Time-lapse Gravity Research Articles

Aquistore Project Measurement, Monitoring, and Verification: From Concept to CO2 Injection

Abstract As an independent research and monitoring project, Aquistore intends to demonstrate that storing liquid carbon dioxide (CO 2 ) deep underground (in a brine and sandstone water formation), is a safe, workable solution to reduce greenhouse gases (GHGs). Managed by the Petroleum Technology Research Centre, Aquistore is built upon the learnings of the IEAGHG Weyburn-Midale CO 2 Monitoring and Storage Project, where over 22MT of CO 2 have been stored in an oil field during EOR operations. As a global leader, PTRC has over a decade of experience in CO 2 storage and monitoring work. Aquistore is Canada's first dedicated CO 2 storage project, and is an integral component of SaskPower's Boundary Dam Integrated Carbon Capture and Storage (CCS) Demonstration project - the world's first fully integrated CCS demonstration project from a coal-fired power plant. As an unique CCS project, Aquistore is providing buffer storage to this commercial CO 2 capture plant and active oilfield EOR operations. In collaboration with SaskPower, Aquistore will be the first integrated project globally to demonstrate deep saline CO 2 capture, transport, and storage on a commercial scale from a coal fired power plant. CO 2 will be captured at unit 3 of the Boundary Dam power- station (BD3), transported via underground pipeline to the Aquistore site, and injected to a depth of 3.4 km. The Winnipeg and Deadwood formations, which constitute the deepest units within the Williston Basin and were chosen as the target zone for CO 2 injection. These geological formations have much greater capacity for storing CO 2 than any oil reservoir in western Canada. The suitability of the storage complex was investigated through 3D characterization using high-resolution 3D seismic images and data from the injection and observation wells. These data show that: there are no significant faults in the immediate area of the storage site; the regional sealing formation is continuous in the area; and lastly, that the reservoir is not adversely affected by knolls on the surface of the underlying Precambrian basement. In the Aquistore project PTRC has brought together internationally recognized expertise and diverse interests. Research in monitoring methods is central to the Aquistore Project. A permanent areal seismic monitoring array has been deployed which comprises 650 geophones installed at 20 m depth on a 2.5x2.5 km regular grid. The objective of this array is to test “sparse array” seismic imaging and to provide continuous passive monitoring. This array is being utilized in conjunction and comparison with time-lapse 3D seismic imaging, continuous microseismic monitoring, vertical seismic profiling, cross-well seismic tomography, and broadband seismographs. Injection-related surface deformation monitoring is provided by InSAR analysis in conjunction with a network of tiltmeters and GPS stations. Electromagnetic surveys and gravimeters are also being used. In an effort to minimize surface impact these monitoring technologies - where possible - have been collocated and solar-powered. Surface electromagnetic methods and time-lapse gravity monitoring will be tested in conjunction with deployment of a fibre-optic distributed acoustic sensing (DAS) line. A fibre-optic distributed temperature sensing (DTS) line is also installed. A unique fluid recovery system (FRS) as well as casing conveyed pressure and temperature gauges will contribute to the project's down-hole monitoring techniques. Aquistore's MMV program will test and develop effective methods for monitoring CO 2 storage sites and ensure conformance of the storage process through continuous monitoring. This program is seeking to minimize the risk associated with any potential leakage of CO 2 from the storage reservoir through early detection. Aquistore's MMV program has included an added focus on integrated monitoring methods such as the inclusion of non-seismic methods and constraints from reservoir flow simulations. Aquistore will provide an efficient, cost effective, and field-tested basis for designing effective MMV programs for other similar projects worldwide.

