk injection of CO2 was limited to 30 years to avoid injection above the fracturing pressure and it has a storage efficiency of 0.65%. The CO2-brine displacement strategy increased the storage efficiency and project life from 30 years to 200 years. The required percentage of power plant capacity was 7.9% for the external brine disposal case and 17.51% for the internal saturated brine injection case.
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A Novel Method for CO2 Injection that Enhances Storage Capacity and Security
Authors S. M. Shariatipour, G. E. Pickup and E. J. MackayIn this novel proposed injection system, brine is extracted from the target aquifer by means of a lateral horizontal completion located near the top of the formation. An Electrical Submersible Pump (ESP) is used to extract the brine and boost its pressure, before it mixes with CO2 that is injected down the vertical section of the well. The mixing takes place in the vertical section of the well below the upper lateral. The CO2 – brine mix is then injected into the same formation in a lower lateral. A down-hole tool would be used to maximise agitation and contact area between CO2 and brine in the vertical mixing section of the well, which may be 10s to 100s of metres long, depending on the thickness of the formation. here are The advantages: 1.Because the CO2 is mixed with brine from the formation, there is no overall pressure increase. 2.The extracted brine is already at high pressure when it mixes with the CO2, greatly increasing the solubility of CO2 and reducing the volume of brine required. Energy is not expended lifting the brine to surface. Nor is there any concern about handling large volumes of acidic brine in the surface equipment.
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Multipole Seismoelectric Logging while Drilling (LWD) for Acoustic Velocity Measurements
More LessIn seismoelectric well logging, an acoustic wave propagates along a borehole and induces electrical signals along the borehole wall. The apparent velocities of these seismoelectric signals are equal to the formation velocities. Laboratory scale-model multipole acoustic and seismoelectric LWD tools are built to conduct measurements in a borehole drilled into a sandstone formation. The tools include either an acoustic receiver array of an electrode receiver array along with four acoustic sources to allow the generation of monopole, dipole, and quadrupole modes. Results show that the standard acoustic measurement of formation velocities are impacted by strong tool wave contamination in most situations. However, because the propagating tool waves do not induce any electrical signals, the seismoelectric measurements can provide a more robust velocity measurement. The multipole seismoelectric logging-while-drilling (LWD) could be used as a new logging method to measure the acoustic velocities of the borehole formations.
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Nuclear Magnetic Resonance Permeability Comparison to Image and Production Data in Secondary Porosity Conditions
More LessThe Bray-Smith equation for direct calculation of permeability from the nuclear magnetic resonance (NMR) T2 response was recently applied in several different, difficult reservoirs with excellent results. The expected production rates from calculated permeability compared closely to actual production rates. Secondary porosity from fractures or vugs complicates the ability to establish an accurate permeability value. Fractures that are open or filled can have the same characteristic log signature on conventional logs, or they may remain undetected by these standard logs. Vugs can develop as isolated features, or can be well connected to create excellent petroleum reservoirs. An adequate description of these characteristics is desirable. This paper compares the magnetic resonance permeability from the Bray-Smith equation to actual production results in open, healed, and drilling-induced fractures. It also demonstrates these responses in connected vugs, as well as in situations in which the vugs do not interconnect (oomoldic reservoirs).
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Seismic Solutions for Drilling - Real-time Pore-pressure Estimation Ahead of the Bit
Authors C. Esmersoy, Y. Yang, C. M. Sayers, C. Parekh, M. Woodward, K. Osypov, S. Yang and Y. LiuOverpressure is one of the important drilling hazards seen globally. Estimates of overpressured zone locations and overpressure magnitudes have a direct impact on well drilling and completion. Formation pressures in a drilling location are estimated from seismic velocities by using rock-physics-based transforms that map formation velocities to pore pressures. Consequently, the accuracy of the estimated pore pressures depends both on the accuracy of the velocities and the transforms. We present a method that makes optimum use of seismic data and information obtained from the well being drilled to provide pore-pressure estimates ahead of the drill bit in real time. During the drilling process, adjustments are made to the rock-physics model based on Logging While Drilling (LWD) data calibrating the model to local overburden geology. Furthermore, we show that by using while-drilling checkshot information, integrated with surface seismic data, we can improve the velocity estimates ahead of the bit. Field study results from a Gulf of Mexico well show that combining these two measurements provides a significantly better pore-pressure estimate ahead of the bit compared with pre-drill predictions.
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Indirect Indicators for Establishing Fracture Treatment Initiation Pressure in Unconventional Reservoirs
By C. SmithAfter drilling, the most expensive component of an unconventional resource play is the fracture treatment of the well. Generally, 10 to 18 stages are attempted along a horizontal wellbore that varies in length from 3,000 to 5,000 ft. Most designs have regular spacing of treatment intervals with the expectation that maximum wellbore exposure to the reservoir can be attained. One of the early calculations in the treatment is the breakdown pressure required to initiate the fracture. This determination affects the size and number of fracture pumping trucks that are brought to the location. Because the pump charges are determined by the horsepower on location, an effective calculation of breakdown pressure can result in a significant reduction in these charges. This paper investigates the potential of two different indirect indications of closure stress to arrive at these breakdown pressure values. The first technique is to apply curvature as determined by surface 3D seismic acquisitions. The second technique is to define closure stress in the horizontal wellbore from dipole sonic measurements. The paper also includes evaluations that compare the indicated and expected results with those actually achieved in the wells investigated.
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Correlation of Brittleness Index with Fractures and Microstructure in the Barnett Shale
Authors Z. Q. Guo, M. Chapman and X. Y. LiThe effectiveness of fracture stimulation techniques depends on the microstructural features which control the rock strength. We analyze brittleness index, fractures, and microstructure of the Barnett Shale for a better understanding of the correlation between them. The complexity of multiple minerals, pore geometries, and pore inclusions are modeled using the self-consistent approximation (SCA) model, with consideration of statistical distributions of pores and cracks in shales. The method is applied to the data from core samples and well logs to evaluate the aspect ratio of pores, and the proportion of stiff pores and cracks. Results show that the aspect ratio for the Barnett Shale varies between 0.01 and 1 and has a dominant value of 0.1. Comparison indicates that a definition of brittleness index including both quartz and total carbonates gives a more accurate evaluation of rock brittleness in terms of mineralogy. A good correlation between inverted aspect ratio with the brittleness index defined by λ-μ and Poisson’s ratio confirms that higher value of brittleness index corresponds to the presence of more natural cracks in the Barnett Shale.
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An Empirical Vp/Vs Shale Trend for the Kimmeridge Clay of the Central North Sea
More LessThe shales of the Kimmeridge Clay formation cover an extensive area of the North Sea and act as both seal and source rock for many reservoirs. Much of the Kimmeridge Clay of the Moray Firth area can be seen to lie off the Greenberg-Castagna (1992) Vp-Vs trend for shales. Here we document the Kimmeridge Clay trend for the Moray Firth area using a database of some forty wells, creating a suggested empirical trend for the Kimmeridge Clay shales in this area. Also, some ideas will be put forward to suggest the causes for the change in shale trend.
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Crude Oil-in-water Emulsion Flooding for EOR
Authors M. M. Moradi, M. K. Kazempour and V. A. AlvaradoInjection of oil-in-water emulsions has been identified as an effective oil recovery method. McAuliffe (1973) introduced emulsion injection as a mean to increase sweep efficiency. McAuliffe compared dilute emulsion flooding to water flooding to show that injection of emulsion into sandstone cores improved sweep efficiency. In this study, alaboratory investigation is performed to characterize crude oil-in-water emulsion and to evaluate the emulsion injection to improve oil recovery. The experiments consist of injecting crude oil-in-water emulsion with known droplet size distribution through Berea cores with different permeability values to explore the effect of drop to pore throat size ratio on the effectiveness of emulsion flooding. Results show that crude oil-in-water emulsion can effectively increase oil recovery. The blockage phenomena caused by emulsion injection can be effective even when emulsion is chased by waterflooding, but the response is a function of capillary number.
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Geologically Constrained History Matching with PCA
Authors M. Prange, T. Dombrowsky and W. BaileyPrincipal Component Analysis (PCA) has been used in a variety of disciplines such as pattern recognition, machine learning and image processing. In this article, PCA is used for history matching, i.e., adjusting the uncertain parameters of a reservoir model in order to match the simulated results with the observed behavior. This approach incorporates geological realism into the history-matching process by limiting the search to models that satisfy the prior geostatistical constraints. The Brugge model is presented as a test case to which the algorithm is applied. The results show an improvement over previously published data, yielding higher net present value (NPV) and lower predictive error.
