Chemical and isotopic studies of the HGP-A geothermal well

Abstract

"The HGPA well, located on the Lower East Rift Zone of Kilauea Volcano, was drilled in 1976. It is completed to a depth of 1966 m 1768 m below sea level and has a bottom hole temperature of approximately 360C. The well is capable of producing approximately 50,000 kghr of geothermal fluid 43 steam, 57 brine at wellhead pressure of 1172 kPa and is currently providing steam to a 3 megawatt electrical generator facility. During earlier testing periods and the present extended production 20 months as of the present date from the well, a number of chemical and isotopic analyses have been conducted on the geothermal fluids discharged. The results of these analyses suggest that the geothermal system sampled by HGPA is in some respects analogous to those observed at seafloor spreading centers such as Galapagos or the midAtlantic Ridge.Chemical data and isotopic data are given.During early production testing stable isotopic analyses were performed for hydrogen and oxygen; radio isotopic analyses of carbon dioxide and aqueous tritium were performed at this time as well. More recently a limited number of stable isotope analyses have also been conducted for reduced and oxidized sulfur species. Tritium H and carbon14 analyses have placed very broad limits on the age of the fluids present in the HGPA reservoir; tritium data obtained after extended well flow tests have yielded values indicating an age of at least 50 years and tritium levels present after an extended pumpdown test of modern water indicated that circulation within the reservoir in the immediate vicinity of the well bore is on the order of at least years rather than months. Two carbon14 analyses conducted on the well discharge several years apart yielded activities of approximately 20 of modern carbon indicating a maximum age of about 12,000 years, This age is considered to be an upper limit primarily because the contribution of magmatic carbon of zero activity to the total produced is currently unknown.Stable isotope analyses of hydrogen and oxygen conducted on the early fluids produced by the well indicated that the source of the meteoric component of the reservoir fluids was from an altitude of a few thousand feet higher than that at which the well was located and that relatively little oxygen isotopic exchange had occurred between the reservoir rock and the geothermal fluids. It is highly probable that the fluids sampled for these analyses were heavily contaminated by surface water pumped into the well during quench operations; however, more recent analyses of these isotopes have not been performed to confirm this interpretation.Sulfur isotopic analyses have been conducted on several samples of hydrogen sulfide from the steam phase, aqueous sulfate, and pyrite and anhydrite collected from the HGPA core samples. The results of these analyses are presented in Table 2, It is apparent that the sulfur isotopic ratios for the reduced and oxidized sulfur species are quite divergent suggesting that the reduced and oxidized species may have different sources. It is also noteworthy that there has been an apparent change in the hydrogen sulfide d34S values with time."