Time-lapse gravity inversion with an active time constraint

Traditional methods used to invert gravity data are generally based on smoothness as a regularizer and the time-lapse inversion of gravity data is traditionally based on sequential inversion. A smooth density model can be in contradiction with the known geology of the target and sequential time-lapse inversion may lead to the presence of artefacts in the sequence of tomograms due to the propagation of errors from the initial dataset and its associated tomogram. We propose a deterministic time-lapse algorithm to invert a sequence of gravity data combining two features: an image focusing technique and the use of a time-dependent regularizer using an Active Time Constrained (ATC) approach. These two features are included directly in the objective function to minimize. The ATC inversion of time-lapse gravity data is efficient in filtering out noise-contaminated data as long as the noise is not correlated over time. Our approach can also be used to incorporate prior information regarding the density model we want to retrieve. The forward solver is based on a commercial finite element solver with a high flexibility in meshing irregular domains, a feature that is important to include realistic topography from digital elevation maps, for instance, and to describe the density distribution of geological structures with complex geometries. We benchmark the accuracy of the forward modelling code using an analytical expression and test the effectiveness of the focusing algorithm. We show the advantage of our approach in the case of the water flooding of an oil reservoir in order to detect and monitor the position of the oil–water encroachment front. We also test the model against synthetic data describing the evolution of a hydrothermal system feed by a magmatic source and the collapse of a mine. In all these cases, the approach we follow is successful in monitoring density changes.

Gravity effect of water storage changes in a weathered hard-rock aquifer in West Africa: results from joint absolute gravity, hydrological monitoring and geophysical prospection

Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology inAfrica) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (θMRS) known to be higher than the specific yield. Hence, we scaled the 2-D model of θMRS with a single factor (α). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of α is obtained with aMonte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to α = Sy/θMRS = 0.63 ± 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.

Continuous Gravity Monitoring for CO2 Geo-sequestration

Our purpose is development of a monitoring method for both lowering costs and increasing the safety in CO2 sequestration by complementing standard seismic survey with various other geophysical techniques, especially gravity monitoring. In many cases reservoirs are relatively thin and deep, resulting in subtle time-lapse gravity signals at the earth's surface. These signals must be separated from gravity signals generated by near-surface hydrological sources, in which case it would be effective to monitor continuously with a superconducting gravimeter. We have started gravity monitoring at a CO2 sequestration field in Utah in collaboration with the US project of Southwest Regional Partnership.

Time-Lapse Gravity for Reservoir Monitoring: Feasibility and Potential Applications

We demonstrate a robust workflow for time-lapse gravity modeling in reservoir sequestration/production monitoring applications. This systematic approach outlines a reliable methodology to understanding the value and limitations of 4D gravity at a particular site, for both pre-acquisition decision making, and as a guide for post-data acquisition interpretation. To demonstrate, we present a multi-faceted feasibility study for monitoring CO2 injection into a reservoir at various injection times using 4D micro-gravity method. The simulations are performed for a currently active CO2-EOR site, the Louisiana Delhi Field in the United State. We construct an accurate representation of the field directly from current seismic data, followed by application of binary inversion technology adapted to the time-lapse gravity problem and tailored to the specific site. Finally, we illustrate a method of resolution analysis to demonstrate the decreased recoverability of fluid movement at the site in the presence of varying data noise.

Feasibility of time-lapse gravity monitoring of producing gas fields in the Northern Carnarvon Basin, Australia

Aquifer influx and pressure depletion are key variables during the production and development of a natural gas field. To obtain an understanding of how aquifer influx and pressure depletion varies across the reservoir, remote geophysical monitoring techniques are commonly used, particularly in offshore environments where well data is geographically sparse. The seafloor time-lapse gravity technique is a candidate technique for remote reservoir monitoring of water influx into producing gas fields in the Northern Carnarvon Basin. We have developed a method to quickly assess the sensitivity of time lapse gravity measurements to water influx or pressure depletion using a vertical cylinder model for gas reservoirs. In strong water-drive gas reservoirs, a field-wide height change in the gas-water contact greater than 5m may produce a detectable gravity response depending on the reservoir depth and rock quality. In depletion-drive gas reservoirs, large pressure changes between 6MPa (~870psia) throughout the reservoir can produce a detectable response. Applying this technique to Carnarvon Basin gas fields where the primary reservoir is the Mungaroo Formation suggests that gravity monitoring of production related changes may be feasible but needs to be assessed on a field-by-field basis. The method employed is both flexible and practical. It can be used in a range of applications, and provides a quick assessment of the feasibility of time-lapse monitoring of subsurface density changes.

