Microseismic mapping of an HDR reservoir

Abstract

Microearthquakes induced by hydraulic fracturing provide considerable information about the flow of fluid through rock. In the Hot Dry Rock (HDR) scheme, two wells are connected by a man-made geothermal reservoir, with cold fluid pumped down one well, and hot fluid produced from the other. The HDR reservoir is created by one or more hydraulic injections into hot, impermeable rock. Microseismicity provides the primary means of characterizing most of the HDR reservoir, which lies outside the limited rock volume interrogated by wellbore logging instruments. Induced microearthquakes can be used to determine the geometry and location of the, prominent fluid flow paths in the reservoir. In addition, the microearthquakes provide information about rock mechanical processes, and about earth stresses within the reservoir. An RDR reservo~r was created at Fenton Hill, New Mexico by a large volume (more than 20,000 m ) injection, termed the Massive Hydraulic Fracture (MHF), done in December, 1983. Since the MHF, four more large fluid injections have been done at Fenton Hill, and each produced thousands of microearthquakes. We describe here the major results of characterizing that HDR reservoir by analysis of the induced microseismicity from the MHF. The MHF was the largest and single most important fluid injection done to create the reservoir that is currently being further developed and tested.