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82nd EAGE Annual Conference & Exhibition
- Conference date: October 18-21, 2021
- Location: Amsterdam, The Netherlands
- Published: 18 October 2021
41 - 60 of 1137 results
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Pure P- and S-Wave Equations in Anisotropic Media
Authors A. Stovas, T. Alkhalifah and U. Bin WaheedSummaryPure mode wave propagation is important in applications ranging from imaging to avoiding parameter tradeoff in waveform inversion. We propose new artifact-free approximations for pure P- and S-waves in a transversely isotropic medium with vertical symmetry axis. Our approximations are very accurate compared to other known approximations as it is not based on weak anisotropy assumptions. As a result, the S-wave approximation can reproduce the group velocity triplications in strongly anisotropic media.
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Suppressing Seismic Record Linear Noise on the Common Offset Gathers Using Mathematical Morphology Filters
More LessSummaryThe attenuation of linear coherent noise is a persistent problem in seismic processing and imaging. Traditional methods utilize the differences in frequency, wavenumber or amplitude between useful signals and noise to separate them. However, in some cases, the differences are too small to be distinguished, and the traditional method are limited or even invalid. So we introduce mathematical morphological filter to attenuate the linear coherent noise utilize the differences in the shape of seismic waves. In the seismic exploration, we can see that the same linear coherent noises exist in some different common shot gathers. After investigation, we find that the trajectories of linear coherent noises in the common offset gather are horizontal continuous ones. So we implement the MMF attenuation the linear noises in the common offset gathers by using the horizontal consistency. And apply the proposed method on field seismic to show the excellent performance.
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Correcting Density/Sonic Logs for Total Organic Carbon to Reduce Uncertainty in Pore Pressure Prediction
More LessSummaryPore pressure prediction in shales undergoing compaction, including mechanical and chemical diagenesis, is customarily related to the mechanism referred to disequilibrium compaction. However, even when this mechanism is established and the normal compaction trend in sonic velocity, as a proxy for shale porosity, is well constrained, the pore pressure prediction may be in error because of the lithological variation in shale composition. Presence of high levels of organic matter in shales that are immature for hydrocarbon generation is an example, causing marked overprediction of pore pressure unless properly accounted for. All published datasets involving TOC and wireline data record a similar relationship between TOC and the bulk density and P-wave velocity log response, in the sense that the measured wireline data shows a decrease (which implies an increase in porosity) as the TOC content increases. In this paper it has been shown that a rock physics model that links TOC and bulk density can be utilised to correct the measured bulk density in immature shales, and, when limited to immature shale, the correction can be extended to velocity data using simple industry-standard models.
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Fault Reactivation Controlled by Elastic Stress Transfer during Hydraulic Fracturing at Preston New Road, UK
Authors T. Kettlety, J. Verdon, M. Werner and M. KendallSummaryWe investigate the physical mechanisms that produced felt seismicity during hydraulic stimulation of the Preston New Road PNR-1z well in Lancashire, England in October – December 2018. While pore pressure increases are typically assumed to be the principal cause of induced seismicity, other factors such as poroelastic stress transfer and aseismic slip have also been proposed as alternative mechanisms. At PNR-1z, a downhole microseismic monitoring array detected and located over 38,000 events during the stimulation, which revealed the interaction between the hydraulic fractures and a pre-existing fault. Here we probe this interaction in more detail, focussing on the role played by elastic stress transfer produced by the tensile opening of hydraulic fractures. We generate stochastic models to simulate the impact of tensile fracture opening on the surrounding stress field, and find that the observed microseismic event locations occur predominantly in regions where these effects moved the stress conditions towards the failure envelope. We therefore conclude that elastic stress transfer from tensile opening of hydraulic fractures played an important role in controlling fault reactivation at this site.
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A Dynamic Model for Non-Newtonian Drilling Fluid’s Filtration in Casing Drilling Technology
Authors M. Vasheghani Farahani, M. Salehian and S. JamshidiSummaryCasing Drilling is a recent technology for simultaneously drilling and casing a well. Using casing instead of conventional drill pipe results in a relatively small annular space between the conduit and wellbore, where the shear rate caused by drill pipe rotation can significantly affect the mud cake thickness. Previous studies have developed dynamic models to estimate mud cake characteristics in conventional drilling operations, however, the detailed information regarding the impact of drill pipe rotation on mud cake and formation damage has not been deeply addressed yet. This study presents a dynamic model for mud filtration in an isothermal radial system, while considering the impact of shear rate on mud cake thickness and filtration radius. Here, we use the Power-Law as the rheological model of the non-Newtonian drilling fluid. Results show the necessity of considering drill pipe rotation effect in dynamic flow calculations, highlighting the advantage of here-developed model for accurate estimation of mud cake characteristics in casing drilling operations.