4D gravity response of compacting reservoirs: Analytical approach

Time-lapse gravity is a technique sensitive to subsurface change in mass and in mass distribution. We attempted to devise a method to predict gravity effects caused by redistribution of subsurface mass induced by reservoir compaction. First, displacements and strains due to compaction were modeled using a geomechanical model. Then, 4D gravity effects were derived from the displacements and the volumetric strains computed in and around the reservoir. A sensitivity study was carried out for geomechanical parameters, such as Poisson’s ratio, as well as for geometrical parameters, such as reservoir radius and depth. Finally, changes in gravity due to compaction were compared to changes induced by reservoir fluid substitution. Given a rigid basement close below a strongly compacting reservoir, our modeling case showed that the deformation itself could give gravity changes comparable to changes caused by reservoir fluid density changes, which are the changes traditionally targeted by gravity monitoring. For a homogeneous subsurface, the compaction gave negligible gravity changes for our modeling case.

Time-lapse gravity: A numerical demonstration using robust inversion and joint interpretation of 4D surface and borehole data

There are a number of ongoing developments in the 4D gravity method for time-lapse production and sequestration problems. Complex model construction is an essential component for meaningful feasibility studies and data interpretation in 4D. In the case of oil reservoirs, for example, the 4D gravity method must deal with very small density contrasts at depth, and overly simplistic model representations of a field site may not properly guide monitoring efforts for effective reservoir management decisions. The tracking of injected fluid through inversion can be significantly improved with the joint interpretation of surface and borehole gravity data. This is particularly relevant for time-lapse gravity problems where subtle changes in density contrast at depth may not produce surface data that alone contain the necessary information to recover the boundaries of fluid movement. Complex model construction and robust inversion show that time-lapse gravity surveys may contribute to improved production efficiency and reservoir management in-between the more traditional, and expensive 4D seismic surveys. Results hint that the true value of time-lapse gravity as an additional tool for production efficiency and reservoir management may be greatly undervalued. Data simulated within current and ongoing sensor technologies, combined with advances in computing power and robust inversion, can extract much more meaningful information about fluid movement in reservoirs that are geometrically complex and relatively thin and deep. To illustrate these points, simulations are performed using a representation of the published Jotun Field in the Norwegian North Sea, a well-studied site demonstrating successful application of the time-lapse seismic method, and reconstructed for 4D gravity understanding.

Understanding the applications and limitations of time-lapse gravity for reservoir monitoring

There is a strong need in our industry for advancing the understanding and applications of multiple geophysical techniques for time-lapse problems, such as for [Formula: see text]S sequestration and enhanced oil recovery (EOR) projects. Currently, monitoring efforts for such activities are dominated largely by 4D seismic, and there are numerous developments and successful applications throughout the geophysical literature to justify this state of affair (e.g., Lumley, 2010). The primary advantage of 4D seismic for these endeavors is that it can provide a high-resolution imaging tool that allows better reservoir management decisions and improved production efficiency.

4D gravity modeling: Integrating seismic data with highly constrained gravity inversions for effective reservoir monitoring

We demonstrate a robust workflow for time-lapse gravity modeling in reservoir sequestration/production monitoring applications. This systematic approach outlines a reliable methodology to understanding the value and limitations of 4D gravity at a particular site, for both pre-acquisition decision making, and as a guide for post-data acquisition interpretation. To demonstrate, we present a multifaceted feasibility study for monitoring CO2 injection into a reservoir at various injection times using 4D micro-gravity method. The simulations are performed for a currently active CO2-EOR site, the Louisiana Delhi Field in the United State. We construct an accurate representation of the field directly from current seismic data, followed by application of binary inversion technology adapted to the time-lapse gravity problem and tailored to the specific site. Finally, we illustrate a method of resolution analysis to demonstrate the decreased recoverability of fluid movement at the site in the presence of varying data noise.