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Predicting Productivity in the Eagle Ford through the Integration of Seismic, Geologic and Engineering Attributes
Authors T. Royer and R. PeeblesThe Eagle Ford Shale in Texas is one of the more exciting unconventional resource plays in the United States at the current time due to its size and production numbers. The key issues for the exploration companies are finding where to focus acreage acquisition and how to improve drilling plans for optimal gas and oil recovery. This can be accomplished by applying the workflow demonstrated here. Seismic data provides information about stress and fractures from azimuthal anisotropy, rock strength predictions from elastic inversion, and fault/flexure analysis from volumetric curvature. Next, an integration of geologic and engineering data with the geophysical data allows for an analysis that establishes the most important data types, in terms of production, by comparing relationships between well performance metrics and these data. Once a portfolio of performance indicators has been established, they can be brought together in multivariate space to produce a predictive model of well prospectivity and performance.
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Geohazard Prediction and Real-time Updates with Uncertainties
Authors K. Osypov, Y. Yang, C. Esmersoy, R. Bachrach and I. A. S. BuendiaLarge-scale capital assets and human life are all at risk in exploration for oil and gas. Pre-drill assessment of geohazards has, therefore, become an essential component of well planning. No well is drilled without such an assessment. Quality seismic data hold the key to accurate pre-drill pressure prediction. The behavior of rock velocities with depth is related to the state of shale dewatering and compaction, and consequently, the pore pressure that can be expected at depth. Clearly, seismic velocity is the key to pore-pressure prediction. However, as velocity estimation from seismic data is not unique, incorporation of velocity uncertainty in drilling risk analysis can yield a better understanding of the range of potential costs. Therefore, quantification of velocity and correspondingly geohazard uncertainties is essential in decision-making during drilling operations, and, thus, managing these uncertainties generates business value. In particular, decisions for casing points should take into account the geohazard risk associated with the uncertainty in our knowledge of the velocity model. This paper introduces workflows for pore-pressure and fracture-pressure prediction that account for seismic and related uncertainties associated with the ambiguity of tomographic velocity model building. Also, we demonstrate how the geohazard uncertainty can be updated in real drilling time.
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Towards Joint Interpretation of CSEM Surveys with 4D Seismic for Reservoir Monitoring
Authors O. Salako, C. MacBeth and L. MacGregorHere, we generate CSEM data from the fluid flow simulator by performing 1D forward modelling. For this purpose we utilise a heterogeneous model where resistivity is been measured using an inline acquisition geometry. It is observed, that timelapse CSEM surveys can be used to monitor realistic water flood fronts. Changes in the amplitude of the horizontal component, and phase of the vertical component, correlate with water saturation change. The sensitivities of the timelapse CSEM and the 4D seismic to water saturation change in a three phase system with pressure variation are compared using cross-plots. The CSEM is more responsive and consistently more linearly related to water saturation change than the seismic. However, direct measurements of saturation change from CSEM surveys still need to be constrained by knowledge of the spatial distribution of porosity, net-to-gross and reservoir thickness.
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The Impact of Low Throw Faults on Fluid Flow Along a Layercake Reservoir
Authors M. Welch, V. Woods and R. J. KnipeThis paper presents the results of a numerical modelling study on the impact of low throw faults on flow along reservoir units. We investigate specifically the case of a permeable reservoir unit of uniform thickness sandwiched between two impermeable seal units, that is cut by a fault with throw less than the thickness of the reservoir. Situations like this are often observed in outcrop at a range of scales, and are also seen on seismic data from producing fields, although often such faults will be below the level of seismic resolution. We use a series of 2D single phase finite difference models to calculate transmissibility along a 100m long section of reservoir offset by a vertical fault. We vary the fault throw, kv/kh ratio and reservoir permeability profile, and also run models incorporating low permeability fault rocks. We use the results to derive quantitative empirical equations for fault transmissibility multipliers and flow resistance as a function of these variables. The results show that the area reduction index, used in many reservoir simulators, will overestimate the impact of these faults on horizontal transmissibility, except where low permeability fault rocks are present, when it will underestimate the impact of the faults.
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Assigning Boundary Flux from a Groundwater Model for Simulating Coalbed Natural Gas Production, Powder River Basin, USA
By A. R. DuttonProducing gas from coalbed natural gas (CBNG) reservoirs can require extraction of a large volume of groundwater to decrease reservoir pressure and allow gas to desorb from coal. Accurately modeling the relation between water production, pressure decline, and yield for a CBNG reservoir requires a reasonably representative and accurate set of boundary conditions that account for lateral and vertical inflow of water. Boundary flux can be assigned from a regional-scale model of groundwater flow and used as a starting value for reservoir-model calibration and sensitivity analyses. A case study demonstrates taking boundary conditions from a regional model for a local-scale model of CBNG reservoirs in the Upper Fort Union Formation of the Powder River Basin, USA, and shows that ignoring boundary flux, e.g., assuming a no-flow boundary, might result in significant discrepancy between predicted and actual amount of water extraction needed to obtain targeted pressure drop for gas desorption because the CBNG reservoirs are interconnected within regional groundwater flow systems.
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Rock Physics Templates for Analysis of Brittleness Index, Mineralogy, and Porosity - A Barnett Shale Case Study
Authors Z. Q. Guo, M. Chapman and X. Y. LiRock brittleness plays a significant role in effective hydraulic fracturing for shale gas production, and is often related to mineralogy, mechanical properties, and microstructure features in shales. We construct a rock physics workflow to link elastic properties of shales to complex constituents and specific microstructure attributes. Multiple compositions and various pore geometries are considered using a self-consistent approximation (SCA) method. The laminated textures due to the preferred orientations of clay particles and possible laminated distribution of kerogen are considered using Backus averaging method to model the anisotropy of shales. Our rock physics model is calibrated on the well log data from the Barnett Shale, and is applied to generate rock physics templates for the interpretation and prediction of shale rock brittleness, mineral constituents, and porosity from elastic properties of shales. Results also show that the lamination of clay particles significantly reduces the sensitivity of shale elastic properties to porosity. Seismic AVO analysis based on the modeling data from top and bottom of the Barnett Shale formation illustrates that AVO intercept and gradient have predictable trends according to the variation of brittleness index, mineralogy, and porosity, which means that we can predict such characterizations from seismic responses.
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Complex Three-dimensional Surface Representation and Linearization for Full Waveform Inversion
More LessFull waveform inversion (FWI) provides a powerful mechanism for building complex seismic velocity models, with the potential to resolve high-resolution structure. FWI methods generally operate on a gridded representation of a seismic velocity model. However, many models used in FWI contain complex bodies (e.g. salt intrusions) that generate a large, sharp contrast in seismic properties. Refining these with FWI can prove extremely difficult when they are represented implicitly within a gridded velocity model. The constraints on model updates required at surfaces to optimise convergence are different from those required elsewhere. It is difficult to account for this difference correctly when the surfaces have no separate representation. The method described in this paper maps full waveform inversion model updates for seismic parameters to updates in the position and shape of surfaces within a 3-D model. This allows surfaces to be represented, refined and constrained explicitly during waveform inversion. The link between structural and gridded models allows ray-based constraints from tomography to be incorporated directly as constraints in FWI to increase the stability and improve the convergence of the inversion process.
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Seismic Full-waveform Inversion of Salt Geometry Using a Level Set Approach
Authors W. Lewis, E. W. Starr and D. VighAn accurate definition of geometry of complex subsurface bodies, such as salt intrusions, is crucial for imaging below such targets. Full-waveform inversion (FWI) is a method for building high-resolution seismic velocity models from nonlinear iterative minimization of the misfit between observed and synthetic seismic data. The ability of FWI to accurately recover the geometry of salt bodies can be limited by several factors, with the lack of low frequencies in the data being one of them, thus, requiring the salt geometry to be often manually interpreted. In this paper we modify the FWI algorithm to use a level set representation to parameterize and invert for the geometry of salt bodies, without adding to computational cost of FWI.