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Time-Domain CSEM Modelling Using Frequency- and Laplace-Domain Computations
Authors D. Werthmüller and E.C. SlobSummaryModelling time-domain electromagnetic data with a frequency-domain code requires the computation of many frequencies for the Fourier transform. This can make it computationally very expensive when compared with time-domain codes. However, it has been shown that frequency-domain codes can be competitive if frequency-dependent modelling grids and clever frequency selection are used. We improve existing schemes by focusing on (a) minimizing the dimension of the required grid and (b) minimizing the required frequencies with logarithmically-spaced Fourier transforms and interpolation. These two changes result in a significant speed-up over previous results. We also tried to further speed-up the computation by using the real-valued Laplace domain instead of the complex-valued frequency domain. Computation in the Laplace domain results in a speed-up of roughly 30% over computation in the frequency domain. Although there is no analytical transformation from the Laplace to the time domain we were able to derive a digital linear filter for it. While this filter works fine for exact analytical responses it turned out that it is very susceptible to the smallest error. This makes it unfortunately unsuitable for iterative 3D solvers which approximate the solution to a certain tolerance.
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Integrated Geophysical Study for Landfill Mining: A Case Study in Denmark
Authors A. Sandrin and J. KeidingSummaryIn the last years the discipline of Urban Mining has been established, and the existing landfills may be now considered as a source of valuable commodities. For the estimation of the presence and amount of recyclable materials, a thorough exploration approach is strongly recommended. Non-invasive investigation methods are obviously preferred, due to the potentially high environmental impact of invasive methods in case of accidents. In this paper it is presented a case history from a landfill in Denmark.
A set of geophysical surveys was performed to determine the presence of metals within the waste, and possibly define their location and depth. The integration of magnetic, geoelectric, and seismic data seem to be able to provide a reliable assessment of the presence of magnetic metals at around 8 m depth in the westernmost part of the landfill. More detailed exploration methods and modelling techniques will be required for a quantitative evaluation of the valuable volumes in place.
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Mechanism of Overpressure Generation in the Paleocene Shahejie Formation in in the Linnan Sag, Eastern China
More LessSummaryIn this paper, the log response–vertical effective stress and acoustic velocity-density crossplots are used to identify the characteristics and generation mechanisms of the overpressure in the Linnan Sag. The analyses of the acoustic velocity/density–vertical effective stress and acoustic velocity-density crossplots demonstrate that the overpressured points consistent with the loading curve. So, the disequilibrium compaction of the thick Paleocene mudstones is the fundamental mechanism resulting in overpressures. Hydrocarbon generation and vertical transfer may be the main unloading mechanisms, that correspond to the overpressure points that deviate from the loading curves. Since organic matter cracking may occur in formations at depths deeper than 3800m, the contribution of hydrocarbon generation to overpressuring should be limited. The transfer of overpressure via opening faults is therefore considered as the main cause of higher overpressure in local sandstones. The results of this analysis provide an indication of the magnitude, mechanism and distribution of the overpressure. This understanding will help to guide further exploration activities in the Linnan Sag and similar geological basins.
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Azimuthal Seismic Difference Inversion for Tilted Fracture Weaknesses
Authors H. Chen and K. InnanenSummaryTilted transverse isotropy (TTI) provides a useful model for analyzing how tilted fractures affect seismic wave propagation in subsurface layers. To determine the TTI properties of a medium, we propose an approach of employing azimuthal differencing of seismic amplitude data to estimate tilted fracture weaknesses. We first derive a linearized P-to-P reflection coefficient expression in terms of tilted fracture weaknesses, and then we formulate a Bayesian inversion approach in which amplitude differences between seismic data along two azimuths are used to determine tilted fracture weaknesses. Tests with simulated data confirm that the unknown parameter vector involving tilted fracture weaknesses is stably estimated from seismic data containing a moderate degree of additive Gaussian noise. Applying the inversion approach to real data, we obtain interpretable tilted fracture weaknesses, which are consistent with expected reservoir geology.
