Major Geochemical Changes in the Tongonan Geothermal Reservoir (LEYTE, Philippines) During Its Twenty Years of Operation

Abstract

After 20 years of exploitation, three major reservoir processes have been experienced in the Tongonan reservoir. The first is the encroachment of cooler injected brine into the production wells. Production wells near the Tongonan-1 injection sink displayed increasing salinity with accompanying declines in geotemperatures, gas levels and discharge enthalpies. As a mitigating measure, the injection load was distributed between the two injection sectors of Tongonan. This led to the recovery of most of the production wells.

The second involved the transformation of the reservoir from a liquid to a steam-dominated field during the commissioning of additional power plants bringing the total installed capacity from 112.5 to 500 MWe. The resultant field enthalpy and the available steam increased significantly. The different sectors of the field however responded differently. The northern section continued to show increase in enthalpy because of limited recharge. In contrast, the southern section induced influx of cooler waters from the periphery of the field which is regarded as the third major reservoir process experienced in Tongonan.