Time-lapse Surveys Research Articles

Evaluation and correction of sources of 4D noise using modeled ocean-bottom-node data

Ocean-bottom-node (OBN) data are being used successfully in the deepwater environment to acquire time-lapse surveys due to their low 4D noise level. To help maximize the benefits of using such data, we built an acoustic model based on a deepwater subsalt field from the Gulf of Mexico. We generated synthetic base and monitor data using a realistic acquisition geometry and a laterally variable, random water-velocity layer. This study focuses on investigating a number of sources of time-lapse noise, including water-velocity variations, source and receiver repeatability, and position accuracy, and on evaluating ways to correct for these errors during processing. Results indicate that variable water velocities, if not corrected for during processing, produce the majority of residual 4D noise observed on the difference images. With currently achievable node placement accuracy, repeatability is fairly good and produces the smallest amount of noise. Sources, which are the least repeatable, produce almost three times the amount of noise compared to nodes in our experiment. We applied a similar processing workflow to the up- and downgoing wavefields and imaged both types of data using one-way and two-way wave-equation-migration techniques. In this particular synthetic example, image quality and 4D noise level were similar in the extra-salt area on both processed up- and downgoing images, but both measures in the subsalt were much better on the upgoing data.

An inverse method for estimating thickness and volume with time of a thin CO2‐filled layer at the Sleipner Field, North Sea

AbstractMigration of CO2 through storage reservoirs can be monitored using time lapse seismic reflection surveys. At the Sleipner Field, injected CO2 is distributed throughout nine layers within the reservoir. These layers are too thin to be seismically resolvable by direct measurement of the separation between reflections from the top and bottom of each layer. Here we develop and apply an inverse method for measuring thickness changes of the shallowest layer. Our approach combines differences in traveltime down to a specific reflection together with amplitude measurements to determine layer thicknesses from time lapse surveys. A series of synthetic forward models were used to test the robustness of our inverse approach and to quantify uncertainties. In the absence of ambient noise, this approach can unambiguously resolve layer thickness. If a realistic ambient noise distribution is included, layer thicknesses of 1–6 m are accurately retrieved with an uncertainty of ±0.5 m. We used this approach to generate a thickness map of the shallowest layer. The fidelity of this result was tested using measurements of layer thickness determined from the 2010 broadband seismic survey. The calculated volume of CO2 within the shallowest layer increases at a rate that is quadratic in time, despite an approximately constant injection rate into the base of the reservoir. This result is consistent with a diminished growth rate of the areal extent of underlying layers. Finally, the relationship between caprock topography and layer thickness is explored and potential migration pathways that charge this layer are identified.

The variety of subaerial active salt deformations in the Kuqa fold-thrust belt (China) constrained by InSAR

Surface salt bodies in the western Kuqa fold-thrust belt of northwestern China allow study of subaerial salt kinematics and its possible correlations with weather variations. Ephemeral subaerial salt exposure during the evolution of a salt structure can greatly impact the subsequent development and deformation of its tectonic setting. Here, we present a quantitative time-lapse survey of surface salt deformation measured from interferometric synthetic aperture radar (InSAR) using Envisat radar imagery acquired between 2003 and 2010. Time series analysis and inspection of individual interferograms confirm that the majority of the salt bodies in western Kuqa are active, with significant InSAR observable displacements at 3 of 4 structures studied in the region. Subaerial salt motion toward and away from the satellite at rates up to 5 mm/yr with respect to local references. Rainfall measurements from the Tropical Rainfall Measuring Mission (TRMM) and temperature from a local weather station are used to test the relationship between seasonality and surface salt motion. We observe decoupling between surface salt motion and seasonality and interpret these observations to indicate that regional and local structural regimes exert primary control on surface salt displacement rates.

