Influence of Reservoir Stress Path on Deformation and Permeability of Weakly Cemented Sandstone Reservoirs

Abstract

Summary The influence of production-induced changes in reservoir pore pressure on compressibility and permeability of weakly cemented sandstones has been analyzed. Laboratory experiments simulating reservoir depletion have been conducted over a range of stress paths that a reservoir may follow. The results suggest that compressibility of weakly cemented sandstones is stress path dependent. Compressibility measured under uniaxial strain conditions, or a stress path defined by a lower ratio of the rate at which the effective horizontal to effective vertical stress were increased than the one associated with uniaxial strain, is more than twice the corresponding value found from the hydrostatic loading experiment. In contrast, matrix permeability measured in the maximum stress direction show no significant stress path dependence. Experimental results suggest that a better understanding of the stress-sensitive behavior of weakly cemented sandstones can only be gained by dealing more directly with the microstructure of the rock. The stress-path-dependent nonlinear behavior of weakly cemented sandstones is related to effects of shear-enhanced compaction. Increasing cementation has been experimentally shown to reduce stress sensitivity. The observed nonlinearity is attributed to dilatancy rather than shear-enhanced compaction, also reflected by permeability measurements made in the maximum stress direction.