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 ∝ ϕ3. These changes have been compared as κ/κ0, where κ0 was the initial value at 0.1 MPa, 19°C. For example, κ/κ0 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/κ0 decreased ∼25% with T at 19°–100°C, then increased approximately twofold by 350°C.