Effectiveness of Radon Concentration Features in Soil Gas for Specifying Hydrothermal Ascent Paths in the Wayang Windu Geothermal Field, Indonesia

Abstract

Indonesia is a typically volcanic country with one of the world's largest geothermal resources and plans to greatly increase geothermal power output. For this, high introduction cost for geothermal power generation, the cost for exploration to identify the geothermal reservoir in particular is a big issue. We started a project in 2014 aiming at development of advanced exploration schema for geothermal resources using several geoscientific methods. As one of the effective methods for our purpose, measurements of soil gas component have been applied to estimate the condition of geothermal reservoir from the ground surface. Wayang Windu Geothermal Field (WWGF), which is one of the greatest geothermal potential area in Indonesia, was selected as the case study site. Total 24 measurement wells up to the 5 m depth were constructed in WWGF. We focused on Rn-222, the longest half-life radon isotope, in soil gas, because Rn-222 is an effective indicator of volcanic activity, thermal manifestation and fractures due to its chemical features. The periodical measurements using DURRIDGE RAD7, an electronic radon detector with real-time monitoring have been carried out through over one year including rainy and dry seasons. The remarkable result is a temporal variation of radon concentration with elapsed time within one hour, which is classified into two patterns, stability or decreasing pattern. In addition, several wells showed the seasonal variation of radon concentration. These variation patterns probably indicate differences in soil permeability including the fracture system and vapor amount derived from the geothermal reservoir. Thus, radon concentration can contribute to identify main flow path of geothermal fluids that is a target of drilling for geothermal power generation by integrating with short and long term variation. In addition, other gas components such as H2S, O2 and CO2 were different with the wells. Moreover, satellite image analyses in WWGF support the interpretations from the soil gas measurements by indicating the presence of permeable fractures and hydrothermal alteration minerals. This integration of several analyses is expected to locate accurately high potential resource zones and increase the success ratio of production wells.