Monitoring the movement of the cooler fluids in the deep geothermal reservoirs of Greater Tongonan geothermal field, Philippines

Two types of cooler fluids are present in the Greater Tongonan geothermal field, namely the reinjected waste brine in the Tongonan reservoir and the shallow groundwater in the Mahanagdong reservoir. The influx of reinjection fluids in the production area of Tongonan became apparent in 1990 after seven years of commercial operations of the first power plant. The wells that previously delivered highly two-phase fluids started to become liquid-saturated. As a consequence, the fluid temperatures dropped by 10 deg. C and the field enthalpy declined from 2,200 kJ/kg to 1,600 kJ/kg. The new distribution of stable isotopes shows isotopic enrichment in the western part of the field [+1.00 per mille δ18O] where waste brine injection takes place. In Mahanagdong, the field started to generate 180 MW of power in July 1997 that brought about changes to the physical and chemical characteristics of the reservoir. After about one year of commercial operation, the production sector closest to the western part of the field sustained decline in fluid temperature and salinity ultimately leading to decline in bore output. The present stable isotope distribution across the field illustrates invasion of isotopically depleted fluids from the west to the production part of the field, passing through a northwest-trending structure. (author)