Response of Bao-Bamat-i Thermal Areas to Commercial Operations in Tongonan Geothermal Field, Leyte, Philippines from 1983-1996

Abstract

The chemical trends of the Bao-Banat-i hotsprings are reviewed to resolve whether these springs are the outflow of the Tongonan or Mahanagdong geothermal systems and to study the response of these springs to Tongonan I commercial operations. Mineralization of the Bao-Banat-i springs had shown declining trends from their pre-exploitation values with the onset of commercial operations in Tongonan I in 1983. This trend is attributed to the effect of decreasing reservoir pressures leading to a reduction in contribution of outflowing deep reservoir fluids to the surface discharges. However, there was no indication of reinjection fluid returns porn the Tongonan I sector to Bao-Banat-i springs based on decreasing chloride porn 3000-3500mgkg in 1981-1982 to 1983-1989 levels of 1000-2000 mgkg. Moreover, CUB ratio was maintained for most of the springs, at baseline levels of 30-35. The observed decline in CUB for some of the springs is due to increasing contribution of ground waters, which have Cl/B ratio of 5-1 0.

Tracer test using Iodine-I31 in 5R1D, which was conducted on December 1982-January 1983 yielded negative results in the Bao-Banat-i springs. This was conducted while 5RID was having reinjection load of 65 kg/s from Well 509. However, an increase in spring flowrates was noted when the load to 5RID was increased to 164 kg/s in January to September of 1983; the flow rate subsequently declined when there injection well was shut. The recent utilization of 5RID for the pre-commissioning activities of the 77 Mwe Malitbog Power Plant again reflected an increase in mass discharges of the Bao-Banat-i springs and thermal activities in the area. Moreover, this is paralleled by increase in the chloride level of the hotsprings and by the positive returns of Na-Flourescein tracer to the monitored springs, concluding that the Bao-Banat-i is outflow of Tongonan and that the springs respond only to 5R1D injection at high RI load (>140 kg/s).