Variation of Fracturing Pressures with Depth near the Valles Caldera

Abstract

Hydraulic Fracturing at the Fenton Hill Hot Dry Rock Geothermal site near the Valle s Caldera has yielded fracturing pressures from 14 to 81 MPa (2030 to 11750 p s i ) at depths ranging from 0.7 to 4.4 km (2250 to 14400 ft). This data can be fit to a fracture gradient of 19 MPa/km (0.84 psi/ft), except for an anomalous region between 2.6 to 3.2 km where fractur in g pressures are about 20 MPa lower than estimated using,the above gradient. This anomaly coincides with a biotite granodiorite intrusive emplaced into a heterogeneous jointed metamorphic complex comprised of gneisses, schists and metavolcanic rocks. Microseismic events detected with sensitive downhole geophones suggest that shear failure is an important process during hydraulic fracturing of such jointed rock. Consequently the usual relation between minimum earth stress and fracture opening pressure, based upon classic tensile failure, cannot be used a priori; fracture opening pressure is instead a complex function of joint orientation and all three components of principal earth stress.