| Abstract |
The natural state of Tongonan geothermal reservoir in 1983 typifies a liquid-dominated geothermal field. Deep geothermal brine with temperature of at least 220░C was present at the depth of about 800 m, with the exception of the upflow sector that hosted natural upper two-phase fluids. The distribution of chemical species across the field showed highly saline, high-gas and isotopically enriched fluids in the upflow sector; and diluted, low-gas and isotopically depleted waters in the outflow sector. From 1983 to 1995, the first power plant lying southwest of the upflow sector was operated with rated capacity of 112.5 MW. The waste brine generated from the operations was injected adjacent to the production field, and this scheme resulted to the return of cooler injected waters to the production wells starting in 1988 and peaking in 1995. Moreover, the twelve-year continuous production led to slight decline in the reservoir pressure, not only in the active production sector, but also within the entire field. Decline in pressure resulted to near-well boiling for the wells not affected by mixing of injected fluids which were indicated by increases in gas concentration and mineralization of the discharge fluids. Commercial operations in Tongonan field started to peak in 1996 from 112.5 MW to about 500 MW. The current rate of mass withdrawal further induced decline in reservoir pressure and retreat of water level to deeper parts of the reservoir. Thus, the Tongonan reservoir transformed to a thick steam zone atop deeply seated liquid-phase fluids. With the disappearance of the liquid-phase in the present discharges of most of the wells, the chemistry of the fluids became less saline but enriched with gases. The present state of the field also resulted to lesser volume of waste brine and increase in steam availability, and this condition is projected to persist in the immediate future. |