Karner, Stephen L.¹, Judith S. Chester¹, Frederick M. Chester¹, Andreas K. Kronenberg¹, Andrew Hajash¹
(1) Texas A&M University, College Station, TX

ABSTRACT: Granular Deformation of Quartz Sand: Critical State Failure Model and Implications for Clastic Rocks

We performed isostatic and triaxial deformation experiments on St Peter quartz sand and disaggregated Arkansas Novaculite to quantify elastic and inelastic compaction by mechanical processes of grain rearrangement, grain breakage and poroelastic deformation. Throughout each experiment we monitored applied stresses, sample strains, and acoustic emissions (AE). Deformation of quartz sand at low stresses is accommodated by granular flow (without significant grain breakage) whereas at high stresses granulation and cataclastic flow is dominant. For isotropic conditions, samples show compactive strains with quasi-elastic deformation at low stresses that transitions to pervasive brittle grain failure at a critical stress, P*. Experiments on a range of sieve fractions show an inverse power-law dependence of P* on grain size. Triaxial tests at high effective pressures exhibit compactive strains and large AE rates coupled with grain size reduction that corresponds to macroscopic sample failure. For triaxial loading at low effective pressures, samples exhibit dilatant granular flow with minimal grain fracturing and breakage. Our results quantify an elliptical failure envelope for macroscopic failure consistent with critical state models and allow us to predict both elastic and inelastic strains under sub-yield stress conditions. Qualitative comparison of inelastic strains in our experiments are consistent with microfracture density data from natural reservoir sandstones from the Gulf of Mexico. We use our results to quantify compaction of porous sands in both extensional and contractional basins and predict limits on the generation of fluid overpressure solely by mechanical processes.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.