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A Scattering-based Reparameterization of Sensitivity Kernels for Full-waveform Inversion
Authors D. L. Macedo, I. Vasconcelos and J. SchleicherWith the increasing demand in complexity for subsurface models in environments such as subsalt, sub-basalt and pre-salt, full-waveform inversion (FWI) is becoming one of the model-building methods of choice. While it can, in principle, handle all of the nonlinearity in the data, in practice nonlinear gradient-based FWI is limited due to its sensitivity to the choice of starting models. To address model convergence issues in FWI, here we analyze the role of nonlinearity in sensitivity kernels, which are the centerpiece of gradient-based FWI algorithms. Using a scattering-based approach, we reparameterize the subsurface model in terms of smooth and sharp components for both compressibility and density. This leads to a decomposition of the data into a reference field that is sensitive only to the smooth model, and a scattered field sensitive to both model components. Focussing on the model backprojections from the scattered data, we provide expressions for the Frchet-derivative sensitivity kernels of all model parameters. Our results decompose current FWI kernels into several subkernels that have explicitly different levels of nonlinearity with respect to both data and model parameters. This capability to discern levels of nonlinearity within FWI kernels is key to understanding model convergence in gradient-based, iterative FWI.
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Depth Extent - A Practical Example in Magnetic Depth Estimation
Authors G. Flanagan and J. E. BainCorrections for depth extent, or the thickness of the assumed magnetic body, are critical in obtaining accurate magnetic depth to basement estimates in common exploration environments. However, determination of the correct thickness to depth (T/D) ratio would seem to require a pre-supposition of the approximate depth to basement and magnetic layer thickness. We show by application to the "Bishop" model that significant improvements in depth to magnetic basement can be made by incorporating a thickness to depth (T/D) ratio in our depth calculations by making reasonable geologic assumptions even when no a-priori knowledge exists as to the appropriate T/D to use. Using basic assumptions of isostasy combined with known geologic information gained through integration with seismic, gravity and well control yields even better results with limited additional effort. Better magnetic depth to basement estimates can have a direct impact in improving regional structural and basin models resulting in an improved understanding of the regional tectonic elements and play characteristics.
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Depth Extent - An Overlooked Parameter in Magnetic Depth Estimation
Authors G. Flanagan and J. E. BainMagnetic depth estimation has been an essential component of geophysical interpretation since its earliest days. Virtually all depth estimation techniques are based in some measure on determining curve characteristics of relatively simple body geometries such as infinite dikes, contacts and thin plates. Such techniques worked reasonably well in continental areas of thick crust and relatively shallow basins. However, as exploration has moved into the offshore and other remote areas where basins are extremely deep and correspondingly the crust is significantly thinner, these methods tend to break down and yield depth estimates which are generally much too shallow because the assumed infinite thickness model doesn’t fit the geology. We present modeled results to demonstrate the thickness effect, develop corrections to account for these effects and demonstrate the value of these corrections in improving the results on simple models.
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Egregious Euler Errors - The Use and Abuse of Euler Deconvolution Applied to Potential Fields
Authors A. B. Reid, J. Ebbing and S. J. WebbEuler deconvolution is commonly applied to magnetic and gravity interpretation problems. For the deconvolution to be successful, care must be taken to choose parameters properly. 1. The interpretation problem must be appropriate to the method (only one depth to be estimated at each point). 2. The field must be adequately sampled, with no significant aliasing. 3. The grid interval must fit the data and the problem. 4. The gradients (measured or calculated) must be valid. 5. The deconvolution window size must be at least twice the original data spacing. 6. The structural index in use must be appropriate to the expected geology. If these are not done, the process will yield grossly misleading results. An example from southern Africa is used to demonstrate the deleterious effects of poor parameter choice.
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Pressure Dependent Electrical Resistivity Anisotropy in Carbonate Reservoir Rocks
Authors L. J. North, A. I. Best and J. SothcottWe present laboratory measurements of electrical resistivity anisotropy from a suite of 37 carbonate samples. These laboratory results show the occurrence of resistivity anisotropy in lithologies ranging from biomicritic limestone to hydrothermal dolomite. Our results also suggest that the anisotropy we observe on the centimetric scale of our samples is an intrinsic property of the rock resulting from fabric scale heterogeneity as apposed to micro-fracturing.
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Geophysical Pore Type Characterization from Seismic Data in Carbonate Reservoir
More LessPore geometry in carbonates control the fluid flow properties and geological story, the purpose of our work is to predict pore type distribution from well observations and seismic data based on geologic understanding of the reservoir. Based on developed rock physics model which can take into account volume fraction of pore type quantitatively, we bridge the three defined geophysical pore type with extracted seismic properties from seismic data. The approach is evaluated on real well log data and seismic data from offshore Brazil carbonate reservoir, we obtain the distribution of reference pores, stiff pores and cracks which can be used to predict reservoir permeability.
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Use of Micro-X-ray CT Analysis for Porosity and Permeability Characterisation of Volcanic Reservoir Rocks
Authors C. R. Couves, S. Roberts, A. I. Best, A. Racey and I. TrothApplication of micro-focus X-ray computed tomography (μCT) to a suite of variably altered volcanic rocks allows quantification and 3D visualisation of the pore space down to a μm scale. This provides unprecedented insights into pore morphologies and when combined with traditional petrographic observations, becomes a powerful tool to analyse the interaction of secondary mineralisation with porosity. We determine porosity is controlled by the amount of vesiculation, the abundance of quench fractures and the original glass content. Permeability is often controlled by the degree of vesiculation, whereby samples with vesicle densities greater than 10% show significantly higher permeabilities due to the development of inter-vesicular micro-fractures. Total porosity is enhanced by the hydration of mesostasis and the alteration of phenocryst phases, however μCT visualisation show these micro-pores are not contributors to the volcanic rocks permeability, because they are isolated.
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How Multicomponent Data Enable Effective Seismic Interference Elimination from Marine Acquisitions
Authors M. Vassallo, K. Eggenberger, D. J. van Manen, S. Rentsch, W. Brouwer and A. ÖzbekWe present a fast and effective method to detect and eliminate seismic interference from 3D marine data measured by four-component (4C) streamers. The method we propose acts on each shot record independently from the others, relying on the pressure wavefield being reconstructed (eventually deghosted) on a 2D grid, densely sampled in both the inline and the crossline directions. Such reconstruction is enabled by matching-pursuit-based signal processing techniques proposed recently in the literature that have the capability to explicitly use the information of the multicomponent measurements. Without these measurements, the reconstruction capability in the receiver’ domain would be seriously compromised by the strong crossline aliasing. We show that the interference can be easily isolated and removed from the data, without affecting the seismic signal of interest after the data are reconstructed on a dense grid of receivers. When supported by vector based seismic interference detection, this technique has the potential of being automated and applied directly during the acquisition timeframe.
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L1 Pseudo-Vz Estimation and Deghosting of Single-component Marine Towed-streamer Data
Authors R. Ferber, P. Caprioli and L. WestWe present a novel technique to estimate the data of the vertical component of particle motion from marine single-component pressure data. The particle motion data, bar an angle dependent obliquity factor, is computed by convolution of the output from L1 deconvolution of the pressure ghost wavelet with the corresponding ghost wavelet of the particle motion. The estimated particle motion data is then used in a conventional two-component technique for receiver ghost attenuation by combination with the original pressure-wave data. We applied our technique to deep-tow streamer data of a 3D over/sparse-under marine survey, in which six streamers were towed at a shallow depth, with two further streamers towed deeper. This data set enables us to compare the results from our single-component deghosting technique with optimum deghosting of the over/sparse-under data. We will show that our technique achieves improvements in bandwidth of the single-component pressure data, while not fully reaching the quality of the optimally deghosted data from the over/sparse-under survey.
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Separation of Signal and Noise by Signature Deconvolution
More LessThe paper proposes a method for signature deconvolution that also separates signal from noise and provides a measure of the quality of the estimated source signature. Classical signature deconvolution designs an inverse filter for the estimated source signature and convolves this filter with the measured data to obtain an estimate of the earth impulse response. There is no measure of the quality of the result. This paper formulates recovery of the earth impulse response as the calculation of a Wiener filter in which the estimated source signature is the input and the measured seismogram is the output. There is thus a separate Wiener filter calculation for each trace. Convolution of this filter with the estimated source signature is the component of the measured data that is correlated with the estimated signature. Subtraction of this correlated component from the measured data yields the uncorrelated component: the estimated noise. If the estimated source signature contains errors, the estimated earth impulse response is incomplete, and the estimated noise contains signal, recognizable as trace-to-trace correlation. The method can be applied to many types of geophysical data, including transient and electromagnetic data; it is illustrated with an example of marine transient electromagnetic data.
