Providing solutions to the rapid changes induced by stressed production in Mahanagdong geothermal field, Philippines

The prevailing rapid changes in the Mahanagdong geothermal field brought about by the present commercial operations find their roots in the inherent features of host rocks and deep fluids. The thick sequence of lavas and pyroclastics confined the convective system of commercial hot fluids below 1200 m depth, where the high-pressure environment prevents boiling at depths. This prevented development of a natural steam cap and further boiling of the fluids resulting from pressure decline since the start of commercial operations. Instead of phase separation, the depressurized central part of the field invited an inflow of fluids from the over-pressured peripheral areas through the permeable structures. The inflows have reversed the short-lived rise in enthalpy from 1997 to 1998, thus resulting in a decline in the available steam. The cooler fluids from the western front are one of these inflows, the others being the injected waste brine coming from the northern and southern injection sinks. The fluids in the southern part of the field are, moreover, saturated with calcite. Since the start of commercial operations in 1997, calcite blockages have been forming inside the well bores, and this process is the major contributor to steam decline in Mahanagdong. The type of fluids in Mahanagdong influenced the development scheme of the field. The acidic fluids in the north, cooler fluids in the west and high-gas fluids in the east confined the productive sector of Mahanagdong-B to a tiny area in the central part of the field. As a consequence of this scheme, rapid pressure decline occurred in the area where densely spaced wells were drilled. Several solutions have been implemented to address the declining supply of available steam. One is the installation of an injection system to inhibit calcite deposition inside the well. Another is waste injection management by prioritizing the injection wells sited furthest from the production sector. This practice has prevented rapid returns of the injected wastewater. However, the ultimate solution to the continuing steam supply shortfall is importation of steam from the nearby Tongonan field through a 17 km-long pipeline connecting the two fields.