Monitoring and Repairing Geothermal Casing Cement: A Case History

Abstract

In the hot dry rock (HDR) concept of extracting geothermal energy, as developed by the Los Alamos Scientific Laboratory (LASL), a manmade geothermal reservoir has been created by drilling a deep hole into relatively impermeable hot rocs, 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 completion of HDR geothermal wells presents some very severe cementing problems. Current well depths are from 10,000 to 14,500 ft with bottom hole temperatures up to 525°F (275°C). During investigation, development, and flowing the reservoir, water injections may lower the temperature of the entire wellbore to 104°F (40°C). As result, the casing string may be subjected to many temperatures stress cycles representing differential temperatures of up to 455°F (235°C). 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 these cyclic stresses, and rapid deterioration of casing to cement land cement to formation bonds occurred, which allowed significant flow in the resulting mircoannulus. This paper will describe (a) the performance history of the casing cement for the existing HDR EE-1 injection well, (b) the subsequent remedial cementing pgroam, (c) the cement bond logs, and (d) the radioactive isotope tracer injections tests, used to monitor the conditon of the casing cement.