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The 4D microgravity method is becoming a mature technology. A project to develop practical measurement and interpretation techniques was conducted at Prudhoe Bay, Alaska, from 1994 through 2002. Beginning in 2003 these techniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a [Formula: see text] area are reoccupied in each survey year with sub-[Formula: see text] precision absolute gravity and centimeter precision Global Positioning System (GPS) geodetic measurements. The 4D gravity measured over epochs 2005–2003, 2006–2003, and 2007–2003 has been successfully modeled to track the mass of water injected since late in 2002. A new and improved version of the A-10 field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology. The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There are substantial differences between a 4D absolute microgravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the 4D gravity signal in each epoch is less than [Formula: see text].

Summary The world's first 4D surface-gravity surveillance of a1 waterflood has been implemented at Prudhoe Bay, Alaska. This monitoring technique is an essential component of the surveillance program for the Gas Cap Water Injection (GCWI) project. A major factor in the approval process for the waterflood was to show that we could monitor water movement economically where a very limited number of wells penetrated the waterflood area. The drilling of numerous surveillence wells to monitor water movement adequately would have been cost-prohibitive. Field surveys now show conclusively that density changes associated with water replacing gas are being detected readily with high-resolution surface-gravity measurements. The gravity methods used to monitor the water-flood include time-lapse (4D) measurement of surface gravity over the reservoir followed by inversion of the 4D signal for mass-balance calculation and flood-front detection. This paper will focus on field results of time-lapse surface-gravity surveys. Differences in the gravity field over time reflect changes in the reservoir-fluid density. The inversion procedure was formulated and coded to allow for various constraints on model parameters such as density, total mass, and moment of inertia. The gravity survey was designed to permit the inversion for reservoir mass distribution, with resolution on the order of hundreds of meters in the presence of uncorrelated noise of reasonable magnitude (12-μGal standard deviation). Time-differenced gravity-survey results clearly show an increase in surface gravity that is a result of the injected-water mass. Density-change maps deduced from measured gravity change show that water movement is reasonably similar to the reservoir simulations and to the water detected in observation wells. The overall ultimate gravity signal is predicted to increase to approximately 250 μGal, ultimately resulting in accurate maps of the water movement.

Between 1994 and 2002, a series of experiments was conducted at Prudhoe Bay, Alaska, aimed at the development of an effective 4D (or time-lapse) gravity technique. Theoretical investigations had pointed out the potential for monitoring water injection in the [Formula: see text]-deep reservoir, but it was not clear that gravity measurements of sufficient accuracy could be made in the arctic environment. During the course of these experiments, new techniques and instrumentation were introduced and perfected for both gravity and position measurements. Gravity stations are located using high-precision global positioning system (GPS) techniques without permanent monuments. Robust methods for meter drift control have improved noise resistance in relative gravimeter surveys. Absolute gravity measurements with a field-portable instrument maintain absolute gravity levels among surveys. A 4D gravity-difference noise of [Formula: see text] standard deviation has been established at Prudhoe Bay for GPS-controlled relative gravimeter surveys. The lessons learned are now being applied to full-scale waterflood monitoring at Prudhoe Bay. The basic technique is applicable to microgravity surveys and 4D microgravity surveys for any purpose.

