Time-Dependent Subsidence Associated with Compaction in Reservoir-Bounding Shale
Subsidence in coastal Louisiana has been recognized as one of the major causes that result in regional land loss. Extensive leveling data indicate that fluid withdrawal from ubiquitous oil and gas fields in this region and consequent reservoir compaction can be a part of the reason responsible for the observed subsidence. An interesting aspect of the observed subsidence is its increased rates after reservoir depletion and cessation of production, as revealed by two consecutive leveling surveys (epoch 1: 1965-1982 and epoch 2: 1982-1993). In order to explain the accelerated post-depletion subsidence, we propose a possibility of compaction in the reservoir-bounding shale after depletion, which is induced by slow drainage of pore fluid from the shale to depleted reservoir. We estimate the significance of post-depletion compaction in the bounding shale using a simple analytic model that consists of sand reservoir bounded by shale formations. To describe time-dependent shale compaction driven by pore pressure diffusion, we use two sets of rheological constitutive equations (poroelastic and viscoplastic). The model result shows that despite low permeability, the amount of time-dependent compaction in the bounding shale associated with slow pore pressure diffusion can be comparable to that occurs in the depleted reservoir itself within a decade. The calculated subsidence rate due to the shale compaction is higher than the subsidence induced by reservoir depletion, which demonstrates that the post-depletion compaction mechanism in the bounding shale is a convincing source that results in the observed acceleration of subsidence after depletion.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013