Feasibility study of seismic monitoring for gas hydrate production in the Ulleung Basin in the East Sea

During gas hydrate production, the upper parts of the gas hydrate-bearing layer (GHBL) behave as a seal. If parts of the upper gas hydrates are dissociated, free methane gas present in the lower parts of GHBL may leak into the upper parts of the layer. Therefore, the dissociation of methane gas in the GHBL must be monitored to ensure strong stability during gas production. To investigate the feasibility of monitoring the dissociation of gas hydrates with a seismic survey, we established a rock physics model incorporating the geological characteristics of the region around the UBGH 2–6 well in the Ulleung Basin based on LWD data and core analysis data. We performed fluid substitution modeling to build a velocity model with the dissociated parts of the gas hydrates, and generated synthetic data. Diffractions from the edges of the dissociated parts allowed detection of gas hydrate dissociations, and the dissociated locations could be identified in the migrated image. In addition, sensitivity testing showed that the dissociated part was recognizable in the migrated image, even at a width of half the wavelength. Therefore, we conclude that the stability of gas hydrates during production can be monitored using a time-lapse seismic survey.

Subsidence Inferred from a Time Lapse Reservoir Study in a Niger Delta Field, Nigeria

Production-induced subsidence due to compressibility and fluid property changes in a Niger delta field has been investigated using well log and 4D seismic data sets. The objective of the study is to evaluate changes in time lapse seismic attributes due to hydrocarbon production and infer to probable ground subsidence. Petrophysical modeling and analysis of well data revealed that Density (ρ), Lambda rho (λρ) and Acoustic impedance (Ip) are highly responsive to changes in reservoir properties. These properties and water saturation attribute were subsequently, extracted from time-lapse seismic volumes in the immediate vicinity of well locations. Result show that monitor horizon slices exhibit appreciable increases in ρ, λρ, Ip and water saturation values compared to the base data, especially around the well locations. These increases in relative values of rock/attribute properties between the time-lapse surveys for a constant overburden stress are obvious indications of pore pressure and fluid depletion in the reservoir. Depletion in these properties increases the effective stress (pressure) and the grain-to-grain contact of the reservoir matrix, with a corresponding decrease in compressibility. Consequently, pore and matrix volume decreases, the reservoir compacts and the ground subsides. However, this is suspected to be small and at the reservoir scale due to low initial reservoir porosity and the relatively large lateral dimension compared to the thickness of the reservoir.

High-resolution imaging and characterization of a CO2 layer at the Sleipner CO2 storage operation, North Sea using time-lapse seismics

L arge-scale underground storage of industrially produced carbon dioxide is the most effective way of keeping cumulative man-made emissions of greenhouse gases within safe limits (IPCC, 2005). The CO2 injection operation at Sleipner, in the central North Sea between the UK and Norway, commenced in 1996 and is the world’s longest-running industrial-scale storage project. It is also the first example of underground CO2 storage arising as a direct response to environmental legislation (Baklid et al., 1996). CO2 separated from natural gas produced at the Sleipner field is being injected into the Utsira Sand, a regional saline aquifer of late Cenozoic age, in excess of 200 m thick in the Sleipner area (Figure 1a). The aquifer comprises mostly clean unconsolidated sand of high porosity (> 0.3) and high permeability (> 1 Darcy). A number of thin intra-reservoir mudstones, typically 1-2 m thick, are evident from geophysical logs acquired in wells around Sleipner (Figure 1b). The CO2 is injected in a dense phase via a deviated well at a depth of 1012 m below sea level, approximately 200 m beneath the top of the reservoir. Injection commenced in 1996 at a roughly constant rate, with around 16 million tonnes of CO2 stored by 2015. A comprehensive deepfocused monitoring programme has been deployed of which time-lapse seismic has proven to be the key tool (Arts et al., 2008). A baseline 3D survey was acquired in 1994, with repeat surveys in 1999, 2001, 2004, 2006, 2008, 2010 and 2012. The plume is imaged on the seismic data as a tiered structure some 200 m high comprising a number of bright sub-horizontal reflections (Figure 2a). These are interpreted as reflections from thin layers of CO2 trapped beneath the intra-reservoir mudstones which are partially but not wholly sealing. The reflective layering had formed by 1999 with each individual reflection traceable on all of the subsequent surveys. As a general rule the middle and upper reflections in the plume have increased in amplitude and lateral extent on successive time-lapse surveys, whereas the lower layers have ceased growing, in some cases shrinking and dimming. A key objective of the monitoring at Sleipner is to demonstrate that geological storage of CO2 is a safe and viable technology. One aspect of this is to quantitatively verify or constrain predictive flow simulations of plume development. However, because the injection well is near-horizontal, no wellbore penetrates either the CO2 plume or the exact stratigraphy that the plume now occupies, and quantitative analysis is challenging.