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Gabor Deconvolution for Seismic Resolution Enhancement
More LessSince viscoelastic attenuation processes are ubiquitous in subsurface media, the source wavelet rapidly evolves as the wave travels through the earth. The traditional Wiener’s deconvolution assumes that seismic data is stationary, and only deals with a limited class of attenuation effects in an average sense. In this paper, the Gabor deconvolution is implemented to enhance seismic resolution by eliminating the source wavelet and the attenuation process simultaneously. This algorithm is derived from an approximate factorization of a nonstationary trace model based on Gabor transform. The Gabor deconvolution operator is obtained by assuming white reflectivity and minimum phase and attenuation. An energy balance between the input trace and the Gabor deconvolution result is needed since there is a smoothing process. Synthetic and real data examples demonstrate that the Gabor deconvolution produces a considerable high-resolution image.
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High-resolution Moveout Transform - A Robust Multiple Attenuation Technique
Authors H. Masoomzadeh and A. HardwickAs an alternative to the conventional high-resolution Radon transform, we propose a time-domain approach to transform a gather of pre-stack seismic data into a gather of highly-resolved traces in the transformed domain. Using a range of various velocity functions in a standard NMO correction routine we iteratively identify the most energy-bearing functions and transfer the corresponding stackable energy consecutively. Iso-moveout functions can be used to avoid the distortions related to the NMO stretching. Application to synthetic and real data has shown improvements in resolution and performance. Higher resolution results in less ambiguous aperture compensation and therefore more successful reconstruction of stackable seismic events in the large gaps of missing data. This feature helps to improve the accuracy of modeling multiple events particularly in the near offset zone.
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Enhanced Shallow Water Demultiple with Water Bottom Reflection Modeling
Authors H. Wang, Y. Sun, S. Boyer, G. Yu, J. Stein, S. van Reenen and K. HellmanShallow Water Demultiple (SWD) is a very challenging problem for marine seismic data processing. In shallow water environments, water bottom reflections are recorded only on a few near offset traces because critical reflection angle is reached quickly. In very shallow water, water bottom reflections may disappear completely. This poses a limitation to any convolution based demultiple methods such as Surface Related Multiple Elimination (SRME) and SWD to predict first order multiple. In this paper we propose a way to enhance these aforementioned methods by modeling the water bottom reflection and then adding it to the recorded seismic data. The modified data can then be used to predict first order multiple using SRME and/or SWD. We call these methods enhanced SRME and enhanced SWD, respectively. We will also demonstrate that an optimal way to perform the multiple elimination is to cascade the enhanced SWD followed by SRME. We call this methodology Cascaded Enhanced Shallow Water Demultiple (CESWD). Our test results show that enhanced SWD is better than enhanced SRME, and CESWD is better than enhanced SWD. Finally a comparison of these methods is presented by applying them to a real data example. The enhanced methods produce better than their conventional counterparts.
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Application of MWD for Shallow Water Demultiple - Hibernia Case Study
Authors H. Jin, M. Yang, P. Wang, Y. Huang, M. J. Parry and Y. Paisant-AllenModel-based Water-layer Demultiple (MWD) is a recently-developed method aimed at tackling the challenge of multiple attenuation in shallow water. MWD works by modeling the Green’s function of the water-bottom primary reflections based on a user-supplied water-layer model, then convolving it with the recorded data to predict water-layer-related multiples. In this paper, MWD is applied to Hibernia field data which has a water depth of around 70-90 meters. The results show that while SRME by itself has limited success, MWD is effective in attacking water-layer-related multiples. The effectiveness is attributed to the fact that MWD predicts the multiple models with correct relative amplitude and a spectrum similar to the input data’s. SRME, on the other hand, suffers in shallow-water situations, primarily due to cross-talk between multiples. Once the water-layer-related multiples are removed by MWD, SRME can then be applied to predict and eliminate other types of surface-related multiples which tend to have longer periodicity and less cross-talk. The combination of MWD and SRME is demonstrated as an effective demultiple package for shallow-water data and results in fewer residual multiples and better-preserved primaries over tau-p gapped deconvolution. This, in turn, contributes to a more realistic velocity model and, finally, higher quality images.
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Multidimensional Land Noise Elimination Technology - 5D Semblance Based Algorithms in Exploration - Theory and Practice
Authors R. Wojslaw, J. A. Stein and T. LangstonA generalization of the standard semblance equation is used to develop a 5D noise elimination algorithm. After explaining the methodology we illustrate its power by applying it to a real seismic data set. Additionally, we will compare the results to the 3D version of the same program and show that the inclusion of the extra dimensions greatly benefits the noise cancelation process.
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Case Study - Residual Scattered Noise Attenuation for 3D Land Seismic Data
Authors P. Bilsby, D. F. Halliday and L. R. WestWe show that residual scattered noise can be predicted and subsequently subtracted from a point-source, point-receiver 3D land-seismic dataset. We use a modified form of seismic interferometry that allows wavefields to be predicted between sources and receivers provided one of the inputs can be replaced by a model. We use a simple model of the direct ground roll as one input, and use recorded data containing residual scattered noise as the second input for interferometry. We show that the removal of a layer of low-frequency residual scattered noise leads to better continuity of horizons within the stacked sections, providing more reliable data for interpretation.
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Estimation of Interval Velocity and Attenuation Anisotropy from Reflection Data at the Coronation Field
Authors J. Behura, I. Tsvankin, E. Jenner and A. CalvertAttenuation can be extremely valuable in characterizing gas accumulations in shales and sands. In fractured reservoirs, anisotropy can provide additional information about the distribution of fractures. Here, we apply a layer-stripping approach to wide-azimuth P-wave data acquired over a gas reservoir in the Coronation Field, Alberta. The main processing steps involve estimation of traveltimes from the top and bottom of the target layer followed by computation of the interval NMO velocity and attenuation coefficients using velocity-independent layer stripping and the spectral-ratio method. The vertical attenuation coefficient shows a reasonable correlation with existing gas-producing well locations, and, therefore, serves as an indicator of gas accumulation in pore space. Based on the attenuation information, we conclude that the lower half of the survey area has significant gas reserves. In these areas, the estimated azimuthal velocity anisotropy can be used to plan horizontal wells oriented orthogonal to the fracture direction.
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Monitoring Seismic Attenuation Changes Using a 4D Relative Spectrum Method in Athabsca Heavy Oil Reservoir, Canada
Authors A. H. D. Shabelansky, A. Malcolm and M. FehlerHeating heavy oil reservoirs is a common method for reducing the high viscosity of heavy oil and thus increasing the recovery factor. Monitoring these changes in the reservoir is essential for delineating the heated region and controlling production. In this study, we measure the changes in the seismic wave attenuation of a heavy oil reservoir by constructing time-lapse Q factor maps using a 4D-relative spectrum method. This method estimates seismic attenuation from surface reflection seismic surveys by calculating, for each trace in each survey, the attenuation (Q-factor) using the spectral ratio (Toksoz et al. (1979)) between a reference reflector above the reservoir and a second reflector below the reservoir. The results of our study on a real data set exhibit alignment along the injection wells, indicating that seismic attenuation can be used to monitor changes in a heavy oil reservoir.
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A Phase-unwrapped Solution for Overcoming a Poor Starting Model in Full-wavefield Inversion
Authors N. K. Shah, M. R. Warner, J. K. Washbourne, L. Guasch and A. P. UmplebyWe present a new phase-unwrapped full-wavefield inversion (FWI) methodology for applying the technique to seismic data directly from a poor or simple starting model in an automated, robust manner. The well-known difficulty that arises with a poor starting model is a ‘cycle-skipped’ relationship between predicted and observed data at useable inversion frequencies. The local minimum convergence of cycle-skipped data is one of the root causes for inaccurately recovered models in practical applications of FWI. Further it is why practical applications to date have focussed on favourable datasets possessing very low frequencies and an accurate velocity model already known prior to applying FWI. Here we tackle the cycle-skipping problem by inverting the lowest useable frequency of the data using an unwrapped phase-only objective function. We minimise a smooth, phase-unwrapped residual, extracted from the data by exploiting the spatial continuity existing between adjacent traces. The majority of field datasets acquired today are spatially well enough sampled to be manipulated in this way. An application to highly cycle-skipped synthetic data from the Marmousi model shows the benefit of applying phase-unwrapped inversion to a dataset prior to starting conventional FWI.
