Volcanic Gas Chemistry for Estimating Temporal Changes of Temperature and Pressure in Geothermal Reservoirs

Abstract

The purpose of this study is to clarify the sensitivity and estimation accuracy of Rn (radon-222) for the change of physical properties, in special temperature and pressure, in geothermal reservoirs. The western side of Aso crater in southwest Japan was selected for a case study site, because two reservoirs were estimated by magnetotelluric survey. During two and half years, Rn concentrations have been measured continuously using the volcanic gases pumped from a borehole with 1 or 1.5-m depth and ionization chamber method. Periodic measurements of Rn and Tn (radon-220) by scintillation counter method, and six chemical components data (He, H2, O2, N2, CH4, and CO2) by gas chromatography have been added at one-week intervals. The Rn concentrations by the periodic measurements at the three sites located in the 2-km range clarified the similarity of general fluctuation patterns, which implies an existence of common factor for reservoir condition changes. Because the continuous Rn data at one site were influenced by the atmospheric temperatures, residual components were obtained by subtracting the trend components from the original data. The chemical component data were used to estimate the temperature and pressure in reservoir: resultant temperature range was 230 to 285 C and average pressure was 80 MPa. Main factor for producing these changes is probably volcanic seismicity. It was clarified that the residual Rn components correspond generally with the temporal changes of the estimated temperatures and pressures. Average temperatures at the other two sites were estimated as 265 and 304 C. The differences in the value and changeability of the temperatures may be attributable to source depth of volcanic gases.