Isotopic Response to Changes in the Reservoir in Bacman Geothermal Production Field, Philippines

Abstract

The Bacman Geothermal Production Field typifies a liquid-dominated system. The Osiao-Pangas (OP) wells, located in the Botong area, tap the center of the resource and encounter fluids that are already products of boiling the parent water. The parent water, which closely resembles OP-4D, has an isotopic composition of: 18O +2.0 o/oo and 2H -26.0 o/oo. The fluids at the upflow are isotopically enriched (+4.0 o/oo) and become depleted (-3.9 o/oo) as they outflow towards north-northwest in the central Palayang Bayan, and southwest in the Cawayan sector. The fluids along the outflow path are products of the dilution between meteoric waters and the parent water.
After eleven years of production, the most prominent processes occurring in the reservoir are boiling and dilution. Isotopic trends indicate that the central Palayan Bayan sector has been diluted with fluids coming from the western part. This fluid, also known as the Masakrot fluid, is isotopically depleted, slightly cooler, less mineralized and has lower gas and boron contents. The effect of dilution is manifested by the depletion of 18O and 2H in the central wells. The OP wells, on the other hand, are showing enrichment in the isotopic composition. As pressures draws down, these wells experience boiling in the reservoir.