VSP and Cross-hole seismic surveys used to determine reservoir characteristics of a hot dry rock geothermal system

Abstract

Monitoring microseismic emissions during fracturing and circulation experiments at the Camborne School of Mines 2 km deep hot dry rock geothermal project has been carried out over the past 5 yr. These microseismic locations provide the only information on the position of the stimulated region within the granite rock mass. A well was drilled into the microseismic cloud on the basis that enhanced permeability would be found. However, the evidence for a relation between microseismicity and improved permeability is conflicting. Calculated shear displacements are insufficient to provide the observed permeability and it is known that only a relatively small percentage of the microseismicity that occurs is located. Vertical seismic profiles and cross-hole seismic surveys have been used to help understand the relation between the microseismic and hydraulic reservoirs. Changes in signal attenuation and velocity as raypaths cross the reservoir have been measured. These changes indicate that regions of microseismicity represent permanent increases in crack frequency and/or aperture.