Effects of Injection Returns on Oxygen and Hydrogen Stable Isotope Ratios of the Mt. Apo Hydrothermal Fluid, Philippines

F.E.B. Bayon & B.M.G. Sambrano

PNOC Energy Development Corporation

Abstract

Return of injected brine to the production sector is a common problem encountered in operating Philippine geothermal systems. This phenomenon, although providing positive contribution in the form of increased pressure support, eventually leads to partial cooling of the reservoir fluid resulting in reduced production rates. A major aim of production geochemistry monitoring is to detect early signs of injection returns for appropriate field management strategy adjustments.

In the Mindanao geothermal production field, injection returns from the Kullay and Matingao injection sinks have been identified and detected in the Marbel corridor, affecting nearby production wells. Characteristic geochemical signatures of injection returns such as increases in reservoir Cl, declines in CO₂ total discharge and quartz geothermometer with time, are exhibited by affected production wells. Accompanying shifts (enrichment) in oxygen stable isotope ratios (¹⁸O) are also observed in a number of the affected wells.