Downward migration of shearing in jointed rock during hydraulic injections

Abstract

Large scale hydraulic injections into rock at significant depth are common in the exploitation of hydrocarbon formations, geothermal energy systems and for liquid waste disposal. One example is the hot dry rock geothermal energy project operated by the Camborne School of Mines in Cornwall, U.K., where a cumulative injected volume of over 300,000 m 3 was pumped into 2 km deep boreholes in granite. During the course of these injections the location of pressurized water was followed by means of microseismic detection. It was concluded that the continuing downward growth of the stimulated region was associated with a shearing source mechanism. This result is explained in terms of the anisotropic in situ stress field interacting with critically aligned joints. The downward growth is closely related to the ratio of the maximum to minimum principal effective stress. The existence of the downward growth implies an eventual curvature in the maximum stress envelope if shear failures are not to be incipient under hydrostatic pore pressure conditions. This curvature implies that upward shear growth would occur during injections at greater depths. These results are potentially of general application in strong jointed rock with a markedly anisotropic in situ stress field.