Measuring gravity change caused by water storage variations: Performance assessment under controlled conditions

Summary Subsurface water content is an important state variable in hydrological systems. Established methods to measure subsurface water content have a small support scale which causes scaling problems in many applications. Time-lapse relative gravimetry can give an integrated measure of soil water storage changes over tens to hundreds of cubic meters. The use of time-lapse gravimetry in hydrology has until recent years been limited by the large efforts required to obtain precise and accurate gravity data at the 1 μGal (10 −8 ms −2 ) scale. A typical modern relative gravimeter, the Scintrex CG-5, has a sensitivity of 1 μGal, corresponding to a layer of 0.024 m of water in an infinitely extended horizontal sheet. For gravity surveys using relative gravity meters, the precision is highly dependent on the methods used to operate the gravimeter in the field. Systematic errors, which are difficult to detect, can lead to a loss of accuracy. As a performance test of a CG-5 for applications of time-lapse gravity in hydrology, we have measured the change in water storage in an indoor basin. The experiment was designed to resemble a field application, e.g. a pumping test, a forced infiltration experiment or alluvial aquifer storage change along intermittent rivers, so that the results can be applied to field experiments. The use of a 20 m by 30 m rectangular basin with a known water volume resulted in complete control over the instrument accuracy. Precisions of 3 μGal and accuracies of

Post-eruptive gravity changes from 1999 to 2004 at Unzen volcano (Japan): A window into shallow aquifer and hydrothermal dynamics

We report on results from post-eruptive time-lapse gravity investigations performed between 1999 and 2004 at Unzen volcanic complex, Shimabara Peninsula, Kyushu Island, southwest Japan. One of the most active volcanoes in Japan, Unzen saw its most recent eruption between 1990 and 1995. Here we demonstrate that annual residual gravity variations reflect subsurface mass changes most likely coupled to changes in shallow aquifer levels on the order of several meters. We find a positive correlation between rainfall and gravity data with a 3 to 4 month lag time. We interpret this link as a delayed aquifer response to local rainfall and show that high permeability rocks of the Unzen complex are indicative for low aquifer storativity, which in turn results in substantial water table fluctuations upon deviation from average rainfall budgets. While aquifer dynamics may represent a first order explanation to the observed gravity data, hydrothermal fluid circulation in the feeder system of the recent eruptive episode is also likely to contribute to the gravity signals by deteriorating the signal-to-noise ratio compared to hydrothermally quieter volcanoes. This study shows that shallow hydrological processes may dominate temporal changes in the gravity field after dome-building eruptions identifying a clear need for a better understanding of the interaction between magmatic and hydrological systems at arc volcanoes.

Hybrid optimization for lithologic inversion and time-lapse monitoring using a binary formulation

We have developed a hybrid optimization algorithm for binary inversion of geophysical data associated with lithologic inversion and time-lapse monitoring. The binary condition is designed to invert geophysical data for well-defined physical properties within discrete lithologic units, such as a salt body within a sedimentary host, or temporal changes in physical property associated with dynamic processes, such as the density and conductivity change in an oil and gas reservoir or groundwater aquifer. The solution of such inverse problems with discrete model values requires specialized optimization algorithms. To meet this need, we develop a hybrid optimization algorithm by combining a genetic algorithm with quenched simulated annealing. The former allows for easy incorporation of prior geologic information and rapid buildup of large-scale model features, whereas the latter guides genetic algorithm to faster evolution by rapidly adjusting the finer model features. In examining the performance of the hybrid algorithm, we note its superior performance. Our investigations of the algorithm’s capability with a 3D gravity inversion for the SEG/EAGE salt model and a time-lapse gravity data set from an aquifer storage and recovery process reveal its benefits.