Time-lapse seismic monitoring methodologies applied to the Pyrenees Field, offshore Western Australia

Time-lapse dedicated 3D seismic surveys were acquired across the Pyrenees oil and gas field, Exmouth Sub-basin to map production-induced changes in the reservoir. Rock-physics 4D modelling showed that changes in pore pressure and fluid saturation would produce a time-lapse seismic response of sufficient magnitude, in both amplitude and velocity, to overcome time-lapse noise. The dominant observed effect is associated with gas coming out of solution. The reservoir simulation model forecasted that reservoir depletion would cause gas breakout that would impact the elastic properties of the reservoir. The effect of gas breakout can be clearly observed on the 4D seismic data as a change in both amplitude and velocity. The analysis of the seismic datasets was proven to be enhanced significantly by using inversion methodologies. These included a band-limited extended-elastic impedance (EEI) approach, as well as simultaneous 4D elastic inversion. These datasets, combined with rock physics modelling, enabled quantitative interpretation of the change in 4D seismic response which was a key tool for assisting with the infill well placement and field development strategy.

The impact of a quick 4D seismic survey and processing over the Halfdan Field, Danish North Sea

The quick turnaround of seismic processing of a 4D time lapse survey over the Halfdan oil field allowed for interpretation and integration of the new 4D results seven weeks after completion of the seismic acquisition. Analysis of the first fast track 4D dataset led to improved processing parameters and an additional demultiple application which yielded a significantly improved fast track dataset 11 weeks after the seismic acquisition was completed. The repeat 4D seismic survey was acquired in the summer of 2012 in order to provide a better understanding of the sweep efficiency from the line drive water flood, guide future well interventions and improve the reservoir model. The baseline 3D seismic data was acquired in 1992/1993, prior to field development, and the first monitor 3D seismic was acquired in 2005. The rock physics model shows that increased water saturation due to the water flooding along the injectors in the Tor oil-bearing reservoir dominates the 4D change in acoustic impedance yielding a hardening response. The rock physics model was used to convert the reservoir model pressure and saturation changes from the three time-steps to modelled acoustic impedance changes. Examples will be shown from the 4D time lapse and the reservoir model acoustic impedance changes.

Time-lapse seismic modeling for CO2 sequestration at the Dickman Oilfield, Kansas

Carbon dioxide capture and injection into the subsurface has aroused great interest in the past few years as a method to enhance oil recovery and mitigate [Formula: see text] emissions. The Dickman Oilfield located in Kansas provides two possible [Formula: see text] sequestration targets: a regional deep saline reservoir (the primary objective) and a shallower mature depleted oil reservoir (secondary). We focused on the shallower depleted oil reservoir through a 250-year flow-simulation scenario and a fault leakage test. Seismic responses at various time intervals were simulated to help monitor [Formula: see text] flow paths and injection stability. A complex and realistic geologic model with unconformity was embedded in the flow-simulation model. A regridding technique was used that assigned geologic values to a regular seismic grid that allowed 2D acoustic and elastic finite-difference simulation. Gassmann fluid substitution theory was used to obtain the reservoir properties with different [Formula: see text] saturations, and the vertical seismic profile was used to assist in identifying geologic layers. A [Formula: see text] plume and flow path from the leakage test can be detected from the differences in seismic data (5 to 10 ms time shift) from the first year of injection and the last year of monitoring. This was supported by comparison with the prestack field data available in the Dickman Oilfield. [Formula: see text]-induced reflectivity changes are relatively larger for PS events than for PP events, implying that multicomponent data acquisition and processing may give added value to characterization and monitoring of carbon capture and storage projects. We assessed 4D seismic monitoring in the evaluation of long-term [Formula: see text] containment stability for the Dickman Oilfield and suggested that time-lapse surveys will be useful.

