| Abstract |
In about a year after the Mahanagdong Geothermal Field went into commercial operations in July 1997, the Mahanagdong reservoir was faced with problems such as calcite scaling and the natural inflow of groundwater coming from the northwest part of the field. The entry of groundwater as in the case of well MG-23D was marked by a decline in reservoir chloride concentration, silica (quartz) geothermometer and a corresponding increase in reservoir calcium concentration. Its output decline of 10 MWe, however, was recovered by cement plugging the identified cold-water conduit. Meanwhile, wells with calcite scaling problems were revived through mechanical clearing operations and the installation of a calcite inhibition system. In the last quarter of 1999, wells MG-3D, MG-14D, MG-30D and MG-31D, located in the northeast part of the field, showed indications of the incursion of injection returns. The transfer of brine injection from well MG-21D, to an off-field reinjection pad, MN-2RD, in the north was implemented as a countermeasure to reduce the impact of injection returns in the production wells. The countermeasure implemented, however, proved to be more detrimental to production as inflow of groundwater as a consequence of pressure drawdown became the dominant reservoir process. The incursion of groundwater into the production sector has been a major contributory factor to the significant decline in the in-situ steam supply in Mahanagdong, which declined to its current level of 135 MWe from a high of 192 MWe in March 1998. The reservoir management strategy to sustain steam production in Mahanagdong involved the shift of brine injection back to well MG-21D. Simulation runs conducted show that in-field hot brine injection could significantly reduce pressure drawdown in the center of the field consequently suppressing the entry of groundwater. Moreover, condensate injection was also moved to an off-field injection pad, southeast of the production field. The construction of a 14-kilometer steamline was also implemented to divert excess steam from Tongonan to Mahanagdong to sustain the 180 MWe production steam requirement. As a long-term strategy, the eastern sector beyond the current production block will be developed for additional steam production requirements. |