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.

Matrix permeability of reservoir rocks, Ngatamariki geothermal field, Taupo Volcanic Zone, New Zealand

“…11 has also been observed in other studies (Heard and Page 1982;Klug and Cashman 1996;Saar and Manga 1999;Rust and Cashman 2004;Stimac et al 2004;Mueller et al 2008;Wright et al 2009;Heap et al 2014;Farquharson et al 2015;Wadsworth et al 2016;Heap and Kennedy 2016). There is scatter in the relationship, which results from the geometry and connectivity of the pores (as noted in Mueller et al 2008 and others), amongst other factors.…”

Cold compaction of water ice

“…At zero confining pressure, the force of the piston probably deforms the sample a small amount, which can be ignored. This technique was first used by Heard and Page [1982] to measure thermal expansivity of rocks under pressure.…”

Numerical models of caldera deformation: Effects of multiphase and multicomponent hydrothermal fluid flow

“…Typical values for the thermal expansion coefficient for saturated rock at high temperature and pressure range from 5 Â 10 À6°CÀ1 to 1.5 Â 10 À5°CÀ1 [Bauer and Handin, 1983;Heard and Page, 1982;Wong and Brace, 1979], and we use a value of 1 Â 10 À5°CÀ1 in all simulations. Crustal deformation scales inversely with shear (rigidity) modulus, and laboratory experiments at high pressures and temperatures indicate that the intrinsic shear modulus of crystalline rocks varies from 0.2 to 50 GPa depending on rock type, temperature, pressure, and porosity [Heard and Page, 1982]. Although large-scale crustal values of 30 GPa [Turcotte and Schubert, 2002] are commonly invoked in elastic deformation models, several studies have suggested that the rigidity of warm volcanic rocks should be as low as 0.3 GPa [e.g., Davis, 1986].…”