Navigating marine electromagnetic transmitters using dipole field geometry

The marine controlled source electromagnetic (CSEM) technique has been adopted by the hydrocarbon industry to characterize the resistivity of targets identified from seismic data prior to drilling. Over the years, marine controlled source electromagnetic has matured to the point that four-dimensional or time lapse surveys and monitoring could be applied to hydrocarbon reservoirs in production, or to monitor the sequestration of carbon dioxide. Marine controlled source electromagnetic surveys have also been used to target shallow resistors such as gas hydrates. These novel uses of the technique require very well constrained transmitter and receiver geometry in order to make meaningful and accurate geologic interpretations of the data. Current navigation in marine controlled source electromagnetic surveys utilize a long base line, or a short base line, acoustic navigation system to locate the transmitter and seafloor receivers. If these systems fail, then rudimentary navigation is possible by assuming the transmitter follows in the ship's track. However, these navigational assumptions are insufficient to capture the detailed orientation and position of the transmitter required for both shallow targets and repeat surveys. In circumstances when acoustic navigation systems fail we propose the use of an inversion algorithm that solves for transmitter geometry. This algorithm utilizes the transmitter's electromagnetic dipole radiation pattern as recorded by stationary, close range (<1000 m), receivers in order to model the geometry of the transmitter. We test the code with a synthetic model and validate it with data from a well navigated controlled source electromagnetic survey over the Scarborough gas field in Australia.

Time-Lapse Electrical Resistivity Investigations for Imaging the Grouting Injection in Shallow Subsurface Cavities

The highway of Yongweol-ri, Muan-gun, south-western part of the South Korean Peninsula, is underlain by the abandoned of subsurface cavities, which were discovered in 2005. These cavities lie at shallow depths with the range of 5∼15 meters below the ground surface. Numerous subsidence events have repeatedly occurred in the past few years, damaging infrastructure and highway. As a result of continuing subsidence issues, the Korean Institute of Geosciences and Mineral Resources (KIGAM) was requested by local administration to resolve the issue. The KIGAM used geophysical methods to delineate subsurface cavities and improve more refined understanding of the cavities network in the study area. Cement based grouting has been widely employed in the construction industry to reinforce subsurface ground. In this research work, time-lapse electrical resistivity surveys were accomplished to monitor the grouting injection in the subsurface cavities beneath the highway, which have provided a quasi-real-time monitoring for modifying the subsurface cavities related to ground reinforcement, which would be difficult with direct methods. The results obtained from time-lapse electrical resistivity technique have satisfactory imaged the grouting injection experiment in the subsurface cavities beneath the highway. Furthermore, the borehole camera confirmed the presence of grouting material in the subsurface cavities, and hence this procedure increases the mechanical resistance of subsurface cavities below the highway.

High-resolution 3D seismic investigations of the overburden above potential CCS sites of the inner Texas shelf, Gulf of Mexico, U.S.A.

Much CCS research has focused on the injection interval (storage reservoir) and the immediately overlying primary seal. Considerations related to potential long-term leakage have turned attention toward the geologic overburden between the primary seal and shallow intervals containing protected groundwater. Typically the overburden interval is poorly imaged in commercially available seismic data given acquisition and processing optimized for deeper reservoir systems. Recent advances in high-resolution 3D (HR3D) seismic imaging have allowed for more thorough investigation of this critical interval in marine settings, and are well-suited for evaluating potential leakage pathways for prospective CCS sites. HR3D datasets can serve to reduce risks prior to project development as well as provide containment assurance via staggered time-lapse surveys. HR3D is likely to become a fundamental technology in offshore CCS.