Time-lapse gravity monitoring: A systematic 4D approach with application to aquifer storage and recovery

We studied time-lapse gravity surveys applied to the monitoring of an artificial aquifer storage and recovery (ASR) system in Leyden, Colorado. An abandoned underground coal mine has been developed into a subsurface water reservoir. Water from surface sources is injected into the artificial aquifer during winter for retrieval and use in summer. As a key component in the geophysical monitoring of the artificial ASR system, three microgravity surveys were conducted over the course of ten months during the initial water-injection stage. The time-lapse microgravity surveys successfully detected the distribution of injected water as well as its general movement. Quantitative interpretation based on 3D inversions produced hydrologically meaningful density-contrast models and imaged major zones of water distribution. The site formed an ideal natural laboratory for investigating various aspects of time-lapse gravity methodology. Through this application, we have studied systematically all steps of the method, including survey design, data acquisition, processing, and quantitative interpretation.

Long-term seafloor tidal gravity and pressure observations in the North Sea: Testing and validation of a theoretical tidal model

Seafloor gravity and pressure measurements for 4D reservoir monitoring require precise models of the time-varying tidal signals. Current seafloor instrumentation can resolve [Formula: see text] in time-lapse gravity differences and [Formula: see text] [Formula: see text] in pressure. To verify model accuracy, a seafloor gravimeter and pressure gauge were operated continuously for [Formula: see text] next to the Troll A gas platform in the North Sea (60.64227° north, 3.72417° east) at a depth of [Formula: see text]. The seafloor gravity and pressure time series were filtered and corrected with estimates from the tidal model, which predicts the solid earth tide, ocean loading, and direct gravitational attraction of the varying water level. The rms difference between the observed tidal gravity signal and the prediction is about [Formula: see text] during periods when there are no surface storms. A slight difference is observed for the direct attraction of the water overhead as computed from the tidal prediction versus that computed from direct seafloor pressure measurements when the entire [Formula: see text] record is analyzed; it shows an rms difference of [Formula: see text], equivalent to [Formula: see text] of water-height variation, yielding a gravity effect of [Formula: see text]. We conclude that existing theoretical tide models in combination with in situ pressure records are sufficiently precise for correcting time-lapse gravity observations.

Precision of seafloor gravity and pressure measurements for reservoir monitoring

Changes with gravity over time have proven to be valuable for inferring subsurface density changes associated with production from oil and natural gas reservoirs. Such inferences allow the monitoring of moving fluid fronts in a reservoir and provide an opportunity to optimize production over the life of the reservoir. Our group began making time-lapse seafloor gravity and pressure measurements in 1998. To date, we have surveyed six fields offshore Norway; we have made three repeat surveys at one field and one repeat survey at another. We incorporated a land-gravity sensor into a remotely operated seafloor housing. Three such relative gravity sensors mounted in a single frame are carried by a remotely operated vehicle (ROV) to concrete benchmarks permanently placed on the seafloor. Reference benchmarks sited outside the reservoir boundaries are assumed to provide stable fiducial points. Typical surveys last from a few days to a few weeks and cover from 8 to 80 benchmarks, with multiple observations of each. In our earliest surveys, we obtained an intrasurvey repeatability of approximately [Formula: see text], but recently we have been achieving [Formula: see text] repeatability in gravity and approximately [Formula: see text] in benchmark depth (deduced from simultaneously recorded ambient seawater pressure). We attribute the improved precision to several operational factors, including the use of multiple gravity sensors, frequent benchmark reoccupation, precise relocation and orientation of the sensors, repeated calibrations on land, and minimization of vibrational and thermal perturbations to the sensors. We believe that high-precision time-lapse gravity monitoring can be used to track changes in the height of a gas-water contact in a flooded reservoir, with a precision of a few meters.

