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68th EAGE Conference and Exhibition incorporating SPE EUROPEC 2006
- Conference date: 12 Jun 2006 - 15 Jun 2006
- Location: Vienna, Austria
- ISBN: 978-90-73781-00-9
- Published: 12 June 2006
101 - 200 of 462 results
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Edge Enhancement Using the Tilt Angle
More LessMeasures of the local phase of potential fields can be a useful aid to their interpretation. There are several variations in use, such as the tilt angle, tilt derivative and the Theta map. This paper introduces some new phase based filters which show improved performance as edge detectors in different ways. The filters are demonstrated on aeromagnetic data from South Africa and Australia.
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Wavelet Based Semblance Analysis
More LessSemblance filtering compares the phases of two datasets as a function of frequency. Because it is based on the Fourier transform its application suffers from all the problems associated with that transform, in particular its assumption that the frequency content of the data does not change with time. Semblance is here extended in two ways, using the continuous and the discrete wavelet transforms. When calculated using the continuous wavelet transform, semblance analysis allows the local phase relationships between the two datasets to be studied as a function of scale (or wavelength). Additionally, the efficient inverse transform of the discrete wavelet transform allows datasets to be filtered based on their local semblance, which offers considerable advantages over previous Fourier methods.
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Using Gravity Gradiometry to find Gold Deposits in Weathered Terrains
More LessAirborne gravity gradiometry (AGG) is a new geophysical tool that has been limited in its ability to find many mineral deposists in weathered terrains due to regolith variations swamping the signals from subtle changes in basement rock density. Previous approaches to adress this issue have tried and failed to compensate for the regolith. We have simulated (in 3-D)the response of a number of gold deposits as examples of using gravity gradiometry to find mineral deposits indirectly. The result is that most near-surface deposists in weathered terrains have a detectable negative gravity gradient anomaly due to preferential weathering. Thus, we demonstrate a new way of interpreting AGG data to find gold deposits in areas normally considered unsuitable for AGG.
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Interpretation of Electrical Properties for Humid and Saturated Hematitic Sandstone Sample
More LessEffects of water saturation on electrical properties, of humid, partially, and satrurated hematitic sandstone sample, were investigated experimentally. These data were discussed theoretically using models that account for the electrical polarization. We change the saturation of a hematitic sandstone sample from normal relative humidity (~50 RH) to fully saturated. Complex impedance measurements were performed in the frequency range from 1 Hz to 100 KHz. Experimental data indicates that the electrical properties vary strongly with water saturations. As the frequency increases the rate of dielectric constant change decreases and the change is nearly constant at high frequencies. The rate of decrease of the dielectric constant with frequency decreases as the water saturation decrease. The changes of the electrical properties due to saturation were attributed to the ionic surface charge at grain interface. The interaction between the matrix of a porous material and a small quantity of water located in its pore space induces polarization phenomena and enhances the charge transport. The results can be explained as proton conduction in the humid case and low saturations and electrolytic conduction along with protonic transport in the fully saturated case.
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SP Monitoring of an Acid Stimulation of the Soultz Geothermal Reservoir
Authors G. Marquis and A. GerardSP monitoring of a sequence of water-acid-water injections over a span of three weeks in the Soultz geothermal reservoir shows a decrease of the surface electric potential during the acid stimulation. This requires a negative electrokinetic coupling coefficient, which is predicted by the theory for fluids with very low pH. Post-acid injections also yielded negative signals that are likely caused by electrochemical processes during the increase in ionic content of the injected water. These results have some importance when considering using acid stimulations for geothermal reservoirs in a production phase.
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Potential Field Continuation between Irregular Surfaces - Problems and Applications
Authors B. Meurers and A. AhlThe field continuation procedure by Ivan (1994) providing potential field continuation between arbitrary surfaces has been implemented for use of unevenly scattered rather than gridded data. Errors induced by under-sampling are investigated by synthetic case studies. The basic assumption of dipole magnitudes linearly varying on the triangle facets of the polyhedral topography approximation turns out to limit the accuracy of Ivan’s algorithm. The method is applied to combine aeromagnetic data acquired by different partly overlapping surveys in order to remove unknown biases and to refer all data to a common surface with constant topography clearance.
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Predictive Environmental Geophysics in Israel
By M. RybakovThis paper presents the unpublished early data with an analysis of the new sinkholes opened after the surveys. That confirms predicting power of the microgravity and micromagnetic methods for sinkhole hazard assesement.
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Preliminary Research of Possibility Gravity Monitoring Use at Gas Deposits
More LessThe researches consisting in numerical modeling of gravitational effects of operation of a gas deposit are carried out.
The modeling with use of parameters of a real deposit allows to estimate close to true size of possible variations of a gravitational field and to estimate a degree of influence of a number of major parameters. The measurement of variations of gravity (gravity monitoring) is represented to one of most reliable and cheapest way of the control behind moving, gas or injection of water at operation of deposits. The carried out experimental accounts have shown a basic opportunity of the gravimetric control of gas withdrawal.
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Analytical Continuation of Geophysical Fields into the Area of Anomaly Sources by the Continued Fraction Method (CFCM)
More LessMany works of the leading geophysicists are devoted to the method of analytical continuation of geophysical field from the measuring profile to the lower half-space. But promising results in this area are practically absent. Linear presentation of the field in the form of polynomials, Taylor or Fourier series leads to essential ill position of the inverse problem. Field description, close to reality, is the construction of the fractional-rational type. In this case the presence of function poles in the lower half-space corresponds adequately to the zeros in denominator. Method proposed below is based on the Pade approximants and its particular but important case - continued fractions. The proposed method is called - Continued Fraction Continuation Method.
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Nature of Gravitational Anomaly in the South Caspian Basin
More LessThe South Caspian Basin (SCB) at present attracts increased attention of researchers in connection with its exceptionally rich oil and gas resources.
This paper presents an evaluation for predicted gravity anomaly in the SCB, that may have formed as a result of the continental plate subsidence at its central part. The gravity anomaly formed as a result of subsidence of plate has been appraised through its comparison with Earth crust of the Russian Craton.
This anomaly has been revealed to be equal to -193 mGal and strongly differs from those observed in the SCB. In fact these 193 mGal appear to be possible compensated due to an action of the strata located beneath the Moho partition with considerably higher density than that of mantle material.
The gravitational action of the excessive masses located in the upper mantle was considered as an action of the horizontal cylinder shaped body and depth interval of the consolidated part of the upper mantle has been evaluated. On creating the geodynamic evolution model for the SCB one needs to take into account an astenospheric projection which is predicted to occur at 36-87 km interval.
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Analysis of Crustal Seismic Data in a Geothermal Field (Southern Tuscany)
Authors U. Tinivella, F. Accaino, R. Nicolich and G. RossiSouthern Tuscany is characterised by an extensional regime, crustal thinning and magmatic intrusions. Important geothermal reservoirs and anomalous pore fluids are associated with an intricate geological setting.
The area was investigated by applying integrated methodologies to deep crustal seismic reflection data: amplitude versus offset inversion, and theoretical approaches to model overpressure conditions. The seismic transect CROP-18, split into two lines (18-A and 18-B), has imaged the exceptionally high reflectivity of some markers within the metamorphic crystalline basement units and of the medium-lower crust in this region. Joint analysis of the available data enabled us to distinguish between the lithological reflectors and the reflections caused by a predominant fluids, such as overpressure conditions. We detected the presence of vertical conduits, responsible for magmatic intrusions rising from the crust-mantle transition to the near-surface layers, as well as overpressured reservoirs. Theoretical models are used to quantify the effect of overpressure in terms of porosity increase, and the consequent density decrease in the area where the main marker characterizing the geothermal fields, the K-horizon, becomes shallow.
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An Integrated Geophysical Approach to Investigate the Ancient Copper Mine of Kalwang/Austria
Authors R. Scholger and E. NiesnerThe chalcopyrite deposit Teichgraben is situated 15 km W of Leoben in Austria. There is evidence for first exploitation activity during bronze-age, systematic mining started in the 15th century. The investigated orebody occurs in the Greywacke Zone of the Upper Austro-Alpine nappe. The mineralisations are bound to weekly metamorphic greenschists in the tectonically highest segment of the Greywacke Zone, the Noric Nappe.
Only few information is available on the extension of the mineralised zone to deeper layers. We investigated the extension of this mineralised zone in an integrated geophysical approach ranging from magnetic, electromagnetic, electric, self potential to spectral gamma ray measurements.
The interpretation was done using an electrofacies approach. The lithologies could be specified using the geological mapping and documentations available from the mining activity. A thrust fault between nappes of the greywacke zone intersects the area. The mineralised zone itself is interrupted by small faults. All mentioned features can be recognized on the measured geophysical data and the extension of the mineralised zone has been interpreted based on the integration of all results. The depth extend of the mineralised zone is limited to a maximum depth of about 50 m below the ancient base gallery.
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Thermal Effect of Magma Intrusion on Electrical Properties of Magnetic Rocks from Hamamat Sediments, NE Desert, Egypt
Authors M. M. M. S. Gomaa and R. A. M. ElsayedThermal effects of magmatic intrusion on electrical properties, of magnetic rocks from Hamamat sediments, NE desert, Egypt, were investigated experimentally. An intrusion of granitic magma was intruded in the Hamamat sediments, which is a mixture (mainly magnetite with sandstone) and due to the thermal effect the area around it was altered, with different degrees and magnetite was transformed into hematite with different degrees according to its location. Complex impedance measurements were performed in the frequency range from 10 Hz to 100 KHz. Experimental data indicates that the electrical properties vary strongly with distance. The conductivity of hematite is higher than that of magnetite. As we move from magnetite to hematite it is supposed that the conductivity will decrease, but it was found that the conductivity increases. The increase of conductivity as we move from magnetite to hematite were argued to the heating that make partial or complete melting of the samples, then the porosity of the samples were decreased and accordingly the conductivity and dielectric constant increase. Also, it was supposed that the grains of the conductor in the samples are coated completely or isolated with insulator material.
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From Scarcity to Plenty – Who Shapes the Energy Mix of the Future and What Might It Be?
