Challenges of the Mahanadong Geothermal Field in Sustaining its 10 Years of Operation

Abstract

Reservoir management challenges have been surmounted to address the decline in in-situ steam supply and sustain production in Mahanagdong during its 10-year commercial BOT operations. Short term measures included the following: (1) reconfiguration of injection well utilization by optimizing condensate and brine loading of wells nearest to the production sector and transferring the brine load to other wells: (2) mechanical workover and acidizing of wells affected by mineral blockages and installation of chemical injection system in wells with high decline rate due to calcite deposition. The injection strategy implemented resulted to sufficient reheating of the injected condensate ad brine before mixing with the deep reservoir fluids thereby providing hot recharge to the production sector and blocking the migration of cooler peripheral waters from the west.

Construction of steam line interconnection (SLI) from Tongonan to Mahanagdong was also implemented as a medium term measure to provide flexibility in steam supply between the two fields. In field drilling of M&R wells was also done to increase the Mahanagdong in-situ steam supply. Moreover, Mahanagdong0-B topping cycle plant was shut down to optimize steam utilization and facilitate lowering of the separation pressure from 1.2 to.78 MPag thereby increasing net generation by 3 Mwe and allowing the cut-in of 4 Mwe, for a total increase in MG-B capability by 7 Mwe.

As a long term measure to sustain production I Mahanagdong, PNOC-EDC has started the development of the area east of the existing production sector, with expected 40 Mwe additional steam supply, and the conversion of injection wells in the southeast injection sinks to production, with expected 20 Mwe additional steam. These development plans aim to reduce exploitation in the current producing sector, eliminate importation of steam from Tongonan, and address the decline rate in steam supply due to reservoir processes.