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THE MANITO LOW-ENTHALPY GEOTHERMAL FIELD:
GEOLOGIC SETTING AND DEVELOPMENT HISTORY

Jimmy J. Fragata1, Romerico C. Gonzalez1,
Fidel S. See1
and F.G. Delfin, Jr.2
1
PNOC Energy Development Corp. Philippines
2School of Policy, Planning and Development, University of
Southern California, USA


 

 

 

 

ABSTRACT



The Manito geothermal system is situated in the Manito Lowlands, the northern and poorly-developed ring plain of the Pocdol volcanic range. This low-enthalpy geothermal resource has been harnessed for power generation and direct-use application, and is the first low-temperature (<220C) geothermal field currently under exploitation in the Philippines. The Western Pocdol Mountains, which directly bounds the Manito Lowlands to the south, was built by andesitic eruptions that began at least 0.5 Ma ago, A younger eruptive stage, which constructed the Eastern Pocdol Mountains, began less than 0.10 Ma and may be younger than 0.4 Ma. The entire Pocdol volcanic range is traversed by northwest-trending faults and subsidiary north-south fractures which may be related to the San Vicente-Linao Fault, a major splay of the Philippine Fault.

Unlike typical low-enthalpy systems which have inherently low (<220C) temperature, the Manito resource derives its character from its position on the northern outflow margin of a high-temperature geothermal system beneath the Eastern Pocdol Mountains. Both neutral-pH and acid chloride waters with temperatures of 198-210C are found at depths of 600 to 1000 m. The source of these fluids lies in the high-temperature (>300C) geothermal reservoir more than 12 km to the southeast. The Manito resource was never utilized since exploration drilling in 1982-1984. lt was only harnessed in 1998 to power the Manito Geothermal Livelihood Project (MGLP), an undertaking of the Department of Energy (DOE) intended to provide low-cost electricity and enhanced livelihood to Manito residents. The MGLP produces electricity through a 1.5 MWe Fuji non-condensing turbine-generator supplied by steam from well MO-2, the only neutral-pH well among the three exploration drillholes in Manito. The livelihood component is provided by a drying facility capable of processing three metric tons/day of agricultural produce. This is an example of direct application of geothermal steam where hot air for drying comes from heat exchangers fueled by water which in turn is heated by steam from either the turbine-generator or separator. Since commissioning in September, 1998, several problems related to the resource, the infrastructure facility and the business framework have limited the sustained and effective operation of MGLP. Calcite scaling in well MO-2 constituted the principal problem which led to frequent work-over of the well and consequent shutdown of the power plant and drying facility A calcite inhibition system, which injects a chemical inhibitor in MO-2, has addressed this scaling problem since its operation in October, 2000. Problems related to the breakdown of the turbine-generator and silica scaling in injection well MO-4R are being addressed as they occur. Permanent or long-term solutions to these problems will be put in place once PNOC-EDC secures a more favorable business framework for expanding MGLPs electricity output and subsidizing the operation of the drying facility.
 

 

 

 

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