More LessThe global pattern of primary energy sources will change profoundly during the 21st century. Consumers, businesses and governments together will shape the new energy mix. They all share three strong desires, which will deeply influence their energy decisions: · Sustained strong economic growth · Security of energy supply · A clean and safe environment. Consumer preferences, business solutions, and government demands will drive the change – not the depletion of any energy source. Global demand for commercially traded primary energy might grow from 10.8 billion tOE in 2004 to around 35 billion tOE in 2100. ‘Renewable’ primary energy sources could provide at least 35% of global demand in the year 2100; the solar portion of that might be 15%. Distributed power generation from ‘renewable’ energy sources such as solar, wind, water, and geothermal, is expected to become significant. New ways will have to be found to correctly measure the usage of distributed primary energy since a substantial part will be produced and consumed without ever being commercially traded. Fossil fuels - so dominant in 2004 with 83% - should still be important in absolute terms in 2100 (about 10 billion tOE), but with a much smaller relative share (about 28%). Natural gas and coal are expected to replace oil for many purposes. Nuclear could provide about 20% of the global primary energy demand in the year 2100. Fuel on fuel competition will be strong in the second half of the 21st century. Consumers will be able to choose from many options. This should lead to lower energy prices and to continuous improvements in energy conversion efficiencies. There is plenty of energy in the universe. It is a noble goal to capture enough of this energy so that all may prosper in a clean and safe environment. A lot needs to happen, if we want to get there by 2100. We need to seize the moment.
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Gas Supply in Europe, Gas Demand, Resources and Deliveries to European Consumers in the Upcoming Decades
By J. GallistlAt the beginning of the first decade of this century natural gas primary demand in the European Union with about 470 bcm accounted for more than 20 % of its total primary energy demand (World Energy Outlook 2004, IEA). According to the last “Reference Scenario” of the IEA the European Union is projected to have a natural gas average growth of 1.8 % per year during the period 2000 - 2030. This projected development leads to the total gas demand of more than 780 bcm in the European Union in 2030. The steady increase in gas demand requires an expansion of exploration and production. There is a positive phenomenon in this respect: proven reserves of natural gas have outpaced production by a wide margin since 1970. The world proven reserves account for 180 tcm as of 1 January 2004 (Cedigaz statistics). Russia and the former Soviet republics hold almost a third of global reserves, but its share has decreased steadily over the past decade. The Middle East holds 40% of all reserves and its share is growing. In 2030 the Middle East will be the world’s largest exporting region. Natural gas reserves in Europe come not even to 4% of world total and its share is decreasing. However, natural gas resources which could be imported to the European Union can easily meet the projected increase in gas demand of that region provided we take into account that higher oil prices will stimulate exploration and production of new more remote and challenging reserves. Another precondition is that long term contracts are allowed which will continue to play a decisive role in investment into production and infrastructure. The excellent track record of the European gas industry in the course of last few decades has helped to create a picture of secure natural gas supply to Europe. However, security of supply means not only the physical existence of sufficient and reliable resources, but also the existence of an adequate and reliable infrastructure to bring those resources to the market. At present there are three main corridors for supplying Europe with piped natural gas: from Russia to Central and Western Europe, from North Africa to Italy and Spain and from Norway to UK and Western Europe. To open up a forth corridor to connect the European markets with the world’s second biggest resources in the Caspian region and the Middle East is one of the big challenges of the future to further guarantee security of supply. In addition LNG will also play an increasing role regarding future gas imports to the European Union. All this shows that the European gas industry is facing big challenges to meet future gas demands of European gas consumers.
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Oil resources – Estimates and Uncertainties
More LessOil reserves are reasonably well known, but they are not a reliable indicator of long-term supply. Future oil availability will depend upon reserve additions, which are highly uncertain. New reserves will come from three sources: 1) new field discoveries, 2) growth of reserves in existing fields, and 3) development of unconventional resources. U.S. Geological Survey estimates of global undiscovered conventional oil have a mean value of about 650 billion barrels, but the estimates range from a few hundred billion barrels to 1100 billion and more if the possibility of successful exploration in remote, untested basins is included. Reserve growth in existing fields, which has added more than 250 billion barrels to reserves in the last two decades, is equally uncertain; global estimates of future growth range from tens of billions to 1000 billion barrels or more. Unconventional resources, particularly heavy oil and tar, may also add many hundreds of billions of barrels to reserves, but at unknown rates and costs. Thus estimated future additions to oil reserves range over an order of magnitude, from a few hundred to a few thousand billion barrels. Given the importance of oil to human activity, planners might want to retain the flexibility to adapt to a range of possible scenarios of future oil supply.
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Managing a Field with 24yrs of Production History and More Than 40 Years Still to Go
By G. SundThe Valhall chalk Field, located in the Southern North Sea, is unique in it’s production history, reserves growth linked to technology and expected long future ahead. It was discovered in 1979 and put on production in 1982. Starting with estimated reserves of 250 mm bo, the field today has 2 billion barrels as its ambition and potential. Being a soft, high porosity and essentially uncemented and high pressure chalk, the reservoir has been on primary depletion producing from a strong compaction drive for more than 20 yrs. The crest, hidden under a gas cloud, was the primary target area. Due to drilling problems through the compacting and subsiding, soft overburden and unsuccessful extended-reach drilling two flank platforms were put in place in 2002 and 2003. This has significantly contributed to reserves growth and the reservoir limits are pushed deeper and outwards as we drill on the flanks. In the crest, a water injection program was sanctioned, to build up pressure and increase production through matrix sweep and pressure support. The IP platform was in place in 2003 and water injection started through two converted wells first in the northwest area of the field in 2004. Today a crestal monitoring well (measuring pressure and water induced rock compaction underneath crest installations) and a first crestal water injector are in place as the first parts of a planned water injection programme with a line drive patterns consisting of injectors and producers. A key element in the water injection programme is surveillance. An advanced surveillance program allowing for steering and detecting the pattern of water flow is planned and about to be implemented. Equally important perhaps even more, is the Life of Field Seismic (LoFS) installed at Valhall. Kilometres of cables are covering the field allowing for regular acquisition of seismic data across the field. Primarily installed for following the 4D changes in the field associated with the water injection programme, LoFS has shown to have impact on essentially the entire reservoir management: Well planning, Wells management and for 4D effect in the reservoir. There are challenges to overcome in that details are still hidden underneath the gas cloud and details are still to be unlocked I the images resulting from the successive surveys. However, we are unlocking more and more of the story of the chalk reservoir, changing essentially every day as oil is produced and water is injected.
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Understanding Underground Coal Fires for Utilization
Authors K. H. A. A. Wolf, J. Bruining and R. MeeuwisUnderground coal fires are difficult to extinguish. As an alternative they might be used for heat production. Hence, their controlling factors have to be investigated. Experimental work on in-situ coal gasification and geological surveys on underground coal fires are used to develop a model to understand the combustion process and subsidence behavior. Coal fires are divided in five thermal zones with different thermo-mechanical characteristics. The three zones in and above the seam are used in a convection model. In experiments temperatures, ranging from surface temperature to 1200°C, are applied to claystones, shales and sandstones. Changing mineralogy, thermal expansion, shrinkage, bulk moduli and permeability of grain aggregates are established under defined in-situ T,P-conditions. The results are implemented in finite difference- and creep models to calculate subsidence. Volume changes are converted to permeability fields for a convection model. In this model, seam-, rubble- and overburden thicknesses, their depths and fault densities, are other input parameters. The model calculates the temperature-, convection- and oxygen distribution in the sub-surface. For shallow coal fires (15 m), the overburden, faults and collapsed rubble zone are used for oxygen supply and thermal convection. Deep coal fires (40 m) use the rubble zone and faults for the process.
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Seismic Monitoring the Variations of Open Fracturing of Reservoir Rocks due to Lunisolar Tides
Studied is the dynamics of natural variation of open fracturing of petroleum reservoirs caused by lunisolar tides. Experimental seismic studies were performed by monitoring the fracturing using Side-View Seismic Location (SVSL) and Seismic Location of Emission Centers (SLEC) - active and passive seismic methods, respectively. A periodic variation of open fracturing intensity is established with maximums of open fracturing correlating with maximums of gravity gradients. It is found that the largest variations are predominantly characteristic for subvertical fractures and the role of fracturing variation is crucial for deep-seated reservoirs. The effect of natural time variations of open fracturing should be taken into account when performing reservoir stimulation and during oil recovery and water injection to provide the high efficiency of production especially for mature oil fields where such operations are common.
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Hydrocarbon Microtremors Interpreted as Oscillations Driven by Oceanic Background Waves
Authors R. Holzner, P. Eschle, M. Frehner, S. Schmalholz and Y. PodlachikovHydrocarbon Microtremor Analysis (HyMAS) is an innovative technology for identifying the presence of hydrocarbon containing geological structures by analyzing low frequency background wave signals. A possible interpretation of this reproducibly observable phenomenon is the excitation of hydrocarbon related resonances. Synthetic spectra produced by basic linear and non-linear one-dimensional models of an oscillating liquid filled porous medium show characteristic features of measured HyMAS spectra when oceanic background waves around 0.1-0.2Hz are assumed to be the external driving force.
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Flow-Induced Noise in Fiber-Optic 3C Seismic Sensors for Permanent Tubing-Conveyed Installations
Authors S. Knudsen, G. B. Havsgård, A. Berg and T. BostickThis paper report the test results acquired of flow induced noise in a fiber-optic 3-C seismic station, where the tests represents some of the possible conditions that may be expected in flowing wells. The tests were performed to evaluate a fiber-optic sensor tool for tubing-conveyed installations in a controlled-flow test facility. The results of these tests demonstrated that the active-coupled 3-C fiber-optic seismic station installed on production tubing inside cemented well casing was capable of detecting very small seismic signals at high single-phase flow rates for frequencies in the 10-600 Hz band. Test results also demonstrated that the active 3-C station has a very efficient decoupling from flow-induced noise (compared to a passive 3-C seismic station directly attached to the production tubing), while at the same time having good coupling to the casing and formation. Test results showed that two-phase flow increased the noise at high frequencies over that of single-phase flow; however, for frequencies <100 Hz the noise increase is small. This testing indicates the potential to acquire high-resolution, multi-component seismic data in flowing wells under certain conditions with fiber-optic sensors.
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Change in Horizontal Stress in Soil from Changing Shear Wave Velocity - A Laboratory Investigation
By R. GhoseWe performed a new experiment on an unconsolidated sand sample in a biaxial pressure chamber to observe the effect of horizontal stress on seismic shear-wave velocity. Multi-offset seismic reception was successfully implemented in the laboratory scale. From changes in shear-wave moveout velocity observed in a receiver array we could distinguish the effect of horizontal stress change. The approach appears to be realisable in field with multi-receiver seismic CPT. This should lead to a new methodology for in-situ monitoring of horizontal stress change in soil. Further, we found a good match between the observed and the modelled shear-wave velocity change as a function of horizontal stress. This observation reinforces the possibility of implementing the model-based integration of seismic and CPT data, proposed earlier, for obtaining unique estimates of in-situ porosity and horizontal stress.
