Geothermal Wastewater Reinjection Trials, Mahiao Reservoir Republic of the Philippines

Abstract

Geothermal wastewater ha been reinjected into a shallow (550m) well in the Mahiao reservoir for eighteen months. The fluid is piped directed from the well head separator to the reinjection well at about 178°C and with a silica concentration in excess of 1,000 ppm. Results from the first year of reinjection trial indicate a significant increase in the well injectivity and, despite the high silica concentration, no indication that silica deposition is having any effect on the capacity of the well. The characteristics of the injection well and reservoir have been obtained from injection tests and pressure transient data using the Horner semi-log method and Ramey type-curve matching techniques. Tracer tests have failed to detect the return to the surface of any reinjected fluid, and other wells and thermal springs in the are are apparently unaffected. The trial is continuing and planned additional tests will investigate the effects of injecting cooler fluid with a far higher degree of silica supersaturation.