Surface-Gravity Monitoring of the Gas Cap Water Injection Project, Prudhoe Bay, Alaska
- Jerry L. Brady (BP Exploration Inc.) | John F. Ferguson (U. of Texas at Dallas) | John E. Seibert (Seibert & Assocs.) | Tianyou Chen (U. of Texas at Dallas) | Jennifer L. Hare (Zonge Engineering) | Carlos V.L. Aiken (U. of Texas at Dallas) | Fred J. Klopping (Micro-g Solutions Inc.) | John M. Brown (Micro-g Solutions Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2004
- Document Type
- Journal Paper
- 59 - 67
- 2004. Society of Petroleum Engineers
- 5.4.1 Waterflooding, 1.6 Drilling Operations, 5.4.2 Gas Injection Methods, 5.1.1 Exploration, Development, Structural Geology, 5.5 Reservoir Simulation, 1.9.4 Survey Tools, 4.3.4 Scale, 4.2 Pipelines, Flowlines and Risers, 3.3 Well & Reservoir Surveillance and Monitoring, 6.5.2 Water use, produced water discharge and disposal
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Modeling and field-test surveys indicate that time-differenced surface gravity can be used to successfully monitor the Gas Cap Water Injection (GCWI) Project at Prudhoe Bay, Alaska. Simulation results have shown that more than 95% of the injected water can be accounted for, including reasonable assumptions concerning the noise level in the measurements. The flood front can be reliably detected within 2,000 to 3,000 ft for the Prudhoe Bay reservoir that is buried at 8,200 to 8,800 ft true vertical depth (TVD).
The world's first 4D 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 Water Injection Project (GCWI) at Prudhoe Bay. A major factor in the approval of the waterflood was to show that water movement could be monitored economically with a very limited number of wells. Conventional monitoring techniques would have been cost-prohibitive because of the requirement for drilling numerous new surveillance wells. Modeling studies indicate that density changes associated with water replacing gas can be detected with 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 were 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 more 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 2,000 to 3,000 ft for the Prudhoe Bay reservoir that is buried at 8,200 to 8,800 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 µ Gal and GPS=±1 cm elevation; a µ Gal is approximately one-billionth of the Earth's normal gravity).
This paper discusses the use of surface-gravity measurements as a reservoir surveillance technique, specifically to monitor the gas-cap water injection in the Prudhoe Bay oil field. The fundamental problem of monitoring the gas-cap water-injection project is the sparsity of monitoring wells and the lack of producing wells in the gas-cap area of Prudhoe Bay. The sparsity of wells results from the gas cap being structurally offset to the north from the major oil-producing areas of the field and from the lack of a gas pipeline to market the gas from the North Slope reservoirs. The produced gas is reinjected to aid in reservoir-pressure maintenance. The consequence is that in the gas cap area, very few wells are drilled that can be used for water monitoring. Distances between some monitoring wells are greater than 10,000 ft (3048 m), and it will require years for the injected water to propagate to these distances. Too few wells exist to adequately monitor the water movement using conventional downhole logging techniques. To address this problem, the Prudhoe Bay surveillance program will use 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, provided that there is a sufficient density contrast between the feature of interest and the surrounding rock. The surface-gravity technique can be applied to any field depending on the reservoir thickness and 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 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 either by measurement or by modeling and that noise caused by the measurement process or unaccounted density changes has tolerable characteristics.
The GCWI at Prudhoe Bay will result in an increasing positive gravity anomaly because of the added mass over time caused by water replacing gas in the pore space. Density variations from local geology and topography that do not change with time are effectively canceled when gravity data from different time epochs are differenced. The time-differenced, or 4D, gravity signal is then inverted to obtain a reservoir density model.1 This change in reservoir density represents the waterflood progression.
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