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Source Signature Estimation - Attenuation of the Seafloor Reflection Error in Shallow Water
Authors J. F. Hopperstad and R. LawsZiolkowski et al. (1982) presented a method for computing the monopole pressure field - the notional source signature - of each airgun or airgun cluster in an array from a set of near-field hydrophone measurements. The notional source signatures are then used to compute the far-field signature of the source array. In areas where the seafloor is hard or in shallow water, the seafloor reflection can cause reverberations of significant amplitude in the near-field measurements. Left uncorrected, the seafloor reflections may lead to a significant error in the far-field signature. Kragh et al. (2000) used an adaptive beamformer to remove these reflections. However, in water depths shallower than about 40 m, an alternative method is required. In this paper, we present an extension to the notional source method of Ziolkowski et al. (1982) that includes the water column reverberations measured by the near-field hydrophones in very shallow water.
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Multiple Removal in the Inverse Data Space
Authors D. J. Verschuur and A. J. BerkhoutIn practice, the data of a seismic survey is always discrete. Therefore, seismic data of a complete survey can be conveniently arranged in a so-called data matrix (P). After removing waves that have travelled along the surface (part of the preprocessing step), the data matrix contains signals that can be expressed in terms of wavefield operators describing propagation and reflection in the subsurface.
Considering the dominant role of multiple scattering in seismic data, it is proposed to replace data matrix P by its inverse P-1 before starting seismic processing. In the inverse data space, a natural separation between primaries and multiples occurs, leading to a new type of multiple removal algorithm.
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Partnership in Gas Production and Processing as a Solution for a Marginal Field
Authors G. Palasthy, L. Benedek, L. Erdos, I. Mader, D. Babic and L. Farkas VisontaiC025 Partnership in Gas Production and Processing as a Solution for a Marginal Field G. Palasthy* (MOL Plc.) L. Benedek (MOL Plc.) L. Erdos (MOL Plc.) I. Mader (INA) D. Babic (INA) & L. Farkas Visontai (INA) SUMMARY In 1994 MOL plc. discovered the Vízvár–North gas condensate field in Hungary near to the Croatian border. In the appraisal phase two gas wells were drilled and tested. The composition of the gas is not too advantageous because the CO2 and nitrogen content is high (above 25%) and the condensate has paraffinic behaviour. These circumstances cause technical challenges in the process of
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Experimental & Numerical Investigation of Cap. Press. Hysteresis on Iranian Carbonate Rock
Authors S. M. Hashemi, H. Karimaie, M. R. Esfahani and J. RoodsazC027 Experimental & Numerical Investigation of Cap. Press. Hyst on Iranian Carbonate Rock S.M. Hashemi* (National Iranian Oil Co.-RIPI) H. Karimaie (Norwegia Tech Trondheim (NTNU)) M.R. Esfahani (National Iranian Oil Co.-RIP (National Iranian Oil Co.-RIPI) SUMMARY In this study several experiments were performed on an Iranian carb rock. The bounding imbibition and drainage capillary pressure and sc were determined using a novel experimental setup at high temperat highly accurate measurements of capillary pressure hysteresis on a w show that there is significant hysteresis in capillary pressure bet imbibition curves. Killough and Tan models representing capillary pre based on saturation history
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Increased Gas Recovery by High-Angle Subsea Wells in High-Pressure-High-Temperature (HPHT) Environment at Kristin Field
Authors T. T. Blekastad and O. R. HansenThe Kristin Field is the most extreme high-pressure-high-temperature (HPHT) field in Norway, with a reservoir pressure of 911 bars and a temperature of 175°C. This has led to huge challenges in developing the Field.
The production wells were initially planned as medium inclined wells due to well control issues. Towards field development, strong indications showed that the permeability in the uppermost reservoir was less prosperous than predicted. This would imply only half of the production plateau period committed, reduced recovery from the Field and a substantial deficit in project net-present-value as a result. The need for longer reservoir sections became quite evident.
The scope was extended to near horizontal well designs. The ultimate Kristin well is 6580 m MD long with an 8½" HPHT section of 1083 m with 746 m above 80°. This is the world’s longest and most deviated HPHT subsea gas producer.
By production start 3 November 2005, Kristin has drilled 9 and completed 4 wells, most being high angle wells above 75°. The production simulations show that the plateau production is back up to the original commitments.
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Acid Clathrates for Well Stimulation
Authors V. A. Kuvshinov, L. K. Altunina, A. A. Lobanova and R. K. ShakurovThe paper presents the results of laboratory and field researches of the developed in Russia solid acid reagent - netrol.
From chemical viewpoint netrol is a solid compound of carbamide clathrate and a mixture of inorganic acids. The main part of acid mixture is constituted by nitric acid, which intensifies the effect of acid treatment due to oxidizing ability. Carbamide is a clathrate former and in the netrol solution it provides low interfacial tension, increased detergency and effective penetration, as well as compatibility with water of high salinity. Netrol also includes phosphoric and borohydrofluoric acids, providing stability of a solid netrol form, prolonged effect of acid treatment of the netrol solution and its ability to dissolve silica minerals of rock matrix.
As compared with well-known hydrochloric acid the netrol solution has more effective penetration, especially in mudded carbonate and terrigenous reservoirs with viscous, highly paraffinic and highly resinous oil. It is also compatible with formation waters of high salinity. The netrol solution is easily producible and safe to handle.
In 2004-2005 bottomhole zones of the production and injection wells in carbonate and terrigenous low-permeable reservoirs in Russia and Kazakhstan oilfields were successfully treated with netrol solutions.
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When Geology Meets Geophysics - Optimized Lithoseismic Facies Cubes for Reservoir Needs
Authors P. Breton and O. D. DUPLANTIERWe here present a new strategy for seismic facies probability cubes creation calibrated on petrophysical data. This methodology has been developed on the deep offshore Angola Girassol field data. It proposes to be more consistent with the petrophysical data and to ensure a better geological facies organization and continuity than classical approaches.
In order to infill a geomodel based on seismic constraints, lithoseismic cubes must be fully consistent with the geological interpretation and the petrophysical context. The link between seismic facies probability cubes and the geology is done through the Facies Groups definition. A Facies Group is defined as an interpreted 'log facies classification'. Their definition represents a key stage.
The methodology developed in this paper consists in applying an iterative process between Facies Groups design, their corresponding petrophysical range and seismic response. This new workflow enables to compute optimized lithoseismic cubes as they are fully calibrated on petrophysical data and based on the finest Facies Groups discrimination related to the seismic resolution. The resulting lithoseismic cubes present a much more coherent geological facies organization inside the reservoir model. They provide more suitable data for quantitative estimation of the uncertainties related to the reservoir properties.
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New Approch for Modeling of Two-Phase Flow through Anisotropic Porous Media
Authors V. V. Kadet, N. M. Dmitriev and M. N. DmtrievA new presentation of tensors of relative phase permeabilities for anisotropic porous media is considered.
The relation between the tensors of absolute and phase permeabilities is supposed to be represented by a tensor of 4th rank.
It is shown that in this case the relative phase permeabilities depend not only on the saturation, but also on the ratios of major values of an absolute permeability tensor. The obtained relations for phase permeabilities are used for analyzing the results of numerical modeling of two-phase flow in the orthotropic porous medium. The versions of complex studies for obtaining phase permeabilities in anisotropic porous media from the results of laboratory measurements are discussed.
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Dependence of Elastic Modules on Pore Pressure in Microinhomogeneous Medium
More LessThe paper is devoted to calculation of average elastic modules of microinhomogeneous medium. The new method, which is presented, is based on long-wave approximation, potential method of elasticity and averaging of medium parameters on orientation of structure. The using of fundamental solution of third kind (Weyl H.) as Green tensor gives a possibility to solve regular integral equations instead of singular ones. Velocities of P and S waves are increases with increasing of pore pressure, but increasing of S waves velocities is much more significant with respect to P ones, hence the ratio Vs/Vp is also increases.
Key words. Microinhomogeneity, microstructure, pore space, boundary integral equations.
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Irregular Sampling and Seismic Imaging
Authors H. Keers, T. Standley and A. KaniaMany seismic processing techniques require integration of the data. This integration is commonly done using a binning technique. Binning techniques work well if the integration grid is regular. In practice, however, this is often not the case. We describe an alternative integration technique that takes irregular sampling into account, by assigning weights to the integration points. The weights are computed by triangulating the integration domain.
The two algorithms were tested on a synthetic dataset and a marine dataset from the Gulf of Mexico. The irregular imaging gives better results than the binning method in the cross line direction: acquisition footprints are reduced, resulting in clearer images. In the inline direction there is not much difference between the two methods. Since the irregular imaging algorithm is easy to implement and the additional CPU is negligible, it is an attractive alternative to binning.
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Interpolation of Near Offsets with Multiples and Prediction-Error Filters
Authors W. J. Curry and G. ShanInterpolation of near offsets is an important issue in marine surveys. Pseudo-primary data is created from the Sigsbee 2B model using surface-consistent cross-correlation. This creates pseudo-data in near offsets that were missing from the original data. A non-stationary prediction-error filter is estimated on the pseudo-primaries, and is used to interpolate the missing near offsets in the original data, with promising results. The prediction-error filter can also be used to gauge the benefit of the pseudo-primaries by convolving it with the recorded data.
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Wide Azimuth Interpolation
Authors D. Trad, J. Deere and S. CheadleSeismic processing techniques, such as migration, have strict requirements on information content in the input seismic data. Although not a substitute for well-sampled field data, interpolation can provide useful data preconditioning that allows migration to work better.
Seismic data interpolation has been around for long time, but only recently have we been able to use complex multidimensional and global algorithms that have the capability to infill large gaps in 3D land surveys. This innovation offers great potential for improvement, but for this technology to become useful, many questions still need answers. What are the best domains in which to interpolate? What is the optimal size of operators given a particular level of structural complexity? Should we pursue an ideal geometry for migration or should we stay close to the input geometry in order to minimize distortions? How does sampling in multiple dimensions affect our traditional aliasing constraints? How can we infill large gaps without using a model for our data? Are irregularities in sampling beneficial? Understanding land data interpolation may help to solve many problems in seismic processing.
In this paper, we address some of these issues and show some examples of multidimensional land data interpolation.