캐나다 아퀴스토어 CCS 프로젝트의 이산화탄소 모니터링을 위한 Baseline 탄성파 속성분석

주입된 <TEX>$CO_2$</TEX>가 환경에 영향을 미치지 않고 지하에서 안정적으로 저장되어 있는지를 총체적으로 모니터링 하는 주입 및 주입 후 관리 (Monitoring, Mitigation and Verification, MMV) 기술은 이산화탄소 지중저장 분야에서 경제적 및 환경적으로 매우 중요한 역할을 하고 있다. 특히, 해외 대규모 지중저장 프로젝트 사례를 보았을 때 주입한 <TEX>$CO_2$</TEX>의 거동을 가장 효율적으로 모니터링할 수 있는 방법 중의 하나로 탄성파를 이용한 시간 경과 (Time-lapse) 모니터링 기술이 그 핵심으로 떠오르고 있다. 이 연구에서는 캐나다 Estevan에 위치한 Aquistore 이산화탄소 주입 현장의 3차원 베이스라인 (baseline) 탄성파 자료를 수집하고 분석하여 국내 지중저장 탄성파 모니터링 실증화를 위한 기초 연구를 수행하였다. 이산화탄소 주요 저장 대상층은 탄성파 도달 시간 1,800 ~ 1,900 ms 깊이의 Winnipeg 와 Deadwood 사암층이다. Aquistore 탄성파 자료에 대한 에너지, 유사도(similarity)를 도출하고 주파수를 분해하여 <TEX>$CO_2$</TEX> 주입 대상층의 특성을 규명하였다. 그 결과 등시선도 1,800 ms의 연구지역 북측, 1,850 ms의 남측에 탄성파 에너지가 큰 영역이 집중적으로 분포함을 확인할 수 있었고, 탄성파 에너지 속성을 도시하여 반사계수가 큰 사질 퇴적양상이 우세한 영역을 구분할 수 있었다. 또한 탄성파 기록의 유사도를 도출하여 두 개의 주요한 구조선이 북서-남동 방향으로 지중저장 대상층을 절단함을 확인하였다. 탄성파자료의 주파수를 성분별로 분해하고 5, 20 및 40 Hz 성분을 분석한 결과 연구지역의 중앙에서 동서 방향으로 발달하는 균질한 퇴적 양상이 구체화되었다. 베이스라인 자료의 경우 추가적으로 인위적인 잡음을 제거하고 층서 해석 결과를 통합하여 이산화탄소 지중저장 영역을 묘사한다면 시간경과 모니터링 자료와의 효율적인 대비가 가능할 것이다. <TEX>$CO_2$</TEX> Monitoring, Mitigation and Verification (MMV) is the essential part in the Carbon Capture and Storage (CCS) project in order to assure the storage permanence economically and environmentally. In large-scale CCS projects in the world, the seismic time-lapse survey is a key technology for monitoring the behavior of injected <TEX>$CO_2$</TEX>. In this study, we developed a basic process procedure for 3-D seismic baseline data from the Aquistore project, Estevan, Canada. Major target formations of Aquistore CCS project are the Winnipeg and the Deadwood sandstone formations located between 1,800 and 1,900 ms in traveltime. The analysis of trace energy and similarity attributes of seismic data followed by spectral decomposition are carried out for the characterization of <TEX>$CO_2$</TEX> injection zone. High trace energies are concentrated in the northern part of the survey area at 1,800 ms and in the southern part at 1,850 ms in traveltime. The sandstone dominant regions are well recognized with high reflectivity by the trace energy analysis. Similarity attributes show two structural discontinuities trending the NW-SE direction at the target depth. Spectral decomposition of 5, 20 and 40 Hz frequency contents discriminated the successive E-W depositional events at the center of the research area. Additional noise rejection and stratigraphic interpretation on the baseline data followed by applying appropriate imaging technique will be helpful to investigate the differences between baseline data and multi-vintage monitor data.

