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4D microgravity method for waterflood surveillance: Part IV – Modeling and interpretation of early epoch 4D gravity surveys at Prudhoe Bay, Alaska

Publisher –
Society of Exploration Geophysicists. Geophysics, Vol. 73, No. 6, Nov. – Dec. 2008; Pgs. WA173–WA180.

Authors –
Jennifer L. Hare, Zonge Engineering & Research Organization, Inc., Tucson, Arizona, U.S.A.;
John F. Ferguson, The University of Texas at Dallas, Richardson, Texas, U.S.A.;
Jerry L. Brady, BP Exploration Alaska, Inc., Anchorage,Alaska, U.S.A.

Article – [pdf] O&G_PrudhoeIV

Between March 2003 and March 2007, four high-precision 4D absolute microgravity surveys were performed at Prudhoe Bay, Alaska. These surveys are part of an ongoing effort to monitor the progress of a very large water-injection project in the gas cap of the Prudhoe Bay reservoir at a depth of 2.5 km. These carefully acquired gravity data must be modeled and interpreted in terms of water movement within the reservoir. A constrained linear inversion scheme was tested on reservoir simulations during the planning and development phase of this project preinjection. The inversion methodology has been applied to
data for three epochs 2005–2003, 2006–2003, and 2007–2003, and mass-distribution models have been produced for the reservoir. The time evolution of the water-mass distribution in the reservoir is visualized from these three snapshot models. The waterflood is expanding into the gas cap at the expected rate but is exhibiting non-symmetric behavior that is consistent with a greater degree of structural control than expected. The waterflood seems to be restrained episodically and guided by fault barriers. These barriers are overcome and fault-bounded blocks filled with water in stages.