Mindanao

Mindanao Geothermal Field lies within the watchful eye of Mt. Apo, the Philippines’ highest peak at 2,954 mASL. Mt. Apo is one of the several Quaternary volcanoes that cap Mindanao’s north-south trending Central Cordillera. Mt. Apo rises from the surrounding plain at 300-m elevation and coalesces with two adjoining volcanoes (Mt. Talomo and Mt. Sibulan) to form one contiguous volcanic complex more than 700 square meters in area. Eruptive products of this huge volcanic complex overlie Tertiary sedminets of the Cotabato Basin in the west and the Agusan-Davao Trough in the East. Mindanao geothermal field is divided into three sectors from southeast to northwest: Sandawa, Marbel and Matingao.The hottest part of the field occurs beneath Sandawa, where temperature of greater than 300 degrees Centigrade is measured. Below about 0 mRL, the upflowing reservoir fluid in Sandawa is an acid chloride-sulfate brine. Migration of the deep liquid from Sandawa to Marbel allows neutralization of the acidic brine.

Part of this outflow emerges directly as Cl springs in Marbel sector. For the 52 MW Mindanao-1 geothermal power plant, ten production wells were drilled, while the other 52 MW Mindanao-2 power plant is supplied by seven wells. Injection of separated water and power plant condensate is accommodated in six injection wells. A few wells drilled in the medium-temperature parts of the resource experience calcite scaling during field exploitation. Calcite deposition in well bores is a major operational problem because it diminishes production output. To prevent mineral scaling, and thus maintain power output, EDC installed a calcite inhibitor through a 1/4-inch diameter capillary tubing placed into the borehole. Since its operation in April 2000 in well APO-1D, the CIS has arrested the deposition of calcite scales and has prevented any decline in power output.