Gravimetric monitoring of gas production from the Troll field

Four surveys of relative gravity and depth measurements have been conducted over the Troll field since gas production began in 1996. Precision in gravity (intrasurvey repeatability measured as standard deviation) has improved from [Formula: see text] in two surveys in 2002 and 2005. The scatter of 74 station depth differences (standard deviation) has improved to [Formula: see text] (with modeled subsidence removed) for the same two surveys. Subsidence of up to [Formula: see text] is observed above the thickest reservoir of Troll East between 2002 and 2005. Time-lapse gravity shows a general slight average increase in Troll East (95% confidence for the interval centered at [Formula: see text]), which is likely caused by edgewater influx. One gravity station close to Troll A shows a [Formula: see text] gravity increase, indicating [Formula: see text] water rise. Repeated logging in a nearby well indicates [Formula: see text] rise. These two observations agree within uncertainty bounds. Gravity decrease in Troll West is explained by oil production and a downward-moving gas-oil contact. Gravity increases in two areas in the easternmost part appear statistically significant and are likely to have been caused by edgewater influx. This has not been confirmed by other data, but aquifers located near these areas make the results plausible. The gravity and subsidence data give estimates of total mass influx and pore compaction; these data have been used in and agree with material-balance calculations.

4D Microgravity Surveillance of a Waterflood - Prudhoe Bay, Alaska

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 101762, "Results of the World's First 4D Microgravity Surveillance of a Waterflood - Prudhoe Bay, Alaska," by J.L. Brady, SPE, BP Exploration Alaska; J.L. Hare, Zonge Engineering; J.F. Ferguson, U. of Texas at Dallas; J.E. Seibert, Seibert & Assocs.; and F.J. Klopping, T. Chen, and T. Niebauer, Micro-g Lacoste, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. A 4D surface-gravity surveillance of a waterflood was implemented at Prudhoe Bay, Alaska. This monitoring technique is essential for surveillance of the gas-cap water-injection (GCWI) project. Drilling numerous surveillance wells to monitor water movement adequately would have been cost-prohibitive. Field surveys showed conclusively that density changes associated with water replacing gas are readily detected by use of high-resolution surface-gravity measurements. Introduction The fundamental problem in monitoring the GCWI project is sparcity of monitoring wells and lack of producing wells in the gas-cap area of Prudhoe Bay. Distances between some monitoring wells are greater than 10,000 ft, and years would be required for the injected water to propagate these distances. Too few wells exist to monitor the water movement adequately with conventional downhole-logging techniques. Therefore, the Prudhoe Bay surveillance program uses a combination of conventional downhole logging in existing wells and 4D surface-gravity monitoring. The major monitoring concern with the waterflood is to ensure that water added in the gas cap does not prematurely flow downdip into the oil-producing portions of the field, where it could interfere with a highly efficient gravity-drainage mechanism. Surface-gravity instruments measure the Earth's gravitational field at a specific point, or station. With an array of these measurements, local structural traps, stratigraphic traps, or fluid movement can be identified if sufficient density contrast exists between the feature of interest and the surrounding rock. The surface-gravity technique can be applied to any field, depending on reservoir thickness, size, depth of burial, porosity, and density contrast between the fluids. The surface-gravity technique requires that several time-lapse gravity surveys be made over the life of the field. The first survey should be performed before any change occurs in the fluid volumes to obtain baseline data. The baseline survey can be subtracted from future gravity surveys to obtain the gravity anomaly associated with the change in fluid volumes. The technique assumes that any other time-dependent gravity changes can be accounted for by measurement or modeling, and that noise caused by the measurement process and unmodeled (near-surface) density changes has tolerable characteristics.

Constraints on the in situ density of CO 2 within the Utsira formation from time-lapse seafloor gravity measurements

At Sleipner, CO 2 is being separated from natural gas and injected into an underground saline aquifer for environmental purposes. Uncertainty in the aquifer temperature leads to uncertainty in the in situ density of CO 2. In this study, gravity measurements were made over the injection site in 2002 and 2005 on top of 30 concrete benchmarks on the seafloor in order to constrain the in situ CO 2 density. The gravity measurements have a repeatability of 4.3 μGal for 2003 and 3.5 μGal for 2005. The resulting time-lapse uncertainty is 5.3 μGal. Unexpected benchmark motions due to local sediment scouring contribute to the uncertainty. Forward gravity models are calculated based on both 3D seismic data and reservoir simulation models. The time-lapse gravity observations best fit a high temperature forward model based on the time-lapse 3D seismics, suggesting that the average in situ CO 2 density is about to 530 kg/m 3. Uncertainty in determining the average density is estimated to be ±65 kg/m 3 (95% confidence), however, this does not include uncertainties in the modeling. Additional seismic surveys and future gravity measurements will put better constraints on the CO 2 density and continue to map out the CO 2 flow.