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Characteristics of Azimuthal Anisotropy in Narrow-Azimuth Marine Streamer Data
More LessTowed-streamer acquisition samples a narrow range of azimuths. We discuss the resulting sensitivity of such data to azimuthal velocity anisotropy. Correction for such effects, for example by using a residual moveout correction before migration, can significantly improve the imaging in such situations.
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Analysis of the Common-Reflection-Surface Interpolation
Authors G. Hoecht and P. RicarteThe presented interpolation method is based on the common-reflection-surface (CRS) theory. For each sample that has to be interpolated five parameters are required and estimated from the data. Therefore, the proposed method is computational more expensive than existing methods. We present the potential as well as limitations of the method. The result of a CRS interpolation strongly depends on the accuracy of the detected parameters. We analyze the behavior of the estimated parameters for different situations and investigate the effect of an adapted smoothing of the parameters. For the analysis we use the Sigsbee data set which offers numerous complex reflection and diffraction patterns. For a comparison we show the result of a f-x interpolation.
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Practical Approaches to the Common-Reflection Surface (CRS) Stack
More LessCommon-reflection surface stacking is a time-processing technique for seismic reflection data that improves signal-to-noise at the cost of along-dip resolution. The theory is based on summations across multi-dimensional pre-stack reflections, but the noise reduction benefits can be realized with a purely post-stack process. Dip and azimuth information derived from this post-stack process can yield an improved velocity analysis.
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Elimination of the Spread-Length Bias in the Common-Reflection-Surface Stack
More LessThe Common-Reflection-Surface (CRS) stack yields kinematic wavefield attributes and stacked sections based on a second-order traveltime approximation.
As the CRS stack is performed by coherence analyses this leads to attributes and traveltimes of reflection events that show a spread-length bias, i.e., the quantities obtained are subject to the search aperture.
Obviously, applications based on the attributes and traveltimes will suffer from these misfits and yield incorrect results.
In this paper a technique for the correction of the spread-length bias is presented. Based on the assumption of a linear relationship between spread-length bias and search aperture the attributes and traveltimes are extrapolated to zero aperture and thus to their correct values.
The method is related to multiple coherence analyses with different apertures. The so obtained stack and attribute sections form new pseudo pre-stack data volumes which are used for the estimation of the extrapolation operators. A synthetic data example shows that these corrections are able to significantly improve the results of subsequent applications.
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Hybrid L1/L2 Norm IRLS Method with Application to Velocity-Stack Inversion
By J. JiLeast squares (l2 norm) solutions of seismic inversion tend to be very sensitive to data points with large errors. The l1 norm minimization gives more robust solutions than l2 norm solution does. Iteratively reweighted least squares (IRLS) method which is an efficient approximate solution of the l1 norm problem is widely used for robust inversion. I propose a simple way to implement IRLS method for a hybrid l1/l2 minimization problem that behaves as l2 fit for small residual and l1 fit for large residuals. The boundary between l1 and l2 is decided by a scale factor that is applied to data before and after inversion. Synthetic and real data examples in CMP data enhancing through inversion demonstrate the improvement of the hybrid l1/l2 norm method over least-squares method when there are outliers in the data.
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Perspectives of Chemical IOR/EOR Methods in the 21th Century - A New Thermodynamic Approach
Authors I. J. Lakatos and J. Lakatos-SzabóThe role of IOR/EOR technologies at oil and gas fields is gradually increasing and they form a mainstay to preserve the existing equilibrium between production and the global demand. Thermodynamic interpretation of displacement processes is a consistent basis to classify the EOR methods and anticipate their inherent efficiency. The conclusions confirm the high potential of chemical IOR/EOR methods in the 21st century.
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Applied Geophysics in Shell Europe's Mature Asset
More LessShell Europe's 'Mature Asset' operates some 20 fields in the Northern and Central North Sea with a total oil production of approximately 19000 M^3/day. These fields include ageing fields like the Cormorants, the Dunlin cluster and the younger Tern field.
Geophysics plays a key role in the integrated discipline teams working those mature fields, both for surveillance activities as well as in the pursuit of infill opportunities in those Jurassic and Triassic type reservoir fields.
A wide range of high-end geophysical technologies are routinely deployed. Prestack Depth Migration has a proven track record in these fields despite a relatively benign overburden and is now being deployed as a minimum standard. Time Lapse seismic has been used in particular over the Tern and Cormorant fields, and the results are routinely integrated in the updates of the reservoir models and identification of infill opportunities. Full-field 3D and 4D model driven forward modelling helps us 'close the loop' between static and dynamic models and the seismic data. Advanced inversion technologies are being used for the prediction of reservoir properties and their distribution. This paper discusses some of the results of those technologies todate and illustrates their business value.
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Grid Drilling below Data Resolution in a Waterflooded, Faulted, Fluvial Reservoir
Authors J. Neidhardt and L. RijkelsThe Mulussa F formation in many of Al Furat Petroleum Company's mature reservoirs consists of fluvial sand bodies cut up by faults with a range of throws below seismic resolution. Predicting waterflood efficiency in this 3D labyrinth is complicated. Sands in the low N/G (<35%) column of some 350 m in one field do not water out evenly or bottom-up. Instead, water breaks through in sands higher in the column, above unsupported sands, leading to loss of reserves when plugs are set.
This paper investigates which knowledge is required to predict sweep in the Mulussa and to forecast ultimate recovery of infill wells. Comparison to the actual resolution of seismic and well data shows that deterministic modelling of field development is not possible. Instead, geostatistical dynamic sector models were created, with explicit representation of sand bodies and subseismic faults. A small number of these models with particular parameter sets could explain historical field behaviour. The rewards of an ensemble of wells can be predicted using these particular models.
The formation is now being developed by a grid drilling campaign, with well spacing and orientation defined by the study. The first 7 wells of this campaign found rewards as prognosed.
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Making Flooding More Effective - A New Procedure for Mapping Subsurface Flow Paths
By P. RollinsD028 Making Flooding More Effective - A Procedure for Mapping Subsurface Flo Paths P. Rollins* (Willowstick Technologies) SUMMARY This paper introduces a new method for mapping the flow paths crea flooding activities in mature oil fields. The new procedure uses the flooding substance with a low voltage low amperage audio-freq current. The current creates a distinctive magnetic field that and character of the flow occurring between the electrodes. The field is identified and surveyed from the surface using a highly sens procedure can pinpoint the location of blockages and other problem inexpensively than traditional methods which typically rely on the
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Thermoreversible Polymer Gels for Increased Efficiency of Cyclic-Steam Well Treatment
Authors L. K. Altunina, V. A. Kuvshinov and L. A. StasyevaTo increase the efficiency of cyclic-steam treatment of high-viscosity oil pools we propose to use thermoreversible gel-forming polymer systems with lower critical dissolution temperature (LCDT).
Presented are the results of laboratory research of thermoreversible gel-forming systems with LCDT: cellulose ether - aqueous phase in the temperature range of 20 - 200 °C. The studies carried out on kinetics of gelation and rheological properties demonstrated repeatability of gel rheological parameters at cyclic reversal of temperature conditions. The action of additions increasing temperature of gelation was determined to be additive. The temperature of gel conversion into liquid is 30-50 degrees below than that of gelation. The indicated temperatures linearly increase with additive concentrations and for all reagents under study except carbamide these dependencies are cymbate.
Gel-forming systems (GFS) proved high efficiency on heterogeneous reservoir models, where seam permeability differed 3-5 times, under the conditions simulating operation of a cyclic steam well. We can recommend GFS application to regulate filtration flows of reservoir fluids and to increase conformance of cyclic-steam treatment of high-viscosity oil pools.
In October 2005 GFS were successfully injected into two cyclic steam wells in high-viscosity oil pool in Lyaokhe oil field (China).
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Implementation of Thermal Methods on the Oil Recovery in Romania,
More LessThe application of the thermal oil recovery method is a proven technique solution for oil and enhancement of oil exploitation reserves increasing. Therefore, in Romania numerous laboratory and field experiments in regard modern methods of oil recovery was realized (in situ combustion, steam injection, hot water injection).
The best results in the field were obtained from in situ combustion applied to 26 oil fields in total in Romania. Laboratory experiments and field tests have an important contribution in explanation of the theoretical aspects of in situ combustion, in improving of the technics for application in site in various condition, in monitoring of in situ combustion and in increasing efficiency of this process using water injection and horizontal wells in addition.
In the paper are presented Romanian oil fields which were applied thermal oil recovery methods, parameters for each process and obtained results. Also, they are presented our know-how regarding the tracing of process and extending of applicability for these recoveries methods.
At the end are presented the perspectives of application of recovery thermal oil methods in Petrom SA - Romania, in actual economical and environmental conditions.
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Extraction of Hydrocarbons from Soil and Rocks Using High Pressure Carbon Dioxide
Authors A. H. Al-Marzouqi, A. Y. Zekri, B. Jobe and A. DowaidarCarbon dioxide at elevated pressure and temperature is a powerful solvent capable of extracting hydrocarbons from rocks, soil and mud slurries, which are by-products of large numbers of oil industries. The objective of this study was to investigate the effects of water content and grain size and type on the extraction capacity of CO2 (at 300 bar and 120 oC) and on the composition of extracted hydrocarbons.
The results of this study indicate that CO2 at 300 bar and 120 oC is an effective solvent, which leads to high oil recoveries, except for reservoirs with high water contents (≥20%). The results also show that the extraction efficiency of CO2 from soil is higher than that from limestone particles of the same size and distribution, indicating that Bu Hasa oil holds more strongly to the limestone particles than to the soil particles. Moreover, the compositional analysis show that CO2 at 300 bar and 120 oC is capable of extracting hydrocarbons up to C31 including gasoline and diesel range hydrocarbons.
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Cyclic Waterflooding in Homogeneous and Layered Beadpacks
By M. AraujoCyclic waterflooding has received recent attention since many experimental, simulation studies and field tests have shown that it may lead to additional oil recovery. Even though the process is very similar to conventional waterflooding, there are new effects taking place under pulsed conditions. Two-phase immiscible experiments performed on homogenous and layered packed glass beads cells are compared with conventional continuous injection while doing a visualization of the displacement process. In the pulsed experiments, injection is switched on/off using a timing device in the pumping system. The effect of pack wetting condition was evaluated for the homogeneous case.
