Field and Reservoir Management Challenges in the Sustained Operation of the 125 MW Upper Mahiao GCCU Power Plant

Abstract

The Ormat designed 125 MW Upper Mahiao Geothermal Combined Cycle Unit (GCCU) power plant is the first Build-Operate-Transfe(rB OT) plant commissionedb y PNOC-EDCi n July 1996.A fter 10 years of successfu/ commercial operation, the plant will be turned-over to PNOC-EDC by BOT partner California Energy.

During the 1}-year cooperation period, field and reservoir management challenges were faced and solved by PNOC-EDC resulting to sustained operation of the power plant. Corrosive power plant condensafes disposed through carbon sfee/ pipeline is treated by causfib soda to raise the fluid pH and reduce the corros,on rate to acceptable levels. Sofds discharged in some production wells are captured through wellhead steam washing. Sufficient steam supply was ensured through conversion of idle injection wells into producers, and by drilling make-up and replacement wells in available drill pads. Excess steam supply was diverted to the other power plants through the steamline interconnection. Future expansion plan ,s to drill production wells in a peripheral pad to tap the eastern sector of the resource.

A major reseruoir management challenge is balancing the effects of injecting 300 kg/s cold condensafes from the power plant, and 180 kg/s of separated brine. Srnce both the brine and condensafe injection sinks are proximate to the production sector, injection returns were obserued in the production wells. Chemical front, flow path and rate of returns were identified and quantified through reseruoir tracing and geochemical monitoring. The high mass availability for injection due to use of air cooling by the total condensing power plant is important for recharge and long-term susfain ability of the reseruoir, and in maintaining the reseruoir pressure in the highly twophase Upper Mahiao reseruoir. However, fhis should be carefully done through optimized injection strategy and judiciously se/ected injection locations. Thus, the current strategy is to transfer condensafe injection farther north to allow sufficient reheating before injection returns reach the producing secfor. Likewise, brine injection will be transferred farther north to the currentc ondensafei njectionp ad.