Numerical Study on Downhole Cooling by Mud Circulation in Supercritical Geothermal Drilling

Abstract

Feasibility of supercritical geothermal power generation and the preliminary drilling plans are now studied in Japan as a national research project. The temperature limit of conventional but the latest downhole tools and materials is in a range approximately 250 to 300 degrees C as a whole. This is insufficient for drilling supercritical geothermal reservoirs with temperatures exceeding 400 degrees C. Downhole cooling technique by mud circulation is considered still indispensable for ultrahigh-temperature supercritical geothermal drilling even if the heat-resistant performance of drilling tools will be improved in the future. In this study, first, the computer code of the wellbore temperature simulator GEOTEMP2, which was previously developed at the Sandia National Laboratory, was modified to enable to deal with assumed subsurface formation temperature profiles in supercritical geothermal fields. Using the modified simulator, downhole temperature profiles during drilling were simulated for a model well whose profile was based on the ultrahigh-temperature geothermal exploration well Kakkonda WD-1a that was drilled in Japan 25 years ago. In addition, temperature profiles during cementing operation were also simulated for the model well. Based on the sensitivity studies on various combinations of drilling and cementing operation parameters, downhole cooling strategies in supercritical geothermal drilling are discussed.