Borehole Instruments for Supercritical Geothermal Reservoirs

Abstract

Exploration and exploitation of supercritical geothermal reservoirs requires the use of borehole instruments that can tolerate the reservoir’s high temperature and high pressure environment (above 374 °C and 22 MPa for pure water). In addition, such downhole instruments need to be robust, chemically resistant and present a high level of accuracy. These requirements exceed even that of the aerospace and automotive industries. Mechanical instruments and observation of material processes, such as melting, corrosion and deformation have been used with success in the high temperature geothermal environment, but these methods can only be applied to a limited amount of physical parameters and lack the accuracy needed for proper analysis of the geothermal reservoirs. High temperature tolerant electronic and sensor devices have been developed, along with high temperature batteries, which can be used in the design and assembly of borehole instruments. A description of the requirements and assembly procedures that had to be followed and implemented in new borehole instruments (wireline and slick-line) in the European project HiTI (High Temperature Instruments for supercritical geothermal reservoir characterization and exploitation) is given in this paper and examples of actual measurements using some of these new devices is given. Finally, a description of further implementations, involving both mechanical and electronic aspects, is presented.