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Coalbed Mathane Enrichment Rule and Sweet Spot Optimization-Case Study from Australia North Bowen Basin
By M. LiSummaryThe Bowen Basin in Australia is a typical post-arc foreland coal-bearing basin. There are high Coalbed Mathane resource potential in the North Bowen Basin. The study block in this paper is mainly located in the North Bowen Basin and the Moranbah field was the first developed Coalbed methane field in this block. Combined with the structural characteristics of the North Bowen Basin, the characteristics of coal seam development, gas bearing characteristics and permeability of coal seams. There are following main controlling factors of Coalbed methane in Moranbah coal group, “Coal distributed by sedimentary factor, Coalbed methane accumulated by hydrological factor, Coalbed methane distributed by structural factor and Coalbed Methane permeability is controlled by stress factor”
On the basis of the law of coalbed methane enrichment rule, the optimization evaluation criteria are summarized. The Sweet Spot, the favourable area and the unfavourable area of the Bowen block are determined according to resources basics, enrichment factors and production factors. It is suggested that Sweet Spot should apply for development permit and begin to the trial production, the favorable area should apply for potential commercial area and block temporal preservation and waiting for future development, and the unfavorable area should apply for relinquishment.
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VSP Study Using DAS at the Medipolis Geothermal Well and Implication of a Deep High-Vp/Vs Zone
Authors J. Kasahara, Y. Hasada, H. Kuzume, H. Mikada and Y. FujiseSummaryFollowing our first seismic study at the Medipolis geothermal field in southwestern Japan in 2018, we conducted a second seismic study at the same geothermal field in 2019. We installed an optical-fiber system for distributed temperature sensor (DTS) and distributed acoustic sensor (DAS) measurements. We deployed the optical-fiber system at a 1,545-m depth in the IK-4 borehole. The temperature was measured to be 272.8 °C at a 920-m depth and 152.8 °C at a 1,530-m depth. We operated a MiniVib seismic source at five locations and performed a frequency sweep of 10–75 Hz 480 times each day, for seven days. We cross-correlated the seismic records and the source signature and stacked the correlated data to enhance the S/N. Stacking for 480 or 960 times considerably improved the arrival waveforms. Based on an analysis of DAS data, we constructed the 2D seismic profile. We estimated three major hydrothermal layers, at depths of 800–1,000 m, 1,300–1,600 m, and 3,600 m. The zone around 3,600 m suggests a high Vp/Vs value and the possible presence of a fluid layer.
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Experimental Study on the Effect of Supercritical CO2 on Shaly Caprocks
More LessSummaryPetrophysical properties such as porosity and pore size distribution are critical parameters in seal integrity of the caprock. The effect of interactions between CO2, brine, and minerals constituting the caprock, have a significant influence on the effectiveness of the caprock sealing properties. Alteration of caprock integrity leads to environmental problems and bringing into question the effectiveness of the program altogether. In this study, shale samples were exposed to supercritical CO2 (scCO2) at in-situ pressure, temperature, and salinity condition, representative of a CO2 storage operation in Southwest Hub, Western Australia. Petrophysical properties of the samples are analysed with several methods to track the changes after exposure of samples to CO2. With this approach, we show that in the context of tight samples, the alteration of caprock minerals could result in either porosity enhancement or diminishment. Pore size distribution curves form nuclear magnetic resonance (NMR), low-pressure nitrogen adsorption (LPNA), and mercury injection capillary pressure (MICP) tests indicate an increase in pore volume, except for relatively tighter, clay-rich samples.
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Converted Wave Traveltime Approximation in Elastic Orthorhombic Media
More LessSummaryIn order to approximate the traveltime in an elastic orthorhombic (ORT) medium for converted waves, we define an explicit rational-form approximation for the traveltime of the converted PS1, PS2 and S1S2 waves. For the simplification purpose, The Taylor-series approximation is applied in the corresponding vertical slowness for three pure-wave modes. By using the effective model parameters for PS1, PS2 and S1S2 waves, the coefficients in the converted-wave traveltime approximation can be represented by the anisotropy parameters defined in the elastic ORT model. The accuracy in the converted-wave traveltime for three ORT models is illustrated in numerical examples. One can see from the results that for converted PS1 and PS2 waves, the proposed rational-form approximation is very accurate regardless of the tested ORT model. For a converted S1S2 wave, due to the existence of cusps, triplications, and shear singularities, the error is relatively larger compared with PS waves.
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Traveltime and Relative Geometrical Spreading Approximation in Elastic Orthorhombic Medium
More LessSummaryWe define the Rational Form (RF) approximations for P-wave traveltime and relative geometrical spreading in elastic ORT model. To facilitate the coefficients derivation in these approximation forms, the Taylor series (expansion in offsets) in the vertical P-wave slowness measured at zero-offset is applied. The same approximation forms computed in the acoustic ORT model are also derived for the comparison. In the numerical tests, three ORT models with the parameters obtained from the real data are used to test the accuracy of each approximation. The numerical examples yield the results that, apart from the error along the y-axis in the ORT model 2 for the relative geometrical spreading, the RF approximations are all very accurate for all tested models in both traveltime and relative geometrical spreading that can be performed for the practical use.
