Evolution of Sandstone Properties Undergoing Cyclic Hydrostatic Loading

Abstract:

The stress state of rocks in the subsurface strongly controls their in-situ mechanical and petrophysical properties. Stress state is rarely static in-situ, and rocks frequently experience repeated cycles of loading and unloading. This is of importance for several geophysical and industrial applications, for example wastewater injection and reservoir storage wells, which generate repeated stress perturbations. Laboratory experiments were conducted with Castlegate sandstone to observe the effects of different cyclic loading conditions on a common reservoir analogue. Each sample was hydrostatically loaded in a triaxial cell to a certain confining pressure, and either pore pressure or confining pressure were cycled at different rates. Water permeabilities were periodically measured by halting the cycling and measuring at a pair of fixed pressures. Samples that undergo cyclic loading experience significantly more inelastic (non-recoverable) strain compared to tests without cyclic hydrostatic loading. Permeability tends to decrease initially, but overall remains relatively constant despite repeated cycling of stresses, suggesting permeability is controlled more by the mean effective stress rather than stress state stability. Cycling rate does affect the mechanical behavior; sandstones cycled at faster rates tend to experience lower elastic moduli and strain changes likely due to greater creep occurring in samples with greater cycling rates. The data indicates cyclic loading decreases permeability and porosity more than static conditions over a similar period, but the petrophysics are dictated more by the rate of loading rather than the duration.