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Principal Component Analysis for Seismic High Density Wavefield
More LessThe author analyzed the high density wavefield signal characteristics recurring to the principal component analysis technique in order to optimal the acquisition geometry design.. The high density seismic wavefield (dx=6.25m) and their variance (or PCA spectrum) illustrate that most of the energy focus on the first several principal components. It shows more concentrated trend compared with the principal component diagrams corresponding to spatial interval of 12.5m and 25m. It also conveys the conclusion that the spatial sampling interval of 6.25m discriminates with the sampling interval of 12.5 and 25m intrinsically which could be a guidance for optimal acquisition geometry design.
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3D Seismic Resolution Analysis Using Double Focus Beam Method
Authors M. M. Yang and Y. WangThis abstract simplifies the double-focus resolution algorithm based on seismic ray theory. First, the response of the target point at the surface is calculated according to seismic ray theory, which is called CFP gather. Then the CFP gather is focused to the target layer by downward extrapolating, getting the focus beams. Both the geological models and the 3D seismic acquisition geometry have influence on the resolution of focus beam. This method is proved to be effective and accurate by theory and actual models, which plays a guidance role in the optimization of 3D observation geometry.
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Marine Dispersive Noise Removal Using f-P Modelling
Authors R. O‘Driscoll, P. Bouloudas, D. G. King and A. WythesIn shallow marine seismic data where a hard water bottom is present, post-critical multiple energy can become trapped in the water layer creating dispersive noise modes. The phase velocities of this noise are sensitive to the near surface parameters which can vary significantly across a survey. In certain geological settings, some of these modes dominate the shot record but cannot be attenuated by traditional demultiple techniques such as Tau-P deconvolution as their period is too short. We present a simple method of attenuating these modes by modelling them in the frequency/slowness (f-P) domain. After Tau-P deconvolution, the slowness and dominant frequency of the strongest remnant noise mode is automatically picked for each shot record in a survey. This allows the mode to be isolated, modelled and subtracted from the shot record. Automated f-P modelling was tested on a large marine 3D survey over the North-West Shelf, offshore Australia where shallow carbonates cause complex dispersive noise. This data driven technique was shown to reliably model this dispersive noise, allowing it to be attenuated and reveal underlying primary energy.
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Shallow Water Demultiple Using a Multichannel Prediction
Authors H. Wang, Y. Sun, S. Boyer, G. Yu, J. Stein, S. Van Reenen and K. Hellmanfective ways are needed to eliminate these types of multiples in shallow water environments. Many model-driven methods for Shallow Water Demultiple (SWD) have been developed in the past with partial success. The success of multiple predictions lays heavily on the ability to make an accurate water bottom model. The current trend in the industry for SWD is to derive a 2D predictive operator with a predictive lag calculated from bathymetry, thus making it a totally data driven method. This approach has showed some improvements but much is left to be done. We derive and present here a data-driven method which is inherently more challenging but as we shall see it produces better results. Tests results show that our SWD methodology works better than SRME. Furthermore we will also show that a cascaded SWDSRME approach can give an even better result than either of these applied alone. We have successfully applied this technique in many production environments around the world and we will show some examples in this paper.
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Multiple Prediction through Inversion - GPU Acceleration
More LessMultiple prediction through inversion (MPI) is an effective method for the free-surface multiple attenuation. Due to the large amount of matrix operations involved, the computational cost of MPI scheme is considerably expensive. In this paper, we apply the technology of Graphics Processor Unit (GPU) into the implementation of MPI scheme. As GPU has highly parallel structure and is suitable for matrix related calculations, we can improve the computational efficiency dramatically when applying GPU for the free-surface multiple prediction with MPI scheme.
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Badenian Evaporite Evolution and Methane Entrapment in the Transylvanian Basin
Authors C. Pene, O. Coltoi and S. GrigorescuBADENIAN EVAPORITE EVOLUTION AND METHANE ENTRAPMENT IN THE TRANSYLVANIAN BASIN Constantin Pene1, Octavian Coltoi2, Stefan Grigorescu2 1University of Bucharest, Faculty of Geology and Geophysics, 6 Traian Vuia Str., Bucharest, Romania 2Geological Institute of Romania, 1 Caransebes Str., 70124-RO, Bucharest, Romania Summary Transylvanian Basin is the main producer of methane in Romania. The correlation of data shows that at least during of the Lower to Middle Badenian in Transylvanian Basin and Carpathian Foredeep was a unique sedimentary basin, isolated from Central Parathetys and Mediterranean. The Badenian salt in the Transylvanian Basin has almost continuously development in average thickness of 400m. The top of the salt layer is highly undulating, while the base is nearly horizontal. The salt shapes has been different: in the central part there are pillows, layers and piercements; in east and west, salt flowing created the salt diapirs and salt wall. The salt movement created large dome-shape of the overlying deposits that represent excellent traps for the biogenic methane entrapment. The aim of this paper is to investigate the Badenian evaporites evolution we used a model of an elastic plate overlying a viscous fluid to understand the causes that created zones with different intensity of the diapirism.
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Biofacies, Palaeo-environments and Stratigraphy of the Ratawi, Minagish and Makhul Formations of Kuwait
Authors S. Crittenden, M. Al-Baghli, G. Gegas, A. P. Kadar and P. ClewsThree major micropalaeontological biozones / biofacies assemblages are described from the latest Jurassic (Tithonian) to Early Cretaceous (Berriasian to Early Valanginian) lower part of the Thamama Group in Kuwait. The thin sections studied and analysed are predominantly from core samples but supplemented with a small number of ditch cuttings where core was not available. The sample interval was irregular as the thin sections were originally chosen for reservoir parameter studies that included petrography, microfacies, porosity and permeability. Three major bio-assemblages (defining local biozones) can be identified and approximate the Makhul Formation – radiolarite assemblage of the restricted platform muddy limestones; the Minagish Formation and Ratawi Limestone Member – calcareous algae / foraminiferal assemblage of shallow shelf, clean carbonates including oolite / grainstone shoals; and the Ratawi Shale Member – foraminiferal assemblage of the mixed clastic and carbonate sedimentary environment. Within these three major bio-assemblages a number of subsidiary biofacies subdivisions, temporal and spatial, can also be identified. This allows the recognition of vertical stacking patterns and depositional cyclicity, as observed in the core description and petrographic microfacies studies, of this important hydrocarbon bearing interval.
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Play Identification for Paleogene Rift Sediment in Ngimbang Sub Basin, East Java Basin, Indonesia
By A. AnggunThe purpose of this study is to identify type of play for Paleogene rift sediments in Ngimbang Sub Basin to open a new opportunities to search the future potential of hydrocarbon reserves. Each phase in rift evolution will be subdivided by Prosser (1993) tectonostratigraphy concept. Seismic stratigraphy interpretation is used to distinguish each tectonic phases: rift initiation (S2), rift climax (S3), immediate post rift (S4), and late post rift (S5). Depositional models in Ngimbang Sub Basin will be generated from isochrones maps for S1, S2-S1, S3-S2, S4-S3, and S5-S4 tectonic phase with its own unique seismic expression to recognize its facies. Most of the working process will be focused on the distribution and understanding type of reservoir rock from Ngimbang Formation within the area while other geological factors will be utilize from combination of data interpretation and other publications to conclude play type for Paleogene rift sediments in East Java Basin. In conclusion, there are at least four type of play in this area: facies change play, alluvial fan play, basin floor fan play, and channel fill play. It is expected that these play will open a new exploration opportunity in Ngimbang Sub Basin.
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Upper Jurassic and Lower Cretaceous Reefs of Shatsky Ridge, Black Sea
Authors S. Bachin, M. Skvortsov, V. Gayduk, S. Proshlyakov, A. Mityukov and N. MyasoedovThe main oil and gas prospects of the Shatsky ridge are associated with large anticline structure: North-Black Sea one, Gudauta, Maria, etc. Some of the potential reservoirs are Upper Jurassic – Lower Cretaceous reef buildups. About 4,500 sq.km of 3D seismic survey was shoot within certain potentially productive uplifts by “Rosneft” Oil Company during 2008-2011 time period. This study presents the major results of seismic interpretations of the structure and distribution of reef buildups. The performed study allowed to find out new prospective objects associated with probable reef buildups on the Shatsky ridge and clarify geological structure of previously revealed objects and condition of their formation. The results obtained were used by “Rosneft” Oil Company to design their geological exploration activities in their licenses on the Black Sea shelf.
