Modeling the Reservoir Fluids of Acidic Geothermal Wells in Mahanagdong, Leyte, Philippines

Some production wells of the high-temperature and liquid-dominated Mahanagdong Geothermal Field discharge low pH (<4 at 25 °C) fluids. The acidic discharges pose challenges to operations due to corrosion and scaling on the wellbore and surface facilities. The low-pH discharges have high salinity, and have either high or low sulfate concentrations. To elucidate the nature and mechanism of the acidic discharges, equilibrium and boiling simulations were conducted with the aid of SOLVEQ-XPT and CHIM-XPT geochemical software, taking into account the apparent “excess enthalpy” discharged by the wells and assuming that aluminum levels in the solution are in equilibrium with secondary minerals at depth. The reservoir fluids of the high sulfate acidic fluids have a high concentration of HSO - species, which dissociates at lower temperatures and yields acidic discharges at the wellhead. In comparison, the low sulfate fluids are very acidic in the reservoir, with high levels of HCl inferred to be a major contributor to the acidity of the fluids at depth.