Geochemical techniques in geothermal research — the Hawaii example

Abstract

The Hawaii Geothermal Project (HGP) was organized in 1972 to locate and utilize geothermal energy resources in Hawaii. A geophysical program was designed to select a site of high potential for a geothermal resource and to develop an understanding of the thermal processes of a basaltic volcano and its associated rift zones. The well was completed in July 1976 and after subsequent clearing was producing at 5800 kg/hr with a well head pressure of 11 kg/cm 2 and a temperature of 190°C (equivalent to 3.5 MW of electrical energy). The discharge fluid was only slightly saline despite the termination of the well 1768 m below sea level. The water was nearly depleted in magnesium but contained high concentrations of silica and sulfide. Geochemical thermometers based on the data of Fournier, Rowe, White, Ellis, and Truesdell are in excellent agreement with the measured downhole temperature of 275°C. Stable and radioactive isotope analyses for 14C, 13C and 13O indicate that 66% of the recharge is from local rainfall with a turnover of at least several hundred years. The remaining discharge is derived from sea water (10%) and hydrothermally altered fluids. Analyses for the gases CO 2, He and 3He indicate that the fluids have a deep-seated or magmatic source.