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Giant ACG Field, South Caspian Basin, Azerbaijan - Peculiarity of Oil and Gas Content
Authors A. Narimanov, A. Javadova, L. Alimuradova and E. Rzayev- In spite of detailed study of the field, we are keeping to face with surprises of nature. There are remarking very compound phase conditions of hydrocarbon mixture in some deposits of the field. For example, the presence of gas condensate deposit with w
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Sedimentary Architecture and Reservoir Rocks Origin of the Maykop Formation Challenge Exploration in the Black Sea
Authors A. Kitchka, M. V. Kharchenko, S. G. Vakarchuk and T. E. DovzhokDiscovery of the Subbotin oil field in 2005 has proved commercial productivity of the East Black Sea sub-basin that gave an additional impetus to further exploration hopes as previous exploration success in the region was quite minor due to lack of reservoir rocks and deepwater setting for most promising exploration targets. However, several factors have challenged exploration pace on this field. An analysis of seismic data, core and field studies have shown clinoform architecture of the Maykop formation and turbiditic origin of its reservoir rocks that complicate as exploration as development of the field. Employing the experience gained during exploraion of Subbotin oil field can make high chances to exploit its success basinward, to deepwater part of the Black sea.
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Pliocene Deep Reservoirs in the South Caspian Basin
More LessThe complex analysis done on the offshore South Caspian Pliocene Productive Series demonstrates accumulation of quartz rich sandstones in certain stratigraphic intervals such as PostKirmaki sand, Fasila, Balakhany VIII. The modeling and factual data show that these sediments display moderate to good reservoir properties at the depth below 7 km due to overpressure, formation of secondary porosity and favorable cementation history. The best reservoir properties in the central deeply subsided part of the South Caspian basin are expected for Fasila Suite sediments.
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A Seismic Vertical Vibrator Driven by Linear Motors
Authors R. P. Noorlandt, G. G. Drijkoningen and R. M. SchneiderIn this paper we present a newly developed vertical seismic vibrator driven by linear motors. We explain the different components the vibrator consists of. We show that the harmonic distortion of the linear-motor vibrator signal is very small. We also show that, without applying a feedback loop on the pilot signal, the weighted-sum ground force signal and its harmonics, is heavily depended on the coupling of the vibrator with the ground.
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LWD Sonic - Robust, Reliable and Accurate while Drilling
More LessThis paper looks at how LWD sonic technology benefits the client to get high quality sonic data while drilling to make fast, reliable real time decisions, focusing on how the unique design of the tool and each component allows for this to be achievable. An example from offshore China illustrates how the client used the sonic data for formation evaluation work in exploration and production wells due to the robust stability and ability to provide reliable and accurate data, where it was not previously achievable. In particular, paying attention to how in real-time the client achieved the optimum result by making real-time decisions in pore pressure evaluation and to proactively adjust the well trajectory.
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Wells Property Simulations Driven by the Shape of a Geobody
Authors C. Magneron and J. FelderGeostatistical models used in the Oil & Gas industry are mainly variogram-based models.They enable to build a large number of estimation and simulation algorithms. In the stationary case, variogram-based approaches are rather intuitive as some variographic parameters can be interpreted direcly in terms of structural properties, namely size and orientations. However as soon as the target area becomes large or involves complex structural patterns, the use of stationary variogram-based models leads to a loss of precision and some difficulties to reproduce the strutural complexity of the reality. Geostatistical solutions exist to deal with structural variations inherent in spatial data sets. In the framework of quasi-stationnarity, it is possible to consider spatial variations of the parameters of variogram-based models. Based on it, Moving GeoStatistics (M-GS) gathers a set of techniques dedicated to the local and optimal determination of variographic parameters. The optimal parameters are called M-Parameters. In this paper, we present a new method to compute M-Parameters. It is based on image-processing techniques and consists in throwing and stopping rays in all directions of the space. The method benefit is illustrated through examples of porosity simulations guidded by the shape of a channel.
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Hydraulic Fracturing of Tight Shale Monitored by Acoustic Emission and Ultrasonic Transmission
Authors S. Stanchits, A. Surdi and R. Suarez-RiveraIn this work we study the effect of fluid viscosity on hydraulic fracturing initiation and near-wellbore propagation on block samples of tight shales subjected to representative effective in-situ stress conditions. Combined analysis of acoustic emission, ultrasonic transmission and volumetric deformation indicates that the viscosity of the injected fluid had a strong influence on hydraulic fracturing initiation, fracture propagation and fracture geometry. Injection of high viscosity fluid into the stressed tight shale resulted in fracture initiation at a bore pressure higher than the overburden stress and occurred significantly earlier than the borehole breakdown pressure. After initiation, the hydraulic fracture propagated symmetrically from the borehole in the direction parallel to the maximal horizontal stress, causing significant volumetric deformation of the rock. In the case of injecting a low viscosity fluid into the stressed block, fracture initiation occurred at a borehole pressure significantly lower than it was required with the higher viscosity fluid, and occurred almost simultaneously with the bore pressure breakdown. AE measurements during hydraulic fracturing allowed us to estimate that the average speed of hydraulic fracture propagation was approximately thousand times faster for the low viscosity fluid than for the high viscosity fluid.
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Depositional Environments of Prospective for Shale Gas Silurian Deposits of the East European Platform, Ukraine
Authors I. M. Kurovets and Y. V. KoltunThe marine deep-water sediments of Silurian and Ordovician extend in south-eastern direction from the Polish border to Romania. Nature and intensity of the processes, which had an impact on the formation of the Silurian strata have been determined by the location of the region, which tectonically belonged to the zone of peri-cratonic subsidence, and paleooceanographically was the floor of the peri-continental sea. The continuous Silurian-Lower Devonian sequence shows almost sub-meridional strike of the clearly defined facial zones and significant increase of the thickness of stratones in western direction. Stratigraphically the Silurian sections made up of rather variable poly-facial sequences of the lagoon, shallow water, open shelf and slope sediments of the ancient basin. Their location and boundaries were changing with time and therefore the area of occurrence of similar facies at different stratigraphic levels do not coincide. The continental slope in paleo-basin was geomorphologically formed at the end of Ludlow, when the trough of the Teysseyre-Tornquist Zone rose and an intense subsidence started in it. At the beginning of Devonian the uplift movements at the platform caused the gradual shallowing and regression of the paleo basin, the eastern coastline of which in Pragian has moved to the TTZ.
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A Novel Analytical Method to Estimate the Carbon Capture Potential of Depleted Oil and Gas Fields
Authors E. Valbuena, M. A. Barrufet, G. Falcone and C. Ehlig-EconomidesCarbon capture and sequestration (CCS) is one of “Five Grand Challenges” of E&P industry to develop sustainable use of hydrocarbons. Success of CCS projects depend greatly on selecting adequate target reservoirs for storage. Numerical simulation is time consuming, expensive, and requires detailed input data, yet it remains the accepted method to forecast CO2 storage in depleted oil and gas reservoirs. This investigation presents an analytical method, based on thermodynamic state functions, to estimate ultimate CO2 storage capacity in such reservoirs. The proposed method makes fast and accurate estimates, while considering contaminants such as nitrogen and carbon monoxide contained in the injection stream. Results are used to select target reservoirs, estimate amount of CO2 to be stored, design injection schemes and surface facilities, and evaluate the injection process. Performance of the new method was compared against compositional reservoir simulation for a wide range of reservoir depletion pressure, target pressure, temperature, water saturation, fluid compositions, and reservoir size. Excellent agreement was observed between analytical and numerical models, with only 1.3% molar basis average difference. Average oil, gas, and water saturations were also matched. Analytical method performed orders of magnitude faster than numerical simulation, with an average time per run of 5 seconds.
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Seimic Sensors Orientation Using Earthquake and Noise Recordings
Authors F. Grigoli, S. Cesca, J. Schweitzer and T. DahmIn this work we apply a new technique based on a complex linear least-squares method, to derive orientation of seismic sensors for two different acquisition setups: a borehole sensor array deployed in a gas field located in the Netherlands and a seismic array in Norway. Sensor alignment is performed using earthquake (both regional and teleseismic events) and low-frequency coherent noise recordings. Finding orientation of seismic sensors by complex linear least-squares approach has the advantage that we are dealing with a linear inverse problem. For this reason there are no complications with local minima and it is possible to add more independent data (other seismic events) to better constrain the solution of the inverse problem. Furthermore, our methods is faster than relative orientation methods based on waveform cross-correlation, and allows to estimate simultaneously all relative orientation angles of each sensors pair. An other advantage of our methods is that, unlike polarization analysis based methods, it can be applied to the full waveform and not only to highly linear polarized part of the waveform.