4D reservoir volumetrics: a case study over the Izaute gas storage facility

Dr Duncan Bate (ARK Geophysics) presents a recent oil company sponsored case study in France pointing to the value of 4D microgravity modelling as a cost-effective and unintrusive way of enhancing reservoir information. Measuring the Earth’s gravitational field as a means to mapping subsurface density distributions has been an exploration technique for decades. Gravitational anomalies may be related to subsurface geological structures and thus indicate features including faults, intrusives, and salt structures. High spatial resolution gravity data (as acquired with 3D seismic surveys) may be used in combination with 3D modelling to help define end member salt volumes in seismic imaging processes such as pre-stack depth migration (PSDM). The measurement of the time varying (4D) gravity field as a method of observing subsurface fluid flow in a hydrocarbon reservoir is a more recent application of gravity data. ARK Geophysics (ARK) conducted a time-lapse microgravity field trial over the Izaute gas storage reservoir in the Gascoigne district of South-Western France approximately 60 km north of Pau, between January 2003 and June 2004. The reservoir is operated by Total Infrastructure Gaz France (TIGF). The project was coordinated by ITF and funded by Statoil, Shell, and Total. The reservoir is a domestic gas storage buffer – filled throughout the summer months and depleted during the winter. The objective of this trial was to investigate whether time changes in the observed gravity field could practically be measured and if so, whether the observations correlated to known changes in gas stock levels. Twelve repeat gravity surveys were performed (each comprising over 200 stations) and two continuous gravity meters were employed. Overall the measured time-lapse gravity data correlate well with the predicted gravity changes from the reservoir model. Any observed differences between the predicted gravitational response of the reservoir model and the measured time varying gravity field requires further investigation. Sources of such observed discrepancies include system noise but also geological information that may not have been captured in the reservoir model such as reservoir compartmentalization, porosity, and permeability differentials as well as unmapped thief zones. The continuous data recorded over the reservoir show a good correlation with the reservoir gas pressure data. The aim of this study was to assess whether the time varying gravity field generated by a propagating density interface (e.g. oil-water or gas-water contact) can be measured by currently available cost-effective instrumentation. A two-fold approach was employed, utilizing time-lapse (or dynamic) and continuous gravity acquisition techniques. A dynamic gravity project comprises repeat static gravity surveys and is sometimes referred to as time-lapse or 4D. Workers in this field have successfully applied this technique to hydrothermal energy exploration (Hallinan et al., 1989; Allis et al., 1986) and hazard monitoring (Rymer & Brown, 1986; Yokoyama, 1989). A continuous gravity survey involves recording the variations in gravity continuously over a period of time. This technique has previously been used for long term studies of earth tides and volcano monitoring. There are several designs of gravity meter that can measure to the required accuracy for this project. The LaCoste and Romberg G and D meters with Aliod electrostatic feedback nulling system were used. This system has a range of approximately 100 mgal (+/-50) with a resolution of 0.01, or 0.001 mgal depending on which version is fitted. The Izaute, and its neighbouring Lussagnet, gas storage facilities have been created from porous submolassic sand reservoirs in the upper section of a vast multi-layer reservoir that contains the ‘inframolassic aquifer’ stretching from the Pyrenees to north of Bordeaux. The Izaute reservoir is formed by a gently plunging anticline, with an overburden of approximately 500 m. The reservoir thickness ranges from 50 to 80 m, has a porosity of between 25% and 35% and a permeability of between 6 and 20 Darcys. Gas is injected in the summer months (May to October) and recovered in the winter months (November to April). The total seasonal variation in the depth to the gas-water contact is 24 m.