Effect of temperature and pore fluid on the strength of porous limestone

Abstract

The presence of pore fluid in rocks generally reduces brittle strength. The role of water weakening in porous carbonate rocks and the effect of enhanced deformation on concomitant transport property evolution are poorly understood. The interaction between pore fluid and deformation mechanisms in carbonate rocks is complicated by crystal plasticity and solubility in calcite at low temperatures and pressures. To explore this system, we perform conventional triaxial deformation experiments on Indiana Limestone (~16% porosity) over a range of temperatures (23-75°C) and effective confining pressures (10-50 MPa). Samples are deformed with pore fluids of different saturation states with respect to calcite host rock. Samples tend to fail by distributed deformation. Sample permeability is reduced by up to an order of magnitude during deformation, even in samples exhibiting dilatancy. Compactive yield strength, associated with shear-enhanced compaction, is greatly reduced in saturated samples compared to dry samples. Dilatancy is suppressed and compaction enhanced at elevated pressures. Greater weakening effects are observed when pore fluid is far-from-equilibrium with carbonate rock. Observed weakening results from the interaction between microcracking, mechanical twinning, and pressure solution.