The world's first 4-D surface gravity surveillance of a waterflood has been implemented at Prudhoe Bay, Alaska. This monitoring technique is an essential component of the surveillance program for the Gas Cap Water Injection Project (GCWI). A major factor in the approval process for the waterflood was to show that we could economically monitor water movement where a very limited number of wells penetrated the waterflood area. The drilling of numerous surveilence wells to adequately monitor water movement would have been cost prohibitive. Field surveys now conclusively show that density changes associated with water replacing gas are being readily detected using high-resolution surface gravity measurements. The gravity methods used to monitor the waterflood include time-lapse (4-D) measurement of surver the reservoir followed by inversion of the 4-D signal for mass balance and flood front detection. This paper will focus on field results of time-lapse surface gravity surveys. Differences in the gravity field over time reflect changes in the reservoir fluid densities. The inversion procedure was formulated and coded to allow for various constraints on model parameters such as density, total mass, and moment of inertia. The gravity survey was designed to permit the inversion for reservoir mass distribution with resolution on the order of hundreds of meters in the presence of uncorrelated noise of reasonable magnitude (12 µGal standard deviation). Time differenced gravity survey results clearly show an increase in surface gravity that is a result of the injected water mass. Density-change maps deduced from measured gravity- change show that water movement is reasonably similar to the reservoir simulations and the water detected in observation wells. The overall ultimate gravity signal is predicted to increase to approximately 250 µGal ultimately resulting in accurate maps of the water movement.

The world's first 4-D surface gravity surveillance of a waterflood is being implemented at Prudhoe Bay, Alaska. This monitoring technique is an essential component of the surveillance program for the approved Gas Cap Waterflood Project (GCWI) at Prudhoe Bay. A major factor in the approval of the waterflood was to show that water movement could be economically monitored with a very limited number of wells. Conventional monitoring techniques would have been cost prohibitive due to the requirement for drilling numerous new surveillance wells. Modeling studies indicate that density changes associated with water replacing gas can be detected using high-resolution surface gravity measurements. Field tests at Prudhoe Bay have demonstrated that sufficiently accurate gravity data can be obtained. This paper will discuss both inverse modeling of time differenced gravity maps and four test surveys that have been completed to perfect the gravity measurement technique. Forward and inverse gravity modeling was carried out on a suite of reservoir simulations of the GCWI. Differences in the gravity field with time reflect changes in the reservoir densities, which in turn reflect pore fluid content. A constrained least squares method was used to invert synthetic gravity data for the subsurface density distribution. The modeling procedure has been formulated to allow testing for sensitivity to gravity sampling patterns, noise and various constraints on model parameters such as density, total mass, and moment of inertia. These simulation results have shown that greater than 95% of the injected water can be accounted for in the inverse model. These estimates include reasonable assumptions concerning the noise level in the measurements of both the gravity data and the location data using the Global Positioning System (GPS). The average flood front can be reliably detected within 2000 - 3000 ft. for the Prudhoe Bay reservoir that is buried at 8200 ft to 8800 ft tvd. Time differenced surface gravity and GPS data were gathered over the Arctic Ocean in typical winter conditions (-40 F) accurately enough to monitor the waterflood (gravity = ± 10 ìGals and GPS = ± 1 cm elevation, a íGal is about one billionth of the Earth's normal gravity).

The world’s first 4-D surface gravity surveillance of a waterflood is being implemented at Prudhoe Bay, Alaska. This monitoring technique is an essential component of the surveillance program for the Gas Cap Waterflood Project. A major factor in the approval process for the waterflood was to show that water movement could be economically monitored, with a very limited number of wells penetrating the waterflood area. Conventional monitoring techniques would have been cost prohibitive since they would have required the drilling of numerous surveillance wells. Modeling studies indicate that density changes associated with water replacing gas can be readily detected using high-resolution surface gravity measurements. Modeling of the gravity data determines the mass distribution, mass balance and flood front location. This paper will focus on how to perform an extremely accurate, high-resolution surface gravity survey in the Arctic. Details on survey methodology, meter testing and calibration will be covered. The use of gravity meters in combination with the NAVSTAR Global Positioning System (GPS) for location and elevation will be described. Field tests at Prudhoe Bay indicate that gravity survey accuracy of 5-10 µGal can be achieved using relative gravity meters in combination with GPS. Model results, using reservoir simulations, have indicated that gravity surveys with this accuracy should be able to account for greater than 95% of the injected water.

