Evidence for the Existence of a Stable, Highly Fluid-pressurized Region of Deep, Jointed Crystalline Rock from Fenton Hill Hot Dry Rock Test Data

Large volumes of naturally jointed Precambrian crystalline rock can be stably maintained at pressures considerably above the least principal earth stress in the surrounding rock mass. At Los Alamos National Laboratory’s Fenton Hill hot dry rock (HDR) geothermal test site, tests carried out for a cumulative period of 11 months in the deeper and larger of the two HDR reservoirs showed no evidence of fracture extension at the boundaries of the pressure-stimulated region. These results were obtained while maintaining a very high reservoir inlet circulating pressure of 27.3 MPa (3960 psi) above hydrostatic, considerably in excess of the least principal earth stress in the surrounding rock mass of about 10 MPa above hydrostatic at a depth of 3500 m. We review and summarize information concerning the earth stresses at depth and the test data relative to the containment of pressurized fluid, particularly the data showing the declining rate of water loss and the absence of microseismicity—the two principal indicators of a stable, pressurized reservoir region. We then provide a coherent and concise evaluation of this and other evidence supporting our assertion that one can indeed maintain large volumes of jointed rock at pressures considerably in excess of the least principal earth stress. In addition, a discussion is presented concerning the initial state of stress at depth beneath Fenton Hill and then possible changes to the stress state resulting from the very large volumes of injected high-pressure water and the accompanying shear displacements—and shear dilation—associated with these pressurizations.