Results of the World's First 4D Microgravity Surveillance of a Waterflood—Prudhoe Bay, Alaska

Abstract

Abstract 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.