Geothermal testing and measurements of rock and rock fractures

Abstract

Predicting hot dry rock geothermal energy extraction requires knowledge of the coupled hydro-thermo-mechanical properties of rock and rock fractures. A description is presented of experiments using a geothermal rock mechanics testing system capable of testing medium-sized cylindrical rock specimens at axial loads to 2.67 MN, confining pressures to 140 MPa, pore pressures to 105 MPa and sample temperatures to 400°C. Tests were carried out to determine mechanical and thermal properties of intact granite and the mechanical deformations, fluid mass and heat transport in fractured rock. Descriptions of the tests and typical results are presented and discussed to illustrate the multi-functional capability of the testing system to address required research in the development of a geothermal energy system. Results obtained through tests on the Carnmenellis granite using the geothermal rock testing system show that: the strength of the rock can be reduced by increasing temperature, the permeability of the fracture is controlled by the effective normal stress, and the heat convection rate is a function of flow velocity in the fracture. The results also show that the granite has a thermal conductivity of about 3.5 W/mK.