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On-Axis Triplications in Elastic Orthorhombic Media
More LessSummaryWe derive the second-order coefficients (principal curvature) of the slowness surface for two S waves in the vicinity of three symmetry axes and define the elliptic form function to examine the existence of the on-axis triplication in ORT model. The existence of the on-axis triplication is found by the sign of the defined curvature coefficients. An ORT model is defined in the numerical examples to analyze the behavior of the on-axis triplication. The plots of the group velocity surface in the vicinity of three symmetry axes are shown for the ORT model where different shapes: convex or the saddle-shaped (concave along one direction and convex along with another) indicates the existence of the on-axis triplication.
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Game-Changing AI for Faster and Better Well Trajectory Planning Decisions (An Example Using the Volve Dataset)
Authors N. Dolle, T. Savels, G.C.A.M. Reijnen-Mooij, J.J. Corcutt, O.R. Hansen and E. LandreSummaryWell trajectory planning is a high-stake and complex multi-disciplinary work activity for Oil & Gas operators. The work involves experts from geoscience, reservoir engineering, drilling, completions and facilities. Each are using specialist software to evaluate and define reservoir targets, subsurface hazards and engineering constraints. “Likes” and “dislikes” of trajectory options are expressed in different terms by the various disciplines. This often leads to an iterative and time-consuming process, influenced by human bias. Time quickly becomes a limiting factor, with a business risk of unrealized value due to incomplete understanding of the full option space and associated uncertainties, risks and rewards.
To mitigate the above challenges, we have developed a collaborative game-based approach to well trajectory planning supported by Artificial Intelligence (AI). This approach has been tested using Equinor’s open source Volve dataset, which demonstrates the potential to significantly reduce cycle time and improve decision quality.
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Multidimensional Factorial Kriging for Prestack Filtering
Authors T. Demongin and C. MagneronSummaryThe presence of stationary and non-stationary noise in prestack seismic data, despite reprocessing improvements, can be tackled by an innovative Factorial Kriging technique. Indeed, the data can be considered in a 4D space, counting the offset value. A multidimensional filter, using a 4D variogram model, is then resolved with the same system as in the case of usual 3D Factorial Kriging.
A NMO corrected raw migration gathers case study is presented, with results and interpretation. It appears in the end that noise observed on gathers and offset planes is removed and the data shows an improved signal to noise ratio.
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Working around the Corner Problem in Numerically Exact Non-Reflecting Boundary Conditions for the Wave Equation
By W. MulderSummaryRecently introduced non-reflecting boundary conditions are numerically exact: the solution on a given domain is the same as a subset of one on an enlarged domain where boundary reflections do not have time to reach the original domain. In 1D with second- or higher-order finite differences, a recurrence relation based on translation invariance provides the boundary conditions. In 2D or 3D, a recurrence relation was only found for a non-reflecting boundary on one or two opposing sides of the domain and zero Dirichlet or Neumann boundaries elsewhere. Otherwise, corners cause translation invariance to be lost.
The proposed workaround restores translation invariance with classic, approximately non-reflecting boundary conditions on the other sides. As a proof of principle, the method is applied to the 2-D constant-density acoustic wave equation, discretized on a rectangular domain with a second-order finite-difference scheme, first-order Enquist-Majda boundary conditions as approximate ones, and numerically exact boundary conditions in the horizontal direction. The method is computationally costly but has the advantage that it can be reused on a sequence of problems as long as the time step and the sound speed values next to the boundary are kept fixed.
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Gramian Constraints in Electromagnetic Multi-Physics Joint Inversion
More LessSummaryJoint inversion of multi-physics is used to minimize the non-uniqueness associated with under-determined geophysical problem by some constraints. The Gramian stabilizing constraint has been used to enforce the linear correlation between the resistivity models from the frequency and time-domain airborne electromagnetic (AEM) data. The Gramian is the dot product of the two resistivity models which constrains the nonlinear least square optimization towards more reliable interpretation even in the presence of noise. Both synthetic and field data demonstrations give satisfactory results.
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Analysis of Thin Sand Recognition Using Supervised Multiattribute Classification Based on ANNs
More LessSummaryThe objectives of this work are to identify heterogeneous thin sands via machine learning (artificial neural network) and evaluate the impact of tuning thickness on the recognition. The thin sands within the study interval mainly developed in a complex fluvial to shallow marine environment. Multiattribute classification using supervised Artificial Neural Networks (ANNs) is employed to predict the distribution of these thin sands within six subintervals and the role of tuning thickness in the prediction is evaluated quantitatively.
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