ALBERT

Description: The evolution of permeability and its effective stress behavior is related to inelastic deformation and failure mode. This was systematically investigated in Indiana and Purbeck limestones with porosities of 16% and 14%, respectively. High‐pressure compression tests were conducted at room temperature on water‐saturated samples. At relatively high confinement shear‐enhanced compaction was observed to initiate at a critical stress, accompanied by significant permeability reduction of up to a factor of ~3. Overall, the permeability reduction due to inelastic compaction in our limestones is smaller than that observed in sandstones. At relatively low confinement, dilatant failure was observed, which was accompanied by a decrease and increase of permeability in Indiana and Purbeck limestones, respectively. There seems to be a trend for the correlation between porosity and permeability changes to switch from positive to negative with increasing porosity. The void space of both limestones has significant proportions of macropores and micropores. The effective stress behavior of such a limestone with dual porosity has been documented to be different from the prediction for a microscopically homogeneous assemblage, in that its effective stress coefficients for permeability and pore volume change may attain values significantly 〉1. In contrast, our investigation of damaged samples consistently showed effective stress coefficients for both permeability and pore volume change with values 〈1. This suggests that the behavior in the damaged samples is akin to that of a microscopically homogeneous assemblage, possibly due to pervasive collapse of macropores that would effectively homogenize the initially bimodal pore size distribution.

Description: The samples are cylinders of Adamswiller and Rothbach sandstones of nominal length 80 mm and diameter 40 mm. The average porosities for our Adamswiller and Rothbach sandstone samples are 25.8% and 24.3%, respectively. The permeability and strain were measured under hydrostatic loading.