A 4D-repeatability indicator based on similarity between shots illumination imprints

A new 4D-repeatability indicator is proposed to appraise the quality of positioning during time-lapse marine surveys. Repeatability of illumination between base and monitor surveys is assessed on selected reservoir horizons at fine discrimination scales, ranging from lines down to individual shots. Similarity between corresponding illumination imprints is evaluated from an adapted Partitioned Intensity Uniformity metric. Such repeatability indicator can be used to assess jointly source and receivers positioning during 4D towed streamers surveys and under-shoots, or only source positioning over nodes surveys. It provides a user-friendly tool to qualify the acquisition, or identify and rank preliminary re-shoots needs.

Evaluating time-lapse borehole gravity for CO2 plume detection at SECARB Cranfield

Monitoring of CO2 storage processes provides a renewed challenge of adapting oil and gas monitoring technology to provide measurements required of operator, regulatory authorities and the public. These requirements include providing measurements that will support an understanding of safe containment of the CO2 plume, and inform evidence that injected CO2 remains in the storage complex for the purposes of green house gas accounting, cost effectively. These roles for monitoring demand a re-evaluation of deep reading measurements besides seismic, such as gravitational and electromagnetic technologies, sensitive to density and electrical properties of the fluids. In particular this paper explores the operational and interpretational challenges of borehole microgravity in a CO2 timelapse survey in two wells at the SECARB Cranfield injection site.The Southeast Partnership (SECARB) test at Cranfield, Mississippi, was the first of the commercial scale projects comprising a staged array of field deployments testing key issues of capacity and best methods for assuring storage permanence. More than 3millionmetrictons of injected CO2 will have been injected since the start of injection in July of 2008. The site is an historic oilfield at a depth of 2500m in the Cretaceous fluvial Tuscaloosa Formation, developed by Denbury Onshore LLC. The project was unique in deploying a range of geophysical measurements including cross-well seismic and electrical resistivity tomography, from two monitoring wells positioned in line from the injector and several 100ft down-dip in the Tuscaloosa. The CO2 Capture Project (CCP) took advantage of the multiple field acquisitions of borehole data during the injection phase of this project to acquire timelapse borehole gravity in the two monitoring wells. The objective was twofold; the first to understand the operational aspects and design of the acquisition, while the second was to assess the ability of the surveys to detect the injected CO2. The baseline acquisition occurred in October 2009 and the repeat survey in September 2010.

Denoising multicomponent CSEM data with equivalent source processing techniques

Frequency-domain controlled-source electromagnetic surveys can be negatively affected by near-surface geologic features, resulting in static shifts in the observed electric field amplitude data. We propose using an equivalent source method to denoise electric field data from frequency-domain controlled-source electromagnetic (CSEM) surveys. The equivalent source method can either be applied individually to data from each measured frequency or simultaneously to data from all frequencies measured. We demonstrate the effectiveness of the method by denoising an individual synthetic CSEM survey with data contaminated with noise from near-surface conductive bodies, as well as a synthetic time-lapse survey representing resistivity changes in a target due to, for instance, [Formula: see text] sequestration. We also apply the equivalent source method to denoise a set of controlled-source audio-magnetotelluric data affected by static shift from a field survey in western China.

Pulsed-Neutron Monitoring of the First CO2 Enhanced-Oil-Recovery Pilot in the Middle East

