Gas Breakthrough' During Drilling at Darajat and Implications for Steamfield Management

Abstract

While drilling two production wells without circulation in the Darajat Geothermal Field, West Java, Indonesia, significant changes occurred in gas chemistry of adjacent wells. The drilling of well DRJ-23, without circulation, caused gas/steam ratios in some wells in the southern production area to increase. Whereas, drilling of well DRJ-24 affected wells in the north area. The change in chemistry was presumed to be due to permeable connectivity within the reservoir. Intensive monitoring of well chemistry, wellhead pressure and temperature and power plant performance, was performed to acquire sufficient data for interpretation. A numerical model was used to analyze the transient gas concentrations, with respect to water volumes injected during drilling without circulation of the subject wells. The increased gas to steam ratios and water volumes injected were used to estimate the steam condensation within the reservoir and likely fracture sizes. The assessment of reservoir steam condensation was done using the ratios obtained from the gas analyses. The changes in chemistry of the affected wells were employed to support the interpretation of the numerical model. Isotopic samples, taken during the breakthrough, were useful in confirming the source of water causing the breakthrough. Additional data obtained from a tracer study, utilizing SF6 and Freon-134a tracers, has contributed significantly to understanding the phenomena and validating the interpretation. The results have possible implications for future steamfield management, including condensate injection strategy, potential negative impacts on power plant output and a general steam production strategy.