| Abstract |
The Greater Tongonan geothermal field, consisting of Tongonan and Mahanagdong reservoirs, has a total plant rated capacity of 702.6 MW. The full exploitation of this field to generate electricity for the Leyte-Cebu and Leyte-Luzon grids in July 1996 and July 1997, respectively, has induced fieldwide reservoir changes that could affect the capability of the field to sustain steam production in the long term. Brine injection in the northwestern, southwestern and southern peripheries of the Tongonan reservoir was implemented since it was commissioned for production to the Leyte-Cebu grid. This strategy is necessary to provide fluid recharge to the reservoir particularly at the center of the field which hosts a shallow, two-phase zone. However, field pressure drawdown manifested by high enthalpy discharges (>2000 kJ/kg) of several production wells still persisted. As a consequence, the reservoir steam fraction in the upflow zone increased to as high as 5.0% from its baseline value close to 0%. In order to provide additional pressure support to the Tongonan reservoir and also address the environmental disposal problem, steam condensate fluids and waste brine will be injected in the northwestern and southwestern reinjection sectors of the field, respectively. In Mahanagdong, while brine injection was confined in the northern and southern sectors, there is a natural inflow of cooler, shallow groundwater (<200?C) into the northwestern part of the reservoir. After around one year of commercial operation since the field was commissioned in July 1997, decline in fluid temperatures, salinities and bore outputs were observed in some production wells. Downhole temperatures indicate the invasion of cooler fluids from the west which percolated down through a northwest trending fault structure. Feasible solutions to this problem are the isolation of the conduit channeling the cooler fluid by cement plugging the top zone in the present wells and avoiding this structural conduit in the drilling of future production wells. |