Summary Logging measurements in the borehole are vital for monitoring carbon dioxide (CO2) floods—for assessing the fluid changes in the reservoir rock as well as in the wellbore. The saturation profile at each well location provides the efficiency of the flood process for fluid displacement within the pore and the vertical sweep across and within the reservoir zones. A snapshot from multiple well locations in the reservoir enables the creation of a picture of the flood flow pattern, and the time-lapse surveys track the progress of the flood with time. Pulsed-neutron logs provide essential measurements for the evaluation of saturation in the injectors, producers, and observers. However, the CO2 environment, with the fluid in the borehole, remains uncharacterized in the industry. Hence, reliable inferences require either that the measurement is immune to the borehole environment or that the perturbation is minimal and can be easily corrected. Where corrections are required, suitable benchmarks should be planned in advance to verify the accuracy of the corrections. These corrections should be modeled after the physics of the measurement to the maximum extent possible. On the first CO2 enhanced-oil-recovery (EOR) pilot project in the Middle East—unique in the world because the CO2 flood was implemented with the reservoir at original oil saturation—several pulsed-neutron surveys were recorded in the injector, observer, and producer wells. The surveys included capture and inelastic mode acquisition. Several novel techniques of data acquisition and interpretation were successfully tried. This paper presents the steps in planning and executing the jobs and the results of the surveys. Limitations of existing characterization and those imposed by the measurement environments in the subject wells are discussed, and we show, through comparison with benchmarks, that correction for the unusual borehole environment is possible. The paper illustrates how the different modes of pulsed-neutron data acquisition complement each other in the individual wells in assessing the borehole environment, providing adequate input data to enable a multiphase reservoir-fluid analysis, and yielding independent fluid saturations for effective comparison. The results of the analysis are compared with openhole evaluation to help create a coherent picture of the reservoir. The fluid analysis from the pilot wells confirms the high displacement efficiency of CO2 as an EOR fluid. The saturation profiles from individual wells portray the vertical sweep of the flood, and the snapshot from the multiple wells gives the areal sweep. Combined with the data from production-log sensors and permeability from the magnetic resonance, the flood-breakthrough layers are identified.

The Impact of Geomechanics on Monitoring Techniques for CO2 Injection and Storage

Surface seismic and vertical seismic profile (VSP) time-lapse surveys have demonstrated the capability of temporally and spatially tracking the storage of CO2 within the subsurface geological formation, however quantitatively linking changes in seismic attributes to fluid flow remains problematic. This study uses coupled geomechanical-fluid flow models to generate seismic attributes and compares the impact of geomechanics on seismic attributes to non-coupled models. Prediction of seismic data may be important in predictive modelling of fluid flow properties for CO2 storage projects and aid in the design of monitoring programs, and the calibration of predictive models. Incorporating geomechanical impacts on seismic attribute predictions will increase the level of sophistication of fluid flow models and may improve the predictive capabilities of such model. Future work will involve the analysis of seismic attribute prediction using a full study of material, geometric and fault properties in the existing model, and extension of this work to a full field North Sea depleted field model, using the initial work as a benchmark.

Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K.

The area around the town of Northwich in Cheshire, U. K., has a long history of catastrophic ground subsidence caused by a combination of natural dissolution and collapsing abandoned mine workings within the underlying Triassic halite bedrock geology. In the village of Marston, the Trent and Mersey Canal crosses several abandoned salt mine workings and previously subsiding areas, the canal being breached by a catastrophic subsidence event in 1953. This canal section is the focus of a long-term monitoring study by conventional geotechnical topographic and microgravity surveys. Results of 20 years of topographic time-lapse surveys indicate specific areas of local subsidence that could not be predicted by available site and mine abandonment plan and shaft data. Subsidence has subsequently necessitated four phases of temporary canal bank remediation. Ten years of microgravity time-lapse data have recorded major deepening negative anomalies in specific sections that correlate with topographic data. Gravity 2D modeling using available site data found upwardly propagating voids, and associated collapse material produced a good match with observed microgravity data. Intrusive investigations have confirmed a void at the major anomaly. The advantages of undertaking such long-term studies for near-surface geophysicists, geotechnical engineers, and researchers working in other application areas are discussed.

Time-lapse seismic surveying: a multi-disciplinary tool for reservoir management on Snorre

Today reservoir monitoring is being used for increased oil recovery in about 75% of the fields operated by Statoil. The Snorre Field is a good example of how time-lapse seismic surveys help understanding and drive reservoir management. Even after more than 14 years of time-lapse seismic surveys, interpreting the 4D results is challenging due to the complexity of the reservoir and the drainage strategy. The time interval between seismic surveys is much larger than the water and gas injection cycles, and these variations in production operations between surveys make it difficult to reconstruct the reservoir history, including the inversion of the time-lapse data to estimate pressure and saturation changes. Using time-lapse seismic surveying as a tool to improve the static and dynamic models is important, but understanding the meaning of the 4D signal together with the uncertainties is critical. Here we describe how the asset works with the 4D data and show how time-lapse seismic data are used qualitatively.