Monitoring and repairing geothermal casing cement: a case history

A manmade geothermal reservoir has been created by drilling a deep hole into relatively impermeable hot rocks, creating a large surface area for heat transfer by hydraulic fracturing, then drilling a second hole to intersect the fracture to complete the closed circulation loop. A second generation system, presently being drilled, will entail creating multiple, parallel, vertical fractures between a pair of inclined boreholes. The original completion of injection Hole EE-1, consisting of a conventional high-temperature formulation of Class B portland cement, stabilized with 40% silica sand, did not withstand the cyclic stresses, and rapid deterioration of casing-to-cement and cement-to-formation bonds occurred, which allowed significant flow in the resulting microannulus. The performance history of the casing cement for the existing HDR EE-1 injection well, the subsequent remedial cementing program, the cement bond logs, and the radioactive isotope tracer injections tests, used to monitor the condition of the casing cement is described. (MHR