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

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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.