An Assessment of the Impact of In-Field Injection at the Malitbog Sector, Tongonan, Philippines

Abstract

The onset of continuous brine injection at the Malitbog sector, Philippines with the commissioning of the 231 MWe Visayas Geothermal Power Company main plant and the 14.56 MWe ORMAT bottoming plant have increased the possibility of reinjection fluid encroachment towards the production sector. This paper examines the changes in reservoir characteristic of the nearby production sector that would identify any effect of brine injection on the individual performance of the production wells and establish the possible movement of the injected fluids across the field. Large scale brine injection began in mid 1996 during the commissioning of the Malitbog Fluid Collection and Disposal System (FCDS). Since then, brine injection has been continuous except on some occasions when the FCDS is on shutdown for its preventive maintenance and servicing (PMS). The continuing mass extraction has caused the reservoir pressures to fall, with most production and peripheral monitor wells showing uniform drawdown across the field. The declining pressures in effect increase the possibility of reinjection fluid incursion into the production sector, as the pressure difference between the production and reinjection sectors widens. Historical fluid movement of the field was reviewed and