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Seismic Attributes for Acoustic Full Waveforms
Authors K. Wawrzyniak-Guz and J. JarzynaAcoustic full waveforms (AFW) can be regarded as an elastic wave field generated and recorded in borehole. In spite of significant differences with seismics, physical background of both methods is the same and some seismic procedures can be included in processing of AFW. This paper presents how to calculate seismic attributes for AFW with the use of traditional seismic software. The goal was to get numerical values of the attributes for P (compressional), S (shear), and Stoneley waves and using them in quantitative interpretation. Results were applied to enhance recognition of the facies distribution in the carbonate rocks in the Polish Lowland. Proposed methodology has potential in detail characterization of the rock formations and it proves that the attributes can be included in quantitative interpretation of AFW.
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Application of Optimal Technique of Shear-wave Velocity Prediction based on Xu-White Model
More LessWith the development of reservoir exploration, complex reservoir in complicated geological conditions has been the important exploration target. Pre-stack elastic wave impedance inversion is the effective way to resolve complex reservoir lateral prediction problem. However, there is no shear wave velocity information which is necessary for restricting and calibrating the pre-stack elastic wave impedance inversion result in many areas. In this paper, an optimal technique and its workflow was obtained for shear wave velocity predication using P-wave velocity and conventional logging data. The pseudo-porosity and nonlinear optimal algorithm were applied into Xu-White model. The validity of this technique is demonstrated by comparing the prediction results with measured shear wave velocity in China Bohai Bay. Then the technique was applied to restrict and calibrate the predicted S-wave velocity in an area of Congo. Subsequently, the good results of reservoir prediction were obtained using pre-stack elastic wave impedance inversion.
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Application of the Velocity-deviation Log in Determining Pore Types and Permeability Trends of Nubia S.S Formation
By T. Shazlyvelocity-deviation log is calculated by converting porosity-log to a synthetic velocity log using the time-average equation.The difference between the real sonic log and the synthetic sonic log can be plotted as a velocity-deviation log.The deviation log reflects the different rock physical signatures of the pore types.Positive velocity deviation mark zones where frame forming pore types dominate.Zero deviations show intervals like interparticle or microporosity.Negative deviations mark zones caused by a cavernous,fracturing,or by a high content of gas.By tracing the velocity deviations continuously down hole,one can know the diagenetic zones,that are characterized by these varying pore types.In addition,this method can be used to observe permeability trends.The velocity–deviation log is applied on the Nubia sandstone Formation for nine wells distributed in the study area of Abu Rudeis-Sidri Field located on the eastern coast of the Gulf of Suez.This study showed that the major pore type for the studied Fm.is the dissolution porosity,this means that the S.S rock of this Fm.is affected by leaching processes which,lead to the presence of the unconnected pores so this Fm.can not be acted as a good reservoir rock.
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FWI for Sub-surface Imaging in the Context of Seismic while Drilling - Synthetic Study
Authors A. Romdhane, E. Querendez and H. M. HelsetSeismic data acquired while drilling can be used to complement the information provided by surface seismic and reduce drilling hazards. In this paper, we investigate the potential of full waveform inversion (FWI) to provide an update of the subsurface image ahead of and around the drill bit while drilling. Inversion tests are carried out for three different acquisition configurations: surface to wellbore seismic, well to well seismic and wellbore seismic with sources and receivers placed along the drill string. A 2D visco acoustic FWI code in the frequency-space domain is used. We considered a deep section of a simplified P-wave velocity model based on data from the Norne field. A fault, considered as a target in the numerical experiment, is created by shifting the layers along an oblique line. The method is applied to a smooth initial P-wave velocity model. For the different acquisition geometries, the final velocity models show that the method can provide clear indications about the presence of the fault ahead of the drill bit. The quality of the models depends on the acquisition geometry. The spurious artefacts observed in the low illuminated zones reflect the strong signature of the acquisition geometry in the inversion results.
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Efficient 3D Borehole Modeling of the Reflected Wave Field While Drilling
Authors I. Veile, T. Geerits and T. BohlenBased on a closed form explicit expression for the 3D scattered elastic wave field resulting from an arbitrary multipole force source excitation in an unbounded, homogeneous and isotropic elastic medium with a plane scatterer, we have developed a numerical implementation to verify the validity of the full elastodynamic Born approximation. The concept for this new fast and efficient 3D elastodynamic forward modeling technique was developed with focus on borehole measurements and therefore aims to include the fluid-filled borehole, but it is not limited to these configurations. We present the details and specific features of the numerical implementation and show a comparison with a quasi analytic solution. Furthermore we show a convergence test to establish a sufficient discretization criterion. For simplicity we restrict ourselves to the PP reflection in a zero-offset configuration with a single force source excitation. The results are promising as it seems that the developed method is fast and accurate enough to properly model the scattered wave field (including converted waves).
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The Use of Interval Velocities as a Seismic Attribute
Authors R. Negrete Cadena and A. Reyes PimentelThe use of seismic velocities as an attribute related to hydrocarbon presence is not as common as amplitude-derived attributes (e.g. AVO) because the interpretation of interval seismic velocities depends on a range of subsurface properties such as lithology, porosity, pore pressure and fluid content. High-resolution interval velocities can be used as a seismic attribute because low velocity anomalies can be related to light hydrocarbon content. To complement the use of seismic velocities to detect hydrocarbon traps, we propose two different interval velocity-derived attributes, instantaneous acceleration and secondary velocity. Low velocity anomalies can be better delineated by using a crossplot between both attributes thereby reducing significantly the interpretation uncertainty. In this abstract we report a case study using these new attributes which suggests the presence of a reservoir of light oil or gas.
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A New Method of Multi-level Sedimentary Cycles Classification Based on Seismic Data
Authors H. J. Liu, C. X. Zhao, Y. G. Wang and K. K. ShanIt is difficult to analyze high resolution sequence stratigraphy accurately and scientifically. This paper develops a new method of multi-level stratigraphy sedimentary cycles classification based on seismic data. Seismic signal processed by Hilbert-Huang transform can be decomposed into basic mode components of different resolution. The instantaneous time-frequency characteristics of these mode components can describe the relationship among time, frequency and energy accurately, and their mirror-image contrast can classify different level sedimentary cycles. The analysis results of model data and actual seismic data show that time-frequency analysis of seismic data processed by Hilbert-Huang transform can accurately classify different level stratigraphy sedimentary cycles.
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Accurate Division of Sedimentary Sequences of Lake Delta in the Wheeler Domain
By H. YuIn the Wheeler domain graphics transformed by seismic data, the author analyzed the system tracts and the Datum Plane Cycle of lake delta, and found the method how to use seismic data to divide sedimentary sequences accurately, which made up for the low horizontal resolution of single-well Sequence. On this basis, the author established the interpretation model of sedimentary sequence of the lake delta in the wheeler domain graphics. The method achieved good application effect in the exploration of dongying delta, and it has widely applicability in the division of sedimentary sequence of all lake deltas.
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Improved Measure of Seismic Coherence Using a More Realistic Data Model
Authors Y. Tyapkin and I. MendriiSeismic coherence is a popular attribute intended to image discontinuities caused by variations in structure, stratigraphy, lithology, porosity, and the presence of hydrocarbons. We improve the technology of computing and utilizing this attribute. After analyzing the relation of this attribute to the supposed mathematical model of seismic data, we present a new method for estimating coherence. It is based on a more realistic data model that permits arbitrary variations of both signal amplitudes and noise variances between the traces participating in coherence calculation. Moreover, a generalized approach to enhance the horizontal resolving power of any coherence measure is suggested. The new methods are tested on synthetic and field data.
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Application of Volumetric Curvature to the Reservoir Rock Prediction of a Platform Carbonate within a Buried Hill
Authors X. Wang, Y. Q. Zhang, G. M. Wang, Z. L. Yu, D. Q. Liu, L. Y. Pan, X. X. Li, H. Ma, J. G. Feng and L. MengThe reservoir rock in the studied buried hill is a Paleozoic platform carbonate with the characteristics of a deeply buried rock, including low seismic signal-to-noise (S/N) ratio and a complex contact relationship with other layers; it is difficult to get good results using conventional seismic data interpretation and reservoir prediction techniques. Based on the geological features of the target interval, this paper proposes new methods for platform-carbonate reservoir prediction in terms of fracture development, flowing water solution, and the timings of fractural events by using various seismic attributes including maximum curvature, most positive curvature, most negative curvature, and Euler curvature.The seismic attributes play an important role in reservoir characterization.
