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AAPG Bulletin, V.
Time-dependent subsidence associated with drainage-induced compaction in Gulf of Mexico shales bounding a severely depleted gas reservoir
1Department of Geology, Chungnam National University, Daejeon 305-764,
2Department of Geophysics, Stanford University, Stanford, California 94305; present address: BP, 1-4 Wellheads Avenue, Dyce, Aberdeen AB21 7PB, UK; [email protected]
3Department of Geophysics, Stanford University, Stanford, California 94305; [email protected]
Production from ubiquitous oil and gas fields in coastal Louisiana and consequent reservoir compaction has been proposed as an important process contributing to coastal subsidence and land loss in this region. As revealed by three consecutive leveling surveys (in 1965, 1982, and 1993), an unexpected aspect of the subsidence is that the rate of subsidence actually increased after the cessation of production. To explain the accelerated postdepletion subsidence, we propose a mechanism involving time-dependent drainage and compaction in the overlying and underlying shales after depletion. We show that the shale compaction is induced by slow drainage of pore fluid from the shale to the depleted reservoir. We estimate the significance of postdepletion compaction in the bounding shale using a relatively simple analytic model in which time-dependent shale compaction is driven by pore pressure diffusion with two sets of rheological constitutive equations: one accounting for poroelastic effects and one accounting for viscoplastic deformation of the shale matrix. Our modeling shows that despite its very low permeability, after about 10 years, vertical compaction due to pressure drainage in the shale exceeds that due to depletion and compaction of the sand reservoir. Consequently, the calculated subsidence rate due to the shale compaction is higher than the subsidence induced by reservoir depletion, thus demonstrating that postdepletion compaction in the reservoir-surrounding shale may explain the observed acceleration of subsidence after depletion.
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