Characterizing Damage Evolution and Yield in Sandstone under Triaxial Loading as a Function of Changing Effective Pressure
Experimental rock deformation was used to study 1) the accumulation of microscopic damage preceding macroscopic failure across the brittle-ductile transition (BDT) in granular porous rocks, and 2) how damage induced at one effective pressure (P) affects failure at a different P. Granular porous material is idealized as an elastic-plastic material, where failure occurs by localized dilatant shear at low P and compactant cataclastic flow at high P. Given distinct failure modes in the low and high P regimes, different types of damage may develop prior to failure at different P. Water saturated cylinders of Berea sandstone (18% porosity, 185 µm grain size) were deformed in triaxial compression at a shortening rate of 4 µm/s. During each experiment, the confining and pore pressure were held constant; acoustic emissions (AE), axial stress, axial displacement, and pore volume changes were recorded. Samples were deformed at pore pressures of 10, 20, and 30 MPa, and confining pressures of 50, 180, and 260 MPa to investigate the brittle, transitional, and ductile regimes. Three different load paths were used. The first involved loading to failure to establish a baseline response. The second involved initial loading to 80% of the differential stress at failure, unloading, and reloading to failure at a different pore pressure. The third was similar to the second, except confining pressure was changed between load and reload to cause failure in a different regime than the initial load. AE is used to quantify damage evolution, and the Kaiser effect was used to map damage states in stress space. The experiments show that contours of equivalent damage are subparallel to the failure envelope across the BDT, and that macroscopic failure depends on load path and the cumulative damage state. Damage induced in either the low or high pressure regimes has little effect for failure in the other deformation regime, supporting the concept of distinct processes and damage development in the two regimes. These results could be important in predicting formation damage and permeability changes in over-pressured reservoirs that undergo large drops in pore pressure, as well as for reservoirs undergoing advance recovery methods and sequestration that increase pore pressure. This could also aid in understanding deformation in tectonic basins that experienced P changes due to burial or exhumation, or from cyclic loading along faults.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009