Stress Changes and Mechanisms Associated with Subsidence of Clastic Media: Sampling and Testing Field Behavior

Maurice B. Dusseault

Short-term subsidence associated with fluid withdrawal is the result of intrinsic rock properties and extrinsic factors that cause stress changes. The most troublesome materials are clays and high porosity sands. Commonly, elastic or elastoplastic numerical formulations are used to simulate stress changes, but the assumption of continuum behavior and constant parameters is fundamentally flawed. Stress changes cause changes in microfabric because in an uncemented clastic medium there is a relationship between the spatial distribution of grain contacts and the stress differential. This relationship in an ideal granular medium, shows an anisotropic fabric generated by loading.

Diagenesis alters this relationship as reorientation of grain contacts is constrained by cementation and grain interpenetration, leading to an "elastic" component of deformation. Sampling of a cohesionless sand destroys the grain contact fabric directly as the stresses are relieved. Furthermore, sampling generated expansion in low permeability cohesionless sandstones can totally disrupt diagenetic fabric, leading to loss of any anisotropic material behavior as well as reduction of stiffnesses by up to an order of magnitude. A superior sampling strategy can improve predictions based on lab data, thereby avoiding large-scale unforeseen subsidence problems.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.