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ABSTRACT: The Tiwi Geothermal Reservoir: Geology, Geochemistry, and Response to Production

James R. Hoagland, John M. Bodell

The Tiwi geothermal field is located on the Bicol Peninsula of Southen Luzon in the Philippines. The field is associated with the extinct Quaternary stratovolcano Mt. Malinao, one of a chain of volcanos formed as a result of crustal subduction along the Philippine Trench to the east. The geothermal reservoir is contained within a sequence of interlayered andesite flows and pyroclastic deposits that unconformably overlie a basement complex of marine sediments, metamorphic, and intrusive rocks.

In its initial state, the Tiwi reservoir was an overpressured liquid-filled system containing near-neutral sodium chloride water at temperatures exceeding 260°C. The reservoir is partially sealed at its top and sides by hydrothermal argillic alteration products and calcite deposition. Isolated portions of the reservoir contain a corrosive acid chloride-sulfate water associated with a distinctive advanced argillic mineral assemblage.

Withdrawal of fluid for electricity generation has caused widespread boiling in the reservoir and the formation of steam zones. The resultant solids deposition in wellbores and near-wellbore formation has been mitigated by a combination of mechanical and chemical well stimulation. Mass withdrawal from the reservoir has also caused invasion of cold groundwater into the reservoir through former fluid outflow channels. During 1983-1987, several wells were flooded with cold water and ceased flowing. In response, PGI moved development drilling west to largely unaffected areas and undertook recompletion and stimulation programs. These programs effectively halted the decline in generation by 1988.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990