Typical geologic targets sought after in airborne gravity geophysical prospecting are in the 2 km – 20 km fullwavelength range. Current hardware configurations and well thought-out processing strategies can yield information for wavelengths as short as 9 km. Clearly there is a desire to obtain better resolution of both the amplitude and wavelength of the gravity signal. If airborne gravity provided resolution similar to that obtained in the marine environment, its use as a decision tool in exploration would be much improved. Table 1 depicts the differences in recoverable full wavelength resolution among land, marine and airborne gravimetry at the present time. Though airborne gradiometery seems to hold the most promise for breaking the 9 km wavelength barrier, until a practical gradiometer is designed, other methods to exploit and improve current state-of-the–art must be pursued. One such area for improvement is increasing the sampling rate of the gravity signal.

J.L. Brady, SPE, ARCO Alaska Inc., D.S. Wolcott, SPE, Stephen Dean Consulting, P. H. Daggett, ARCO Alaska, Inc., J. F. Ferguson, J. L. Hare, C.L.V. Aiken, and M. Balde, University of Texas at Dallas, J.E. Seibert, EDCON Inc., G. Mader, National Geodetic Survey Abstract A unique method to monitor gas cap water movement in an Arctic environment has been developed and tested. The novel surveillance technique for monitoring the water movement had to be developed given the very limited number of wells that penetrate the gas cap. Conventional fluid monitoring techniques require drilling numerous observation wells to adequately monitor water movement. Modeling studies indicate that density changes associated with water replacing gas can be detected using high-resolution surface gravity measurements. Modeling gravity effects of water movement mass distribution, mass balance and water front detection are discussed. A test of the gravity meter and essential high precision station positioning under typical Arctic winter conditions is evaluated using the Global Positioning System (GPS). Modeling results have shown the general shape of the water front can be detected using surface gravity with Gal precision. With high precision gravity measurements, greater than 90% of the increased water can be accounted for in the resulting gravity anomaly. These estimates include reasonable assumptions concerning the noise level in the measurements of both the gravity data and the location data using the Global Positioning System (GPS). The average water front can be reliably detected within a producing well spacing (approximately 2000 ft (610 m)). Surface gravity and GPS data were gathered over the Arctic Ocean in typical winter conditions (-44 F) accurately enough to monitor the water movement (surface gravity = 5 Gals: GPS = 1 cm elevation). In fact, this was the first high precision gravity survey using a GPS antenna as an integral part of the surface gravity meter housing. Introduction This study evaluates a novel surveillance technique to monitor water movement in the gas cap of a North Slope oil field. The novel surveillance technique uses surface gravity measurements to define water and gas movements. The primary problem with monitoring gas cap water movement using traditional techniques is the sparcity of monitor wells and the lack of producing wells in the gas cap. Since a gas pipeline has not been constructed to market the gas from the North Slope reservoirs, it is re-injected to aid in reservoir pressure maintenance. This results in very few wells in the gas cap area that can be used for water monitoring. Distance between some monitoring wells is greater than 10,000 ft (3048 m). The major concern for monitoring the water movement is to ensure that water added in the gas cap does not prematurely flow down dip into the oil producing portions of the field. Surface gravity instruments measure the Earth's gravitational field at a specific point. With a grid of these measurements local structural traps, stratigraphic traps or fluid movement can be identified from surrounding regional geology providing there is a sufficient density contrast. The surface gravity technique can be applied to any field depending upon the reservoir thickness and size, depth of burial, and the 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 prior to any change in the fluid volumes to obtain baseline data. Future gravity surveys can be subtracted from the baseline survey to obtain the gravity anomaly associated with the change in fluid volumes. The technique assumes that any other subsurface or near surface gravity anomalies changing with time fall within a tolerable noise level from which the fluid gravity signal can be extracted, or that they be accounted for through direct measurement or modeling. The large reservoir evaluated in this study is buried at approximately 8200 ft (2500 m) and has gas-water density contrast of 0.13 g/cc as the formation gas is replaced by water. P. 381