Application of Sulfur Isotopes in Characterizing the Acid Fluids at Tiwi Geothermal Field, Philippines

Abstract

Development of the Bariis (Bar) sector of the Tiwi Geothermal Field has been hindered by production of acid fluids in the earlier drilled wells. However, the successful isolation of acid fluids in two Bariis wells during 2005 and 2008 has opened up the Bar sector and, in general, the southwestern portion of the field for deep make-up well drilling. Initial estimates suggest that this portion of the Tiwi Field can provide significant additional steam. In preparation for the make-up well drilling in the Bar sector, a sulfur isotope study is being conducted to provide additional insight on the nature/origin of the Tiwi acid fluids. For this study, a total of 19 BaSO4 (ä34SSO4; ä18OSO4) and 21 Ag2S (ä34SH2S) precipitates and 23 äDH20-ä18OH20 samples were collected from both neutral and known acid wells, including nine downhole liquid samples. Results of the sulfur isotope study indicate a non-magmatic origin for the Tiwi acid fluids. The non-equilibrium in some fluids suggests shallow processes affecting the chemistry of the acid fluids. This implies that the risk of encountering deep acid in the target make-up well areas is low, except in the vicinity of the faults, which could act as a conduit for shallower acidic fluids to enter the deep reservoir. The study also indicates potential use of Ä (ä34SSO4 and ä34SH2S) values in identifying higher temperature portions of geothermal. One of the difficulties encountered in the study, however, is the high pH values of samples from idle acid wells which hinders the full characterization of the acid fluids. It is thus recommended to attempt sampling acidic fluids in flowing wells going forward.