Previous Talks

Wednesday, October 14th, 2009

Deformation of Saturated Wilcox Shale: Rate Dependence and Time Scale of Pore Pressure Perturbations

Andreas Kronenberg, Texas A&M University


Mechanical properties of clay-rich shales depend on readily measured conditions of temperature, stress state and rates of deformation, much as observed for deformation of other rock types. However, because fluid transport through the pore space of most shales is limited, mechanical properties may also depend on internal pore pressures that are not readily known. Pore pressures may become heterogenous with deformation, with perturbations in local pore pressure that are governed by compaction, dilatancy, and fluid transport properties.

The mechanical properties of fluid-saturated, illite-rich shale from the Wilcox formation have been investigated as a function of strain rate, effective pressure, and pore fluids that alter clay surface hydration. Triaxial deformation experiments were performed over six orders of magnitude in strain rate (from 10-2s-1 to 10-8s-1) in order to reveal the intrinsic rate dependence of shale failure and to examine effects of pore pressure perturbations. Based on measurements of permeability and one-dimensional modeling, these strain rates span time scales from effectively undrained conditions to fully drained response.

Both permeability and drained failure strengths of Wilcox shale follow simple effective pressure laws where effective pressure Pe = Pc - Pp; Pc and Pp are confining and pore pressures, respectively. Permeabilities k depend on Pe through a cubic law of the form

where k0 depends on orientation, clay content, and pore fluid composition, m is dependent only on orientation, and P1 appears to be similar for all orientations, clay contents, and fluid compositions. Failure strengths of Wilcox shale determined at strain rates sufficiently low (10-7s-1) for pore fluid drainage increase with increasing effective pressure Pe, consistent with a simple Mohr-Coulomb criterion.

Failure strengths determined at varying rates can be fit using an exponential law of the form

with only subtle indications of compaction softening at strain rates of 10-7s-1. Pore pressures of undrained Wilcox shale samples subjected to changing confining pressure follow confining pressure closely, with a Skempton's parameter B ~ 1. However, measured pore pressure generation due to differential stress (Skempton's parameter A) is comparatively small, and volume reductions early in triaxial compression experiments are nearly compensated by late-stage dilatancy near failure. As a result, even at time scales that do not allow pore fluid drainage from the sample, pore pressures at failure may not be large. The strength of Wilcox shale decreases with increasing adsorbed water at clay surfaces. Experimental results indicate that the low strength of fluid-saturated samples, relative to dry sample strengths, cannot be explained by elevated pore pressures alone. Clay mineral hydration may be responsible for reducing intergranular bonding, and decreasing cohesive and frictional forces during deformation.

Speaker Biography

Andreas Kronenberg is Professor and Head of the Department of Geology and Geophysics at Texas A&M University, and Associate Director of the Center for Tectonophysics. After receiving his PhD in Geology in 1983, he joined the US Geological Survey as a National Research Council Postdoctoral Fellow, and in 1985, he joined the faculty at Texas A&M University. He is the author of 42 refereed journal papers, 9 articles and conference proceedings, and 78 presentations at national and international conferences. He has advised 9 MS and PhD students, and 4 postdoctoral fellows, most of whom now work in the energy industry.