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3D Seismic Discontinuity Estimation Based on the Local Structure Entropy
More LessLocating the substructure discontinuity features is one of the most challenging tasks for seismic interpretation. Comparing with the conventional methods based either on semblance or eigenstructure, local structure entropy (LSE) avoids the computation of large covariance matrices and their dominant eigenvalues. By defining a small correlation matrix, the cross correlation of four quadrants of the analysis cube is estimated. The analysis cube moves throughout the whole 3D seismic volume and the local structure entropy cube is obtained under the estimation principal. The application on the 3D seismic volume verify its validity and efficiency.
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The Use of Gas Chimney Detection in Predicting Abnormal Pressure
More LessGas chimneys or gas clouds are a primary mechanism by which deep pressures are transferred vertically in the sedimentary section, through vertical fractures. Wells drilled in gas clouds in the North Sea have been observed to have excess pressure, compared to wells drilled outside of the gas cloud. Thus the top of gas chimney may correspond to the onset of abnormal pressure. A gas chimney probability volume is created by a neural network from a set of seismic attributes extracted at examples of gas chimneys picked by an interpreter. Examples from West Africa will be shown that demonstrate the close correlation between gas chimneys and excess pressure. Traditional tools for predicting excess pressure from seismic velocities may miss these zones, since velocities are normally picked in well imaged areas.
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Confidence Intervals for Microseismic Event Locations
By U. ZimmerMicroseismic event locations are most often represented by individual points in space. Depending on the quantity and quality of input data and the localization algorithm applied each of these locations has an individual uncertainty space. This uncertainty space is often approximated by error bars. The approximated space is often interpreted as the volume where the event is located with 100% certainty. For most calculations of the uncertainty space this is not true. This paper suggests a method where the different input parameters, e.g. traveltime or hodogram information, can be translated into probability density functions (PDFs) which then allow the calculation of intervals of different confidence levels. The resulting space describes the volume where the event is located with the provided level of confidence. For most applications the 95% or 99% confidence interval is a reasonable definition of the error space but any other level of confidence can be specified. The resulting confidence interval does not have to be continuous as it is suggested by standard error bars. Comparing localization algorithms that use only a single phase arrival, e.g. P-waves, with multiple phase arrival algorithms shows the order of magnitude difference in the location accuracy.
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Statistical Analysis of Microseismic Noise during Hydraulic Fracturing
Authors M. V. Rozhkov, A. F. Kushnir, N. M. Rojkov, I. G. Dricker and S. HellmanSpatial and temporal spectral analysis of the background noise before and during hydraulic fracturing shows that surface noise is generally uncorrelated before and after well injections. Noise becomes sufficiently coherent (correlated) during fluid injection and even during a break between fracturing stages. Stacking of seismic array records with proper move-out corrections of Seismic Emission Tomography (SET) helps to suppress non-correlated noise component, but enhances both correlated technogenic noise and signals from microseismic events. Presence of correlated noise can be a factor increasing number of false alarms in microseismic bulletins which are created by semblance-based SET techniques.
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Using S-wave Splitting to Measure Fracture Compliance and BN/BT during Hydraulic Fracture Stimulation
Authors J. P. Verdon and A. WuestefeldThe effect of fractures on seismic waves is controlled by the normal and tangential compliances of the fractures (BN and BT). Rock physics models and laboratory experiments have indicated that BN/BT will be influenced by (1) the bulk modulus of the fluid filling the fracture, (2) the degree of connectivity between the fracture and equant pore space, and (3) the internal architecture of the fracture, such as the roughness of fracture faces, or the presence of detrital or diagenetic infilling material. Therefore, measurements of BN/BT will provide useful information during hydraulic fracture stimulation. We develop a method to invert S-wave splitting (SWS) data, measured on microseismic events recorded on downhole geophone arrays, for the ratio of normal to tangential compliance (BN/BT) of sets of aligned fractures. We demonstrate this method by inverting for BN/BT using SWS measurements made during hydraulic fracture stimulation of the Cotton Valley tight gas reservoir, Texas. When the full SWS dataset is inverted, we find that BN/BT=0.740.04. Windowing the data by time, we have been able to observe temporal variations, finding that BN/BT varies as the stimulation progresses, and most notably when proppant is injected.
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Analysis of B Value from Barnett Shale Microseismic Data
Authors S. Sil, B. Bankhead, C. Zhou and A. SenaIn 2010, ConocoPhillips acquired microseismic data targeting the Barnett shale from Stocker field in Texas, USA. Two horizontal wells were fractured in eighteen stages. More than 7,000 microseismic events were located using two down-hole receiver arrays. The b values were calculated using the magnitude distribution of the events in each stage by maximum likelihood methods. This work analyzed the temporal and spatial variations of b values in every simulation stage. We showed (1) that the spatial variation of the b value may help to delineate reservoir heterogeneity and (2) that the temporal variation of the b value in each stage may be related to the mechanism of fracture growth.
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Target Oriented VSP Imaging - A Sparse-inversion Approach
Authors A. K. Soni, W. Wouters and D. J. VerschuurFor reservoir monitoring surface seismic data usually has lower than desired resolution due to propagation effects of the wavefield through the complex overburden above the reservoir. VSP data, on the other hand, suffers less from propagation loss and usually have a wider frequency content and higher resolution compared to the surface seismic data. However, conventional imaging techniques applied to VSP data cause imaging artifacts due to the limited aperture and irregular subsurface illumination. These images could be improved considerably by suppressing artifacts and improving image reliability, if imaging is approached as an inversion process. Furthermore, including the free surface multiples in this process will extend the illumination, especially away from the well trajectory. A target-oriented VSP imaging procedure, using a sparse-inversion approach, is presented.
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A Correlation-based Cross-well Time-lapse Velocity Analysis
Authors I. Abakumov, D. Kiyashchenko and B. KashtanThe problem of reconstructing time-lapse velocity anomalies using cross-well data is considered. In practice, these subsurface velocity changes can be caused by the oil production activities, and cross-well seismic is a possible tool for their monitoring. The time-lapse cross-well tomography inverts time-lapse time delay between monitor and baseline first-arrivals into a velocity change. However, the cross-well tomography has limited resolution due to specific acquisition geometry. Sometimes it may create problems for determining the lateral extent of anomalies. The use of reflections in cross-well data may improve the resolution and provide additional information to constrain velocity model. We propose to optimize the functional based on the linear combination of weighted norm of correlation of time-lapse reflection images and direct arrivals. The simple numerical experiment demonstrated that the proposed functional allows us to reduce the uncertainties in determining lateral size of an anomaly and it's velocity. Also this functional is not sensitive to possible difference in the waveforms of baseline and monitor data.
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P-wave and S-wave Separation Based on High-resolution Radon Transform
Authors Z. Li, J. G. Song and X. J. MengHere we put forward a method to separate P and S wave field. Three-component seismic acquisition technology used in VSP and cross-well seismic recorded wealthy seismic wave field. The P-wave and S-wave (P/S wave) couple together, which is the difficulty of data processing. Through analyzing borehole seismic acquisition system, and combining with polarity relation of P/S wave in different components, we can associate polarity of P/S wave with apparent velocity and direction of seismic wave reaching the geophone array. According to this relation, we get the distribution relation of P/S wave in domain based on the linear Radon transform (LRT). Considering the convergence of LRT, we design a polarization filtering method using the high-resolution Radon transform, and then accomplish the separation of P/S wave. Processing of simulated data of theoretical model and practical cross-well data from ShengLi oil field show our method of P/S wave separation is effective and reliable.
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Anisotopic Gaussian Beam Imaging of Walk-away VSP Data
Authors M. Protasov and I. BorodinThe approach to true amplitude seismic imaging in anisotopic media for walk-away Vertical Seismic Profile (VSP) multi component data is presented and discussed. This approach is migration procedure based on weighted summation of prestack data. True amplitude weights are computed with application of Gaussian beams (GB). We shoot a couple of properly chosen GB with fixed dip and opening angles from the current imaging point towards acquisition system. This couple of beams is used in order to compute true amplitude selective image of the rapid velocity variation. The total true amplitude image is constructed by superposition of selective ones being computed for a range of available dip angles. The global regularity of Gaussian beams permits one not take care of, is the ray field regular or not. The use of P- and S-wave Gaussian beams provides the possibility to handle raw multi component data without of their preliminary separation to PP and PS waves. The use of anisotopic Gaussian beams allows to take into account anisotropy of the background model.
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