Origin of Acid Fluids in the Cawayan Sector Bacman Geothermal Production Field

A shallow acid assemblage of sulfur and kaolinite +/alunite, dlclate and pyrophyllite dominates the hydrothermal alteration in the seven wells in Cawayan from surface down to 0 mRL. These zones range in thickness from 20 to 220 m. Thts shallow acid zone formed at temperatures < 160°C. Below 0 mFU down to -600 mRL, a deep acid zone of dlaspore and pyrophyllite +/alunite and dlclute appear in thin, cfiscrete zones which coincide with measured temperatures between 180-250°C. Neutral-pH alteration of epidote, calcite, chlorite and illite prevail at deeper levels. Actinolite veins and biotite at -1300 mRL suggest temperatures of about 280°C whtch are consistent with measured stable temperatures INTRODUCTION The Cawayan sector is located in the Bacon-Manit0 Geothermal Production Field in Southern Luzon (Fig. 1). It is presently supplying steam to a 20 MWe Modular Power Plant commissioned in 1994. Less than a year after commssioning, some of the production wells in t l s sector have already suffered casing corrosion and anhydnte blockage deposition due to acid fluids. This paper therefore aims to establish the nature of these acid fluids itl Cawayan through a study of the acid hydrothermal alteration in the wells. their subsurface dlstributiotb and their correlation with acid fluid inflows. Fot future production wells in this sector. ways to avoid or isolate these acid fluids are recommended. The dominance of acid alteration in the shallow portions of Cawayan indlcate its surfkial origin from the oxidation of H2S. These fluid descended through highly permeable structures to levels as deep as -1OOOmRL. Its incursion resulted in mineral blockages and casing corrosion in CN-1. CN-2D and CN-3D. In contrast, the prevalence of neutral-pH alteration at deeper levels of the Cawayan reservoir shows that a neutral type of fluid circulates at these depths. This brine upflows beneath CN-3D and CN-4D and flows laterally towards the CN-2D/ 2RDI3RD sector. As a consequence of this study, future production wells should be dnlled in the CN-3D/4D sector to tap the upflowing, hot, neutral brine. Structures which channel acid fluids should be avoided or intersected above the production casing shoe which should be set below -600 mRL. Figure 1. Location map of Cawa.van sector, Bachlan.