| Abstract |
The Biliran Geothermal Field located on the northern tip of Leyte was the subject of geothermal investigation from 1979 to 1982 by the Philippines geothermal company PNOC-EDC and the KRTA of New Zealand. Results of the geoscientific studies lead to drilling three exploratory wells: .. BN-1 .. BN -2 .. BN -3 Wells BN-1 and BN-2 encountered neutral Cl fluids with TSi02 temperatures of 250oC and 210oC respectively. Both wells have a Cl/B ratio of 10, showing the homogeneity of the source fluid. BN-3, which was drilled closer to the Vulcan Fault, discharged acid Cl-SO4 waters having a weirbox pH of 3.0. The well reached a Clres of 2,000 mg/kg in its 15 days discharge. It has the very high SO4, Mg and Fe concentrations typical of acid fluids. The discharge, which was accompanied by high excess enthalpy (360-1403 kj/kg), delivered enough steam to generate from 5-6 MWe. However, the corrosive nature of the discharge fluid destroyed the well head and forced the organization to shut down the well. Two different conceptual models may be discerned from the interpretations of the Vulcan- Libtong system than have been set forth to date. In the first model, the heat source is a deep magmatic body beneath the Libtong-Vulcan area. The source drives a convection cell with an up-flow channel located beneath the Vulcan thermal area. The fluid in the up-flow channel contains acid derived from gases and volatiles released by the magmatic body. The acid fluid reaches the surface at the Vulcan thermal area above the central up-flow zone through faults. Within the peripheral outflow zone, extensive interaction between the fluid and host rock neutralized the originally acid fluid. The central zone covers an area of about 9 km2and has an acid and highly corrosive reservoir fluid at temperatures close to 300 |