Elastic moduli, thermal expansion, and inferred permeability of two granites to 350°C and 55 megapascals

Abstract

The coefficient of thermal linear expansion α, Young's modulus E , and bulk modulus K have been determined for the Westerly and Stripa granites to temperatures T of 350°C and pressures P to 55 MPa. Using conventional triaxial aparatus, displacement measurements were made on three samples from each of three orthogonal directions for both rocks. Comparison of the directional values at any P, T , and those from the nine‐sample population indicated that within our precision, both granites are isotropic in E , K , and α. Both E and K for both rocks decreased with T and increased with P in a nonlinear fashion. From 19° to 350°C, E decreased by as much as a factor of 2 and K decreased by 2 to 3 times, depending on P . From 6 to 55 MPa, E increased by factors of 3 to 6 and K increased by 3 to 5, depending on T . Values for α were neither constant nor a monotonic function of P or T . In both granites over the P range investigated, α typically increased from 6 to 12×10 −6 °C −1 at 40°C to 10–15×10 −6 °C −1 at 325°C. In both rocks over the T range investigated, increasing P from 6 to 55 MPa generally decreased α by 1–5 10 −6 °C −1 . Most measurements are consistent with microcracks controlling the thermoelastic response by cracks opening with increasing T and closing with increasing P . Changes in crack porosity ϕ due to bulk compressibility and thermal expansion have been calculated for both granites. Because K and α were nonlinear with P and T , ϕ was inferred to be a complex function of both. Assuming that all cracks affect fluid transport, changes in permeability κ with P and T have also been calculated from κ ∝ ϕ 3 . These changes have been compared as κ/κ 0 , where κ 0 was the initial value at 0.1 MPa, 19°C. For example, κ/κ 0 for Westerly granite was inferred to increase by a factor of 3 from 19° to 300°C at 8 MPa. In Stripa granite at 6 MPa, κ/κ 0 decreased ∼25% with T at 19°–100°C, then increased approximately twofold by 350°C.