| Title | The Geothermal Potential of Biliran Island, Philippines |
|---|---|
| Authors | Nilo A. Apuada and Gudmundur F. Sigurjonsson |
| Year | 2008 |
| Conference | Asian Geothermal Symposium |
| Keywords | Biliran, hydrothermal system, geothermal model |
| Abstract | The Biliran Geothermal Field located on the northern tip of Leyte was the subject of geothermal investigation from 1979 to 1982 by then PNOC-EDC and KRTA of New Zealand. Results of the geoscientific studies lead to drilling three exploratory wells; BN-1, -2 and -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 weirbox pH of 3.0. The well reached a Clres of 2,000 mg/kg in its 15 days discharge. It has a 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 shutdown the well. Two different conceptual models may be discerned from the interpretations of the Vulcan-Libtong system that 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. The central zone covers an area of about 9 km2 and has an acid and highly corrosive reservoir fluid at temperatures close to 300°C. The peripheral zone covers an area of about 15 km2, it is a neutral exploitable reservoir fluid and has temperatures of 200-250°C. The second model is the same as the first with the following important exceptions: within the central up-flow zone acid fluids are limited to depths above 2000 + 300 m. Above this depth limit, strongly acidic and gas-rich steam prevails in the reservoir, the gas having accumulated beneath the cap rock over a long period of time. Below this vapor-dominated zone, the reservoir fluid is neutral and similar to the fluid within the peripheral zone. |