Fault-plane analysis of microseismicity induced by fluid injections into granite

Abstract A hot dry rock geothermal reservoir has been created at a depth of 2 km in the Carnmenellis granite by fluid injection, inducing approximately 6000 locatable microseismic events. The onset of seismicity and the direction of growth of the microseismic locations depends upon the in situ stresses, jointing and pore pressure. Analysis of a number of composite fault-plane solutions indicates that the maximum horizontal stress has an orientation of 310–315° N. This is consistent with near surface overcoring stress measurements and in agreement with hydrofracture stress measurements carried out at depths of approximately 2 km. The fault-plane solutions also indicate that failure is on existing subvertical joints that strike NW-SE. Measurement of the in situ stresses and jointing has been relatively successful at a depth of 2 km. However it is uncertain that these measurements will be possible at 6 km, the depth of an economic hot dry rock reservoir. The good agreement at 2 km between the known stresses and jointing and the fault-plane solutions of the microseismic data indicates that analysis of microseismic data induced at a depth of 6 km will become a powerful diagnostic tool for understanding the in situ conditions.