Application of Stable Isotopes in Monitoring the inflow of Cooler Waters in the Mahanagdong Geothermal Field, Philippines

Abstract

The use of stable isotope with other water and gas chemistry tools has been effective in monitoring reservoir processes affecting the Mahanagdong geothermal field. The dominant reservoir process observed in the Mahanagdong field since the start of its exploitation in 1997 is the inflow of cooler fluids which include the injected brine and groundwaters. Examples of these are the following ; a) inflow of cooler groundwaters in well MG-23D believed to be coming from the Paril area; b) injection returns from MG-83 injection sink affecting wells MG- 300, MG-28D, MG-26D and MG-31D c) inflow of degassed and dilute cooler waters observed in wells MG-3D, MG-l4D, MG-30D and MG-31D believed to be condensate fluids from the nearby well, MG-9D. However, recent stable isotope data collected from the latter production wells reveal that the major component of this inflow is groundwater typical of Paril area. Only well MG-3D showed an indication of mixing with condensate type of fluids based on its shift in .s''O. To address these concerns on cooler water inflow, changes in the presenf pmduction and injection well strategies were implemented in order to sustain the operation of Mahanagdong field.