Compaction Localization and Constitutive Behavior of Weak Porous Sandstone
A combined experimental and constitutive modeling program for weak porous sandstone deformation is described. A series of axisymmetric compression tests were performed over a range of mean stresses to study dilatational, compactional and transitional regimes. Experimental results were used both to derive constitutive parameters for testing localization theory and to parameterize a poroelastic-plastic model. Observed strain localization, imaged syn-deformationally using acoustic emissions, includes high- and low-angle shear and low angle compactional features or “bands”. Isotropic elastic moduli measured via unloading loops show a progressive degradation pre-failure as decreasing functions of work-conjugate plastic strains and increasing functions of stress magnitude. The degradation pathway is unique for samples which underwent localization versus those that underwent spatially pervasive pore collapse.
Total shear and volume strains are partitioned into elastic and plastic portions including the “coupling” strain associated with modulus degradation. Plastic strain calculated with and without the coupling term is compared with regard to localization predictions. Both coupled and uncoupled cases predict high angle shear bands for uniaxial and low mean stress conditions on the dilatational side of the yield surface. Uncoupled predictions show progressively lower angle shear bands approaching the transitional regime (stress conditions approaching the “cap” surface). When elastic-plastic coupling is accounted for, compaction bands are predicted for the transitional regime, as are observed in the experiments.
Finite element modeling efforts are described using a 3-invariant, mixed-hardening, continuous yield surface, elasto-plasticity model that includes several features important for porous sandstone constitutive behavior and observed experimentally, including non-associativity, nonlinear elasticity, elastic-plastic coupling, and kinematic hardening. Modeled deformational behavior attending stress paths relevant for several reservoir production scenarios are described.
The authors gratefully acknowledge the U.S. Department of Energy Basic Energy Sciences Program and the National Science Foundation Tectonics Program (award EAR-0711346) for funding. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under contract DE-ACOC4-94AL85000.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009