Development of high temperature instrumentation for use in geothermal boreholes

Summary form only given. The engineering of a hot dry rock geothermal reservoir requires the stimulation of existing fractures or creation of new fractures to increase the flow between two wells. Hydraulic stimulation can be used to increase the joint aperture either by shear sliding on joints or tensile failure depending, on the in-situ stresses and joint orientation. This hydraulic stimulation of fractures produces microseismic events (micro earthquakes). Monitoring of microseismicity is an independent and direct method of determining the growth and size of the stimulated region. The author describes the hodogram method used for locating microseismic events using a 3-axis unit. The hodogram method combines the signal from three orthogonal sensors to give a direction and distance to an event. The 3-axis tool is required to work at 220°C maximum over prolonged periods (up to 3 months). A new 3-axis tool was developed capable of producing the precision required to locate microseismic events. The tool also has inclinometers, a compass, temperature sensors, pressure sensors etc. to monitor the working environment and the orientation of the tool. The tool is under the control of a microcomputer at the surface (MicroVAX II) and is able to change the various amplifier gains and monitor the preferred instrumentation such as temperature and pressure