It is found that the amount of oil recovered at intermediate stages (1-3 PV injected water) is larger for the pulsing mode. In the homogeneous case, final recovery is the same for both injection methods within experimental error, whereas additional oil is recovered for the layered sample. In the pulsed experiments, during off injection period, spontaneous fluid spreading was observed leading to smoother displacement fronts compared to continuous injection. Also, displacement fronts were more stable under pulsed conditions. The study concludes that residual oil saturation under pulsed injection can be reached earlier than under continuous injection, a result very attractive for field application.
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Downhole Monitoring of Electrokinetic Potential in Petroleum Reservoirs
Authors J. H. Saunders and M. D. JacksonWe present results from a new numerical model capable of simulating two-phase flow in a porous medium and the electrical potentials arising due to electrokinetic phenomena. In a series of simulations using an idealised reservoir we show that, during water-flood of an initially oil-filled reservoir, electrical potentials measured at the production borehole may be indicative of the approaching waterfront. The magnitude of the signals is much larger than the expected ambient electrical noise, and water approaching the well may be detected as far away as 100m. This data may be used in conjunction with 'smart' wells to optimise production in real time. Therefore, we suggest that passive monitoring of electrokinetic potentials downhole may be used to enhance hydrocarbon recovery and reduce the environmental impact of production.
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Chasing IOR Potential on the Veslefrikk Field Using Near Well Imaging while Geosteering
Authors M. Wiig, E. Berg, H. O. Laastad, T. O. Sygnabere, J. M. Kjærefjord, M. Saltnes and E. A. StordalVeslefrikk is a mature North Sea oil field where optimal well placement is critical for drainage of the remaining reserves. The two cases presented illustrate optimisation of horizontal wells in different geological settings. In both cases a newly developed directional resistivity logging while drilling tool was used to optimise the well location during drilling. The tool was able to detect resistivity contrasts in any direction up to 5 m from the wellbore. In the first well, the objective was to maximise the distance to the water and ensure that no oil was left above the well, by placing a horizontal section 1-3 m below the top of a lateral continuous reservoir sand. In the second case, the objective was to optimise the amount of oil filled sand along an 1100 m long horizontal section, while drilling perpendicular to the depositional direction of a fluvial channel system. The early detection of reservoir boundaries resulted in an increase of 10-15 % in the recoverable reserves for each well compared to conventional geosteering.
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The Seismic Response of Faroe Basalts from Integrated Borehole and Wide-Angle Seismic Data
Authors G. Bais, R. White, M. Worthington and M. AndersenWe study the seismic response of layered basalts in the Faroe Islands using borehole data and vertical seismic profiles from the Vestmanna borehole, combined with reflection and wide-angle seismic data recorded into arrays of both borehole and land multicomponent receivers. Imaging through the basalt cover in the Faroe-Shetland Basin is a challenge for conventional seismic surveys: scattering caused by the high reflectivity of the basalt as well as intra-basalt multiples and high attenuation from the layered sequence make it difficult to image within and beneath the basalts. This project allows us to correlate ultrasonic-scale velocity and density measurements from the borehole together with ground-truthing from borehole logs and core samples with the seismic-scale velocities and reflection images derived from VSP and surface data. We find a good match of observed travel-times of borehole and wide-angle P-wave data with those predicted from the borehole measurements, suggesting lateral homogeneity over horizontal distances on the kilometre scale, and restricted transverse anisotropy of the layered basalts. A pronounced intra-basalt reflector identified on the multichannel surface seismic can be correlated with lithostratigraphic interpretation of the borehole logs as caused by thick flows near the top of the Lower Basalt formation.
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iSIMM Experience with Peak- and Bubble-Tuned Sources for Generating Low Frequencies
Authors P. A. F. Christie, Z. C. Lunnon, R. S. White, N. Kusznir, A. Roberts, A. W. Roberts, C. Parkin, L. Smith, J. Eccles, D. Healy, N. Hurst, V. Tymms, A. Chappell and R. FletcherThe iSIMM project has demonstrated the effectiveness of low-frequency acquisition in countering the loss due to scattering by the high-impedance contrasts and rough surfaces presented by stacked basalt flows (Spitzer et al., 2005). Both the ocean-bottom seismometer (OBS) profiles and the densely sampled towed-streamer profiles in the iSIMM surveys allowed comparison of deep-towed, peak-tuned, and bubble-tuned airgun sources. Supported by earlier analysis (Lunnon et al., 2003), we find source tow depth has greater impact upon bandwidth than does tuning mode, although, for the configurations tested, the bubble-tuned source produced a more compact signature. Shot-by-shot signature deconvolution was successfully applied to bubble-tuned streamer data in sub-critical regimes, but wide-angle signatures were adversely affected by standard deconvolution. This paper evaluates the performance of the peak- and bubble-tuned sources and presents a robust shaping deconvolution.
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Solving Static Problems with Ultra-Low Frequency Data and Waveform Inversion
Authors R. G. Pratt and C. StorkThe near-surface problem in land data can be address through the use of waveform inversion. Robust waveform inversion is made possible with the use of low frequency data; new digital receiver technologies make possible the acquisition of ultra-low frequency data. Our aim is to encourage development of this technique for application to the near-surface problem with land data.
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Efficient Waveform Tomography with Low Frequency, Wide-Angle Data and Sparse Survey Geometries
Authors A. J. Brenders and R. G. PrattWe present a study of the ability of efficient waveform tomography to recover 2-D velocity models from a synthetic surface experiment. We show that tomographic, frequency domain waveform inversion using only a subset of the available frequencies can give results equivalent to waveform inversion with many frequencies, for a low starting frequency, using the 2003 CCSS dataset and crustal velocity model. Aliasing effects due to sparse survey geometries begin to affect the result only if sources are spaced with an interval larger than or equal to 3 x Nyquist, provided receiver spacings are unaliased. We investigate this effect further using increased starting frequencies, and find that waveform tomography can produce results consistently superior to traveltime tomography at frequencies up to approximately 3.0 Hz for this dataset.
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The Importance of Low Frequency and Large Offset in Waveform Inversion
By L. SirgueProper reconstruction of the velocity field implies the recovery of a continuous range of wavenumbers from the low to the high end of the spatial spectrum. While the very low-wavenumber components of the model can be recovered by reflection tomography, waveform inversion has the potential to recover both intermediate and high wavenumber components of the model by using the full range of offsets and frequencies in the seismic data. However, this process is inherently difficult due to the non-linearity of the inverse problem. To mitigate this non-linearity, it is important to provide an appropriate starting velocity model, and seismic data with sufficient low frequencies and wide-angle wave propagation.
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Full Waveform Tomography and Nonlinear Migration
Authors W. A. Mulder and R. E. PlessixThe least-squares functional measures the difference between observed and modelled seismic data. Because it has a high computational cost and suffers from local minima, it has limited use for the inversion of model parameters. A good initial velocity model is required. Given such a model, the minimisation of the least-squares functional resembles nonlinear migration more than inversion.
Several authors observed that the model could be updated by diving waves, without the risk of ending up in a local minimum. They used frequency-domain acoustic modelling codes to construct a velocity model. This full waveform tomography is limited to a maximum depth, determined here by considering a simple model. This creates a dichotomy. Down to the maximum depth of diving waves, least-squares minimisation combines tomography and migration. Beyond that depth, nonlinear migration dominates. The dichotomy has consequences for the choice of frequencies when using a frequency-domain acoustic modelling code.
The acoustic approximation will lead to a number of problems when using long-offset data. We show that reasonable results can still be obtained on synthetic marine data created by an elastic time-domain finite-difference code. The resulting density is not correct, but the overall geometry is.
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Detailed Seismic Velocity Imaging of Hatton Bank Volcanic Continental Margin by Tomographic Inversion of Dense OBS Data
Authors L. K. Smith and R. S. WhiteData from a dense array of OBS used in the survey of a volcanic continental margin have been used successfully to image sub-basalt crustal structure using a robust tomographic inversion technique. The acquisition of wide-angle data using a low-frequency source allows useful data to be collected in regions where extruded basalts typically hinder conventional seismic imaging; OBS data can provide accurate seismic velocities at depth, therefore improving seismic imaging and interpretation of lithology, as well as providing important crustal structure information.
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Wide-Angle Seismic Study of the Hatton Continental Margin
Authors C. Ravaut, A. Chabert, P. W. Readman, B. M. O‘Reilly, P. M. Shannon and L. GernigonWe present the results of first-arrival traveltime inversion of two multi-fold wide-angle seismic profiles acquired on the Hatton margin (North Atlantic). This dataset is exceptional and unique in Europe with respect to the number and the closely spacing of the OBSs deployed on each profile. Application of classical imaging techniques provides well resolved and detailed images that may be improved in the near future by applying full-waveform inversion technique.
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An Efficient 3D Reverse Time Pre-Stack Depth Migration
Authors M. H. Karazincir and C. M. GerrardReverse time pre-stack depth migration (RTPSDM), which uses the two-way acoustic wave equation, is not a new concept. Conventionally the method has been very computationally intensive and, therefore, has been considered impractical for production 3D depth imaging projects. Here we describe an efficient 3D RTPSDM algorithm that can be used on real 3D production data. To make it practical and efficient we employ explicit 2nd order in time and high order space domain finite differences, and use domain decomposition methods to split the image cube amongst multiple CPU’s. Only a few finite differencing layers are communicated between related CPU’s, by message passing. High order spatial finite differences handle numerical dispersion and allow larger time steps than those possible with the conventionally used pseudo-spectral method. The forward propagated source wavefield is decimated prior to saving it on local disk to be used during application of the imaging condition. We will show an overview of the method along with 2D and 3D synthetic and real data examples.
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Wave Extrapolation with Locally Parabolic Rays
More LessWe propose a phase-shift extrapolation operators extending the straight-ray approximation to locally parabolic rays for depth variations of slowness and also for lateral deviations. This construction leads to second-order accurate wave-propagations operators. Its improvement is verified by numerical experiments, where the synthetic test demonstrate that the proposed method corrects straight ray approximations.
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Explicit Wavefield Extrapolation Using Projections Onto Convex Sets with Application to the SEG/EAGE Salt Model
Authors W. Mousa, M. Van der Baan, D. McLernon and S. BoussaktaIn this paper, explicit depth extrapolation operators are designed using the method of Projections onto Convex Sets (POCS). Resulting migration operators are applied to the challenging SEG/EAGE Salt model data. The migrated result is compared with images obtained via extrapolators based on modified Taylor series, and with other standard techniques such as the Phase Shift Plus Interpolation (PSPI) and the Split-Step Fourier methods. The POCS algorithm provides very stable depth extrapolators. The resulting migrated section is of comparable quality to the expensive PSPI result, and visibly outperforms the other two techniques. Both strong dips and sub-salt structures are imaged clearly.
