Saturating a Tight Rock and Measuring Its Hydromechanical Response

Investigation of hydromechanical behavior of fluid-saturated tight rock is motivated by the need to quantify the effect of changes of fluid pressure p and mean stress P on rock deformation, hydrothermal fluid, and mass transport. In particular, hydromechanical properties of low porosity crystalline rock are required for analysis of geological processes including areal hydration or dehydration, mineral weathering, and fault mechanics. In this study, poroelastic parameters – drained bulk modulus K, and Biot coefficient α – governing the volumetric response of Westerly blue granite, a typical crystalline rock of low porosity are measured. Three additional hydromechanical properties, unjacketed bulk modulus K_s^′, expansion modulus H, and permeability k, are also measured. For the Terzaghi effective mean stress of 1.0 < P′ = P-p < 25.0 MPa, the unjacketed bulk modulus K_s^′ = 57.5 GPa is constant within the range of mean stresses investigated but other poroelastic coefficients exhibit effective mean stress dependency; the ranges are 13.2 < K < 32.3 GPa, 19.0 < H < 60.0 GPa, 0.83 > α > 0.38, and 20 > k > 5 nanodarcy. The agreement between poroelastic coefficients determined from various methods suggests that the underlying linear elastic assumption in Biot’s theory of poroelasticity is applicable to Westerly blue granite over small increments of effective mean stress.