Thermal expansion and cracking of three confined water-saturated igneous rocks to 800°C

Summary

Solutions of engineering problems of very deep drilling, geothermal energy production, and high-level nuclear-waste isolation require adequate understanding of the mechanical and transport properties of rocks at relatively low pressures but high temperatures. Accordingly, the thermal expansions of water-saturated Charcoal Granite, Mt. Hood Andesite, and Cuerbio Basalt have been measured at effective confining pressures (P _e ) of 5, 50, and 100 MPa to 800° C. The mean coefficient of linear thermal expansion (α) is a function of lithology,P _e , temperature (T) and initial porosity (ϕ). For example, for the Charcoal Granite, α increases withT at all pressures. The signature of the alpha-beta transition of quartz is more pronounced at the lower pressures; at 100 MPa α nearly mimics that of a crack-free rock forT<300° C.

α for the andesite atP _e =5 MPa ranges from 10 to 15×10⁻⁶/°C from 200° to 400° C then decreases gradually to 10.1×10⁻⁶/°C at 800° C. At 50 MPa α ranges from 11.7×10⁻⁶/°C at 100° C to 8.6×10⁻⁶ at 200°C, then increases at a much lower rate to 11×10⁻⁶ at 600° C. The basalt, however, has an essentially constant α (11×10⁻⁶/°C) forT>150°C at the lower pressure and shows but a small increase in α from 6 to 9×10⁻⁶ from 100° to 800° C at 50 MPa.

The difference between measured values of thermal expansion and those calculated from simple mixture-theory relates to new crack porosity generated as a result of differential thermal expansion at the anisotropic grain scale. For the granite, a two to three order of magnitude increase in permeability (k) is predicted from the relation,k∝φ ³.