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Robust Illumination Compensation for Shot-Profile Migration
Authors A. Guitton, A. Valenciano, D. Bevc and J. ClaerboutFor shot-profile migration, illumination compensation can be achieved if the imaging condition incorporates a deconvolution (division in the frequency domain) of the up-going wavefield by the down-going wavefield. To avoid division by zero, the deconvolution requires the selection of a damping parameter that turns out to be quite difficult to select. Consequently, a cross-correlation of the two wavefields is often selected. Alternatively, the zeros in the spectrum of the down-going wavefield can be filled with an average of the neighboring points. Therefore, instead of dividing by the wavefield, we can divide by a smoothed version of it. Smoothing is robust and easy to parameterize. It also corrects illumination problems in the migrated images.
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Conventional vs Cepstral Based Amplitude and Phase Only Full Waveform Inversion
Authors J. B. Bednar and C. ShinIn earlier work we investigated a cepstrum based approached to full-waveform inversion of seismic data and compared it to the more conventional least squares approach of Lailly (1983) and Tarantola (1984). We observed that it would be theoretically possible to separate the cepstral based approach into amplitude and phase only inversions. In this paper we perform this separation and compare the resulting algorithms to similar conventional amplitude and phase only approaches. We show that in the amplitude only case, conventional amplitude inversion is theoretically stable while cepstral based amplitude only inversion is not. In contrast, we show that cepstral based phase only inversion is not only stable but produces superior results to conventional phase only inversions. Moreover, we show that phase only inversion produces results that are almost as good as those obtained from the full cepstral based method.
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Azimut-Angle Migration for Node Type Data
Authors L. M. A. Nicoletis, P. Froidevaux, M. Mendes and O. BouhdicheKirchhoff (or Born) migration- inversion formulas of full azimuth data in the azimuth-opening domain have been obtained as integrals over migration dips, azimuth and opening angle, making use of the Beylkin Jacobian that transforms the acquisition coordinates into the four angular coordinates. It happens that this determinant is quite simple and that the final imaging formula is also simple,for P-P and P-SVwaves. The interest of this approach is to form substacks for selected ranges of azimuth and incidence in order to conduct AVAZ studies. We show how to form such classes, introducing a definition always stable of the local dip dependant azimuth and demonstrate the validity of the formula on node type synthetic data. Application to a real node type dataset is also shown.
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Subsalt Imaging of Wave Converted Reflections in Physical Model Data
Authors R. Alai and D. J. VerschuurAt the boundaries between saltbodies and the sediments, large contrasts in P-wave velocity occur and therefore quite a large amount of energy is converted into S-waves. These S-waves show up in the seismic data as echoes of the P-wave reflections. With the aid of physical model data, these effects can be studied in a controlled manner. Experiments on a physical model dataset illustrate clearly that wave conversions appear at saltbody interfaces and that migration of this data for a known "subsurface model" with the correct subsurface velocities (either P or S) will provide the correct images of (multiply) mode-converted energy.
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Imaging Complex Salt Bodies with Turning-Wave One-Way Wave Equation Migration
More LessIn this paper we propose a modified version of the one-way wave equation migration that can propagate wavefields to any possible direction. Also, we incorporate true amplitude corrections in the migration to enhance steep dips. With this new method, turning waves are properly imaged. We will use 2004 BP 2D benchmark dataset to show how the proposed method helps salt body detection and the imaging of complex salt structures.
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Wave-Equation Migration from Topography
More LessWave-equation migration from topography usually involves additional processing to regularize the acquired seismic data. However, these issues can also be addressed using the converse strategy of adapting wave-equation migration algorithms to be conformal with acquisition geometry. In this paper, the latter approach is examined through the development a methodology for imaging directly from acquisition coordinates. After generating acquisition coordinate systems using a conformal mapping approach, wavefields are extratpolated on these computational meshes using Riemanninan wavefield extrapolation. This imaging strategy is tested on a 2D field dataset acquired in the Canadian Rocky Mountain Foothills. After detailing the initial data processing, I present the results of a wave-equation migration from topography test. The test results indicate that the approach generates good images, especially in the near-surface. Comparisions with Kirchhoff migration results from a flat datum indicate that the wave-equation migration from topography images are superior. However, further benchmark tests are required before definitive statements about the relative merits of the wave-equation from topography approach can be reached.
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On the Influence of Offset in Plane-Wave Migration
Authors S. Grion and H. JakubowiczThis paper considers the influence of offset on plane-wave illumination and migration for seismic data. It is shown that, for a single offset, complete illumination and migration can only be achieved using plane waves encompassing all angles of incidence. By contrast, when a full range of offsets is used, it is possible in principle to illuminate and image the subsurface using only two plane waves. These results are confirmed using synthetic data. The number of plane waves required for plane-wave migration has a direct impact on computational cost. In practice, offset limitations, as well as other considerations, make it unrealistic to expect successful imaging using as few as two plane waves. However, the theory does support imaging with fewer plane waves than might otherwise be thought necessary. This is illustrated by a marine data example for which a good migrated image was obtained using only six plane waves.
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Sampling Issues in Delayed-Shot Migration and Plane-Wave Migration
More LessWe discussed the sampling issues for delayed-shot migration and plane-wave migration in a marine single streamer environment. For delayed-shot migration, when the migration input data has unaliased receiver records with enough cable length, there is sufficient coupling between shots in the inline direction for a physically realizable line source to be synthesized, and we can use fewer px’s to produce good images than would be required to construct unaliased impulse response images. On the other hand, conventional narrow swath marine acquisition presents an additional aliasing problem in the post-migration stack step for plane-wave migration, requiring a larger number of py-values. Delayed-shot migration seems more attractive for current narrow azimuth marine acquisition. However, a 3-D plane-wave migration may be a good candidate for migrating wide azimuth data.
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Modulated-Shot Migration
By R. SoubarasThis paper presents a new migration algorithm, which is able to produce p-indexed gathers
and a final image equal to that of a shot-record migration at less cost. This algorithm uses shot composition, just as plane-wave migration but by using modulation rather than time delays on the original shots. This allows using only a minimal number of composite shots for the migration.
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Generalized Imaging Conditions for Wave-Equation Migration
Authors P. C. Sava and S. FomelWe generalize imaging conditions and angle decompositions for wave-equation migration of primary and converted waves. Prestack images are described as functions of space-shifts or time-shifts between source and receiver wavefields at every image location. Angle transformations allow construction of common-image gathers for both primary (P) and converted (C) waves.
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Preserved-Amplitude Migration in Angle by Wavefield Continuation
Authors J. Svay-Lucas, F. Joncour, G. Lambare and B. DuquetWe propose an approach for preserved amplitude wave equation migration in angle domain. This approach is based on shot-receiver wavefield continuation, and successive buildings of common image gathers in depth offset domain, then angle domain. Imaging formulas are derived in 2D for acoustic reflection coefficient and square slowness perturbation, standing as specific weighted slant-stacks from offset domain to angle domain. An application on the Marmousi dataset is presented, with comparison to preserved amplitude multi-arrival ray-based migration.
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True Amplitude Thin Slab Multi-One-Way Inversion - Application to Marine-Type Seismic Data
Authors D. A. Kiyashchenko, R. E. Plessix, B. M. Kashtan and V. N. TroyanTrue-amplitude wave-equation migration can be obtained by using a modified imaging principle and a multi-one-way scheme. In order to avoid the artifacts of the multi-one-way scheme due to the turning rays, a thin-slab approach is investigated. A wave-equation inversion formula is proposed to recover the reflectivity. Under the high-frequency approximation, it is similar to the pseudo-inverse formula. This formula is applied on a synthetic and real example.
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Accelerating Wave Equation Migration by Frequency Decimation
More LessWave equation prestack depth migration is an effective, albeit costly, approach to image the subsurface in complex areas. We demonstrate a method for reducing the computational cost of such a migration while still yielding high quality images, by decimating the input data volume in frequency. Because a monochromatic signal is able to resolve a finite bandwidth of wavenumbers in the subsurface (in the prestack case), only a small number of frequencies is required to construct the depth image. The decrease in signal-to-noise ratio is very slight, owing to high levels of noise correlation across frequencies. We show a North Sea example, in which the migration cost has been reduced by a factor of 5, without noticeable degradation of the migrated depth image.
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Wave-Equation Angle-Domain Hessian
Authors A. A. Valenciano and B. BiondiA regularization in the reflection angle dimension (and, more generally in the reflection and azimuth angles) is necessary to stabilize the wave-equation inversion problem. The angle-domain Hessian can be computed from the subsurface-offset Hessian by an offset-to-angle transformation. This transformation can be done in the image space following the Sava and Fomel (2003) approach.
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Improvement of Kirchhoff Migrated Images by Dip Selection and Steered Stacking on Multi-Angle Gathers
Authors F. S. Audebert, F. Qing, B. Wang and P. ZhangKirchhoff Pre-Stack Depth Migration (PreSDM) can output images according to multiple angle attributes: reflection angles, illumination dip angles, and their combination. The images can be Common Image Gathers (CIG), that is, images corresponding to one or several attributes at a same observation location, or common angle images, that is, structural images for a single angle attribute at a time. Both images and CIGs can be used as interpretive and diagnosis tools. In this paper we demonstrate that the user can perform a selective stacking within a multi-angle CIGs, to produce an improved final image, free of some migration artifacts and undesirable noise. The steered stacking gives more flexibility than migration with a dip model, particularly in presence of multiple dips (faults, terminations, etc.).
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Fresnel-Volume-Migration of Single-Component Seismic Data
More LessIn standard KPSDM implementations of Kirchhoff prestack depth migration (KPSDM) the wavefield is smeared along two-way-traveltime isochrones. In the case of sparse sampling or limited aperture the image is sometimes affected by significant migration noise. Some modifications have been proposed which aim at reducing these artefacts by constructing a specular path of wave propagation derived from the slowness of coherent phases in the seismogram section and the heuristic restriction of the imaging operator to that wave path. Here we propose another physically frequency-dependent approach by using the concept of Fresnel-Volumes. Firstly the emergence angle at the receiver is determined by a local slowness analysis. Using the emergence angle as the starting direction a ray is propagated into the subsurface and the back-propagation of the wavefield is restricted to the vicinity of this ray according to its approximated Fresnel-Volume. We describe the procedure and show applications to a synthetic model as well as a real data set over a salt pillow in North Germany. Compared with standard KPSDM the image quality of our Fresnel-Volume-Migration is significantly enhanced due to the restriction of the migration operators to the region near the actual reflection point.
