| Title | Mt. Apo Acid Fluid Evolution: Exploring the Role of Rock Buffers in the Reservoir pH Changes with Time |
|---|---|
| Authors | ROSELL, Josephine B., TAMBOBOY, Rhemal J. T., SAMBRANO, Benson G., DACILLO, Danilo B., and REED, Mark H |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Mt. Apo, rock buffer, rock titration, water-rock interaction, geochemical modelling |
| Abstract | The Sandawa sector in the southeastern edge of the Mt. Apo Geothermal Project in the Philippines is characterized by geothermal fluids whose acidity has been increasing over time. This is pronounced in well KN2D wherein measured discharge pH at the wellhead dropped from 4-5 to 2.5 (at 20°C) after 12 years in production. The pH of the deep reservoir fluid feeding well KN2D has also declined from 6.1 to 4.7 based on SOLVEQ-XPT modelling of the discharge fluids to downhole conditions. Geochemical evaluation of the discharge fluid shows that the surface acidity observed in Mt. Apo is due to HSO4- dissociation as the H2SO4-rich reservoir fluid ascends to the surface. The increase in acidity of the reservoir fluid, meanwhile, could be due to (1) the drawing in of more acidic fluids from deeper in the reservoir as a result of mass extraction, and (2) the loss of the buffering capacity of the exposed wall rock along fractures that have continuously conveyed the flowing acidic fluids since production began. This study examines the likely role of rock buffers in the evolution of acidic fluids in Mt. Apo over 12 years of production. Rock titration of well KN2D fluid with host dacite rock is carried out using CHIM-XPT geochemical software and the modelled fluid chemistry is compared with the measured fluid chemistry of well KN2D in the last 12 years. Results show a close agreement of simulated rock-titrated fluid with actual historical data, indicating that water-rock reaction plays an important role in the evolution of acidic fluids in Mt. Apo. |