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How to Cope with Smoothing Effect in Ray Based PSDM?
Authors R. Baina, E. Zamboni and G. LambaréAlmost all implementations of ray based Kirchhoff prestack depth migration require a smoothed version of the velocity function. However, most velocity model building tools fit prestack traveltimes using blocky parameterizations with finite velocity jumps. This leads to an inconsistent scheme, in the sense that the smoothed migration velocity model is kinematically sub-optimal. Here, using a simple and efficient perturbation method, we show and demonstrate how we can correct for the smoothing effect in Kirchhoff PSDM.
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Gaussian Beam Migration - A Viable Alternative to Kirchhoff?
Authors C. D. B. Notfors, Y. Xie and S. GrayGaussian Beam migration is an elegant, accurate, and efficient depth migration method. It has the ability to image complicated geologic structures with fidelity exceeding that of single-arrival Kirchhoff migration, and approaching that of wave equation migration. In fact, for very steep dips, the accuracy of Gaussian Beam migration can exceed that of most wave equation migrations. The method also has the important advantage that it can readily account for the presence of anisotropy. We use results from both synthetic and real data to illustrate the advantages of Gaussian Beam migration.
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The Many Benefits of Traveltime Compression for 3D Prestack Kirchhoff Migration
More LessKirchhoff 3-D prestack migration, as part of its execution, requires repeated access to a usually large traveltime table data base. Access to this data base implies either a memory intensive or I/O bounded solution to the storage problem. Proper compression of the traveltime table allows efficient 3-D prestack migration without relying on the usually slow access to the computer hard-drive. Such compression also allows for faster access to desirable parts of the traveltime table. Compression is applied to the traveltime field for each source location on the surface on a regular grid using 3-D cosine transforms of the traveltime field represented in the Celerity domain. We obtain practical compression levels up to and exceeding 20 to 1. In fact, because of the smaller size traveltime table, we obtain exceptional traveltime extraction speed during migration that exceeds conventional methods. Additional features of the compression include better interpolation of traveltime tables and more stable estimates of amplitudes from traveltime curvatures.
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Resolution Analysis of Seismic Imaging
Authors R. S. Wu, X. B. Xie, M. Fehler and L. J. HuangWe define and formulate the resolution of an imaging system based on the inverse theory and local angle domain decomposition of Green’s functions. The resolution defined in this way includes both the effects of acquisition system and imaging (migration) process. The theory and method are based on wave theory and no asymptotic approximation is made in the calculation. It represents and quantifies the actual resolution we observed in the migrated images. The resolution taking into account only the influence of the acquisition system (frequency band and spatial aperture) and neglecting the factors such as the errors in backpropagation (migration) can be considered as the resolution limit of the system, the best resolution an acquisition system can get. Theoretical analysis and numerical examples are given to show the importance of propagator accuracy in the evaluation of resolution in complex media.
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Interpretive Applications of P- and S-Wave Data
By R. H. TathamThis presentation reviews where we stand, as an industry, in the application of multi¬component seismic data, the experience of end-users and how this information can be incorporated into their interpretations to fully exploit the wealth of information contained in these data. To reduce the impact of these limitations, published results demonstrating numerous applicable interpretive models and how they have been successfully applied have been organized into an internet accessible data base. Presently, some 500 application examples are accessible through browsers focused on either interpretive applications (objectives and problems) or by historical and geographic projects. Further improvements to communication among technology implementers and developers should reduce apparent barriers to widespread application of multicomponent seismic technology.
Access to these examples (case histories) and how so search them in the context of intepretive problems and geograpic locations is provided. The end objective is to provide direction in the intepretaiton of multicomponent seimsic data.
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Multicomponent OBC (4C) Time Imaging over Pamberi, LRL Block, Offshore Trinidad
Authors T. D. Johns, C. Vito, R. Clark and R. SarmientoResults from the P-P and P-Sv prestack time migration of a 2.5D four-component (4C) OBC seismic swath test acquired in 2004 over the Pamberi area in Block LRL offshore Trinidad, are presented. A description of the processing applied to the recorded multicomponent seismic data, through curved-ray anisotropic Kirchhoff prestack time migration for both the compressional P-wave and mode-converted PSv-wave, is provided. Mode-converted shear-wave data acquired from 4C surveys allow for imaging where conventional seismic are perturbed due to the presence of shallow gas and/or fluid in the pore spaces of the rock. Furthermore, mode-converted shear waves propagate with a different raypath than that of the compressional wave, thereby providing an alternative illumination of the subsurface target. As both the P-wave and S-wave record independent measurements of the same subsurface, more reliable rock properties can be uniquely determined, allowing for improved reservoir characterization and lithology prediction.
A conventional towed-streamer 3D was acquired over the Pamberi area in 2003, but failed to image or resolve adequately the target reflectors comprising the reservoir under the main growth fault. Therefore, the purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving stratigraphic interpretation in complex areas with challenging geology.
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Delineating Reservoir Sands Using PP and PS Seismic Data
Authors S. L. Roche, J. Gibson, B. Mattocks, R. Montgomery and M. B. HollandThin reservoir sands less than 40m thickness at an approximate depth of 2800m were successfully delineated using both P-wave (PP) and converted-wave (PS) shear data. Reflectivity associated with the reservoir sands is more clearly seen on the PS data than on the PP data. 58 wells with SP logs were used to train a neural network using both PP and PS data as input. Including the PS data was required to successfully predict the distribution of the reservoir sands. One well has been drilled on these results, with two others planned in early 2006. Training the neural network to predict the reservoir sands using P-wave data (migrated data and AVO attribute volumes) could not accurately delineate the reservoir sands without the PS data.
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Multicomponent PP and PS Seismic Response from Volcanic Gas Reservoirs
More LessHere we present an example of using PP and PS converted-waves for characterizing volcanic gas reservoirs in Daqing Oilfield in Northeast China. The volcanic targets are buried at depth raging from 2800m to 3600m, which often give rise to incoherent P-wave response. To overcome this problem, a multicomponent seismic experiment was set up to evaluate the converted-waves recorded by digital MEMS (micro-electro-mechanical system) sensors. The experiment includes six 2D lines, passing through ten boreholes drilled for the volcanic reservoirs. Several multicomponent VSPs have also been acquired for correlation purposes. Analysis the P- and S-waves from the target formation at the ten borehole locations reveals very consistent P- and S-wave amplitude anomalies. From the gas producing wells, the P-wave reflection is consistently weak and scattered, whilst the PS-wave reflection are consistently strong and continuous. In contrast from the non-producing wells, both PP- and PS-waves show continuous and strong reflections. The gas reservoirs can then be delineated from joint PP- and PS-amplitude analysis and the results agree with the drilling results in the study area. This provides conclusive evidence demonstrating the benefit of multicomponent seismic data from digital MEMS sensors.
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3D PP/PS Prestack Depth Migration on the Volve Field
Authors T. J. Szydlik, S. Way, P. Smith, L. Aamodt and C. FriedrichOcean-bottom seismic (OBS) methods record both compressional (PP) and converted shear- wave (PS) data. PS data have successfully been used to image through gas clouds (Xiang-Yang Li et al., 2001) and image reflectors that are weak on PP data ( MacLeod et al., 1999), and can help constrain rock property estimates (Özdemir et al., 2001). Thus, in principle, the use of PS data can significantly mitigate risk in hydrocarbon exploration and production. However, imaging of PS data is reliant upon accurate compressional and shear velocity estimates, as both the moveout and offset of the imaged point depend upon these parameters. Velocity anisotropy is also a critical factor. The previously used processing flows have not adequately comprehended this, and the results have often been of poor quality and difficult to tie with the PP volumes. This can preclude their use for lithology prediction. We have implemented an imaging workflow that can resolve many of these issues, and here describe its application to OBS data from the Volve field, Norwegian Central North Sea.
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Observations of Time-Lapse Converted-Wave Anomalies on Valhall LoFS Data
Authors S. Maultzsch, F. Mancini and S. PayneWe analyse a subset of the converted-wave data from the Valhall LoFS surveys 1 and 3 in order to investigate whether any time-lapse signal can be seen on the converted waves. Measurements of shear-wave splitting indicate a change in the magnitude of azimuthal anisotropy in the overburden between the surveys. After PSTM we observe a significant time-lapse anomaly at the reservoir level at the location of a production well. The anomaly is only found for azimuth sectors around the average polarization direction of the slow shear wave in the reservoir interval.
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Multicomponent Test for Assessing Anisotropy and Imaging in a Deep Carbonate Reservoir in the Caspian Sea
Authors M. S. Aitchison, E. Loinger, R. Miandro and T. ProbertKashagan is a giant carbonate reef reservoir located in the Kazakh sector of the North Caspian Sea at a depth of approximately 4000 m TVDSS. A multicomponent 2D test was designed to evaluate the detection of horizontal anisotropy and fractures in the reservoir using shear-wave splitting. In addition, the mode-converted (PS) data was tested for imaging the intra-reservoir characteristics in an area with limited shear velocity information. The multicomponent data were processed through prestack time migration, with particular emphasis on coherent noise and multiple attenuation. VSP data were used to calibrate the processing sequence. PS waves were able to provide clear indications of anisotropy, which were consistent with other independent measurements (VSP, geological and structural studies), and they also imaged the intra-reservoir architecture. This successful result, in a difficult area with complex geology, will be used to design further multicomponent surveys in the Kashagan field.
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4C Seismic Monitoring on Land in The Netherlands - Results for Acquisition Design
Authors G. G. Drijkoningen, J. Brouwer, J. Kooijman, G. Steenbergen, B. Dost and A. HuijgenIn this paper experimental results are shown for 4C seismic monitoring on land. First, the depth dependency of receiver responses are shown, where the effect of burial is most significant on the horizontal components for shallow depths (< 10 m). At larger depths, the effect of burial on the vertical components becomes significant. Second, in deltaic areas where water is abundantly present (such as in The Netherlands) it is shown that using hydrophones for monitoring is advantageous compared to geophones when P-wave energy is the main desired signal. Third, the monitoring stations easily pick up the local earthquakes, induced by gas extraction in the area under consideration. These earthquakes can be used as seismic sources in the future, e.g., for seismic interferometry experiments. Fourth, in a new era of fast internet (> Tbit/s) it becomes possible to send all the seismic data real-time to a very fast supercomputer (IBM Blue Gene). The network in the Netherlands becomes equipped with dealing with such large data streams and data sets.
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Multicomponent Analysis of a 2D/3C/9C Survey over an Oil-Bearing Field - Case Study from Russian Platform
2D/9C together with 3C offset VSP project was carried out in the central part of Russian platform. Vibrosources with guided horizontal directivity were implemented for generation of differently polarized shear waves. Full-wave registration allowed receiving compression, converted and monotype shear waves data. 3C uphole survey was acquired for near-surface velocities investigation. All information was processed and interpreted together. Mid- to long-wavelength statics derived from monotype shear wave data dramatically simplified statics solution for converted waves data. Offset VSP allowed exact identification and tie evens of different wave types to a geological section. Structure details in target interval which are not visible on P wave data are observed on PS sections. Anomalous polarization of SH and SV waves indicated possible fractured zones within zone of interest. Integrated interpretation results allowed predicting presence of sand bodies and fractured objects in Vendian-Riphean deposits which can be productive for the given area.
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Full-Wave Imaging Projects Using Multicomponent Digital Sensors - Russian Experience
Authors T. E. Galikeev, A. P. Zhukov, I. P. Korotkov and A. BurlakovTwo 3-D multicomponent datasets were acquired in Western Siberia and analyzed. The same type of digital 3-C accelerometers used for data recording were used with different types of sources (dynamite and VibroSeis). Data analysis suggests that the source and near surface conditions can impact the quality of recorded converted wave data, while the vertical P-wave component is of a high quality and equally interpretable when compared to geophone data acquired in the same area. Absorbing properties of the overburden in Western Siberia limit frequencies of converted waves up to 20-30 Hz depending on intensity of near-surface frost penetration. Therefore, an adequate effort should be spent on generating low frequencies in the source’ spectrum. Production sweeps rarely start below 10 Hz due to equipment restrictions and applied sweep tapers further reduce full energy effort spent on low frequencies. Absence of low frequencies in the source manifests itself by a very narrow frequency spectrum of converted waves in the data acquired with VibroSeis. Although, during processing and utilizing data rotation (data were recorded with 3-C digital sensors) the quality of C-wave stacks were improved.
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Characterisation of Gas Reservoirs in Polish Carpathian Foredeep Using Complex Multicomponent Seismic Data Analises
Authors A. Przybylo and E. GrzywaThe paper presents results of multicomponent seismic data interpretation obtained from different analyses: AVO method, Seismic Inversion, Vp/Vs Calculation, Combined Attributes Analyses. All performed analyses clearly showed reservoir zone. Comparison of all results from both PP and PS data is helpful for delineation of gas reservoir range and allow to reduce false anomalies determined on singular analyses.
Multicomponent seismic data come from Polish Carpathian Foredeep carried out in 2004. Presented examples concern Upper-Miocene sediments.
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Analysis of Converted-Wave Splitting in Volcanic Rocks - A Case Study from Northeast China
Authors L. F. Wang, X. Y. Li and X. Y. SunConverted shear-wave splitting provides a practical means for evaluating azimuthal anisotropy in hydrocarbon reservoirs that may give some insights into the internal architecture of the reservoirs. In this paper, we evaluate converted-wave seismic data acquired over volcanic gas reservoirs. The converted-wave data reveals a significant amount of shear-wave splitting over the volcanic formation, and we develop a technique to extract the shear-wave polarization and time delay from the data. The technique is particularly designed for evaluating converted-wave splitting for 2D or 3D data with a narrow-azimuth distribution. We adopt a rotation-scanning procedure that maximizes the separation of the fast and slow split shear-waves. It is interesting that the amount of splitting determined from the data can be correlated to the known gas reservoirs, revealing a potential to use shear-wave splitting to delineate gas reservoirs in volcanic rocks.
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Comparison of Two Physical Modelling Studies of 3D P-Wave Fracture Detection
More LessTwo 3D physical seismic data were acquired and analyzed to verify the physical basis of using P-wave attributes for fracture detection, to understand the usage of these attributes and their merits, and to investigate the effects of acquisition geometry and structural variations on these attributes. One model was designed to maximize the data quality, and another model was designed to increase the offset-depth ratio to the top of the fracture layer. The study of both datasets reveals that the P-wave attributes (traveltime, amplitude and velocity) exhibit azimuthal variations. For the data with high quality, the amplitude from the top of the fracture layer yields the best results that agree with the physical model parameters, but the results from other attributes (traveltime, velocity, AVO gradient) are either contaminated by the structural imprint, or by the acquisition footprint. For the data with larger offset-depth ratio, the traveltime attributes yield the best results, but the results from the amplitudes are affected by the noise and are less reliable.
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Dual Inversion Applied to 2D Multi-Component Seismic Data Onshore Libya
Authors C. Hanitzsch, L. de Vincenzi, J. M. Michel and D. SemondIn line with the theme of the conference 'Opportunities in Mature Areas' (Sirt basin onshore Libya), 2D3C (multi-component) surface seismic data have been acquired, processed and inverted on behalf of Wintershall Libya in concession C97-I. In this area, presence of non-reservoir is the main risk for new wells. Reservoir sandstone can be partly or fully missing because of shallow basement or intrusions of basalt or deposition of volcanoclastics. From integrated reservoir characterisation studies it was concluded that it is the ratio of compressional to shear wave velocity (Vp/Vs) that allows differentiating reservoir sandstone from non-reservoir. Vp/Vs can be estimated by simultaneous ("dual") inversion of multi-component surface seismic data. In areas of good seismic data quality, the Vp/Vs inversion results correlate very well with well data. In areas with medium to poor seismic quality, Vp/Vs sections can still be interpreted in a relative sense. Therefore, the technology has the potential to help reduce the risk of dry wells in this area.
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PP and PS Joint Inversion - A Case Study from West Africa
Authors F. Mancini and J. ZhangWe present results of a joint PP- and PS-wave AVO inversion from a multicomponent dataset acquired offshore West Africa. We compare these results with the output of the single, P-wave only, inversion. In both realistic synthetic examples and real data the joint inversion produces a better estimation of the shear impedance reflectivity and of the pseudo Poisson’s ratio reflectivity compared with the single inversion.
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Case Study Example of Multiple Attenuation for Over/Under Streamer Data
Authors K. M. Schalkwijk, C. Kostov and E. KraghIn a recent experiment in the North Sea, data were acquired with a streamer towed at conventional depth, as well as with a pair of deeper streamers, towed in so-called over/under configuration. The main benefits expected from the over/under streamers are lower levels of swell noise in the data and suppression of the receiver-side ghost. In the theoretical section of the paper, we discuss the implications of deghosting on predicted surface multiples; the absence of deghosting during preprocessing may introduce inconsistencies between the model of the multiples and the data, which render the subtraction of multiples less reliable. In the case study example, we build upon previously published results by adding attenuation of free-surface multiples (2D SRME) in the processing sequence. As in previous work, the quality of the stack from the over/under data is better than the stack from the shallow streamer data. Improvements in multiple attenuation on the over/under data are not as readily identified as expected, but the detailed interpretation and comparison of multiple attenuation results is still in progress.
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Identification, Interpretation and Attenuation of Interbed Multiples from West Africa
More LessDespite the advances in 2D and 3D surface multiple suppression, interpretation difficulties persist where long period interbed multiples interfere with seismic reflection data. In this paper we study interbed multiples using a 3D re-processing case history from West Africa. We address the understanding of the problem using simple ray-trace seismic modelling to generate prestack data volumes which are processed and then compared with the real data volume. The modeling confirms that non-hyperbolic multiple arrivals and subsequent imperfect removal contributes to the interpretation difficulties. Finally we attempt to suppress the modeled multiples using a new radon domain based adaptive subtraction technique.
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Suppression of Apex-Shifted Multiples with a Shifted-Hyperbola Radon Transform
Authors E. Duveneck, E. Causse, D. Lokshtanov, K. Hokstad, R. Fillon and J. F. DutzerApex-shifted multiples in CMP gathers are difficult to suppress and can seriously degrade seismic imaging results. To address this problem, we present a high-resolution Radon transform based on shifted hyperbolae with an additional apex-shift parameter. This transform allows focusing of apex-shifted events with a wide range of shapes, leading to a better separation and suppression of apex-shifted multiples. In order to make the transform computationally efficient, while obtaining good focusing in the multi-dimensional Radon domain, a restricted set of Radon parameter values is determined using a frequency-domain high-resolution Radon transform. A time-domain Radon transform is then performed on the restricted set of parameter values. The method is demonstrated on a synthetic data example involving apex-shifted pegleg multiples associated with a dipping seafloor. The multiples are successfully removed by muting in the Radon domain.
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Attenuating Diffracted Multiples with 3D SRME - A Feasibility Study
Authors A. Baumstein, D. L. Hinkley, K. D. Andersen and T. G. FarringtonThe problem of diffracted multiple attenuation is tackled through an application of 3D SRME. The impact of changes in acquisition geometry is assessed by performing multiple attenuation on synthetic data. We demonstrate that 3D SRME can successfully attenuate surface-related diffracted multiples, provided the necessary full wavefield is either acquired or reconstructed. We focus our study on the impact of acquisition geometry on the quality of multiple suppression and show that acquiring higher-density data can lead to significant improvements in diffracted multiple attenuation. A large gap in the quality of multiple suppression remains between the case when the full wavefield is obtained directly through modeling and the case when it is obtained using DMO-based data reconstruction. This observation points to the need for further research to determine whether alternative data reconstruction methods can yield greater improvements in SNR while simultaneously allowing sparser, less costly acquisition geometries.
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Wave Equation Based Multiple Modelling - Comparison of Nominal and Dense Acquisition Geometries
Authors T. Weisser, A. Pica and P. HerrmannMarine seismic data acquired over seafloors with rough topography are characterized by the presence of complicated multiple energy patterns. We present a model-based surface-related multiple modelling technique free from any constraint relating to the shot position and distribution. This technique may require streamer interpolation/extrapolation, but does not require any sail lines reconstructions. We are comparing results with conventional and dense acquisition geometries on a Norwegian Sea dataset. The sail lines were shot with 50% overlap for testing of 3D SRME techniques with special attention to the acquisition geometry. The results achieved with nominal and dense acquisition geometries are equivalent, as a consequence the proposed approach does not require dense cross sail line acquisition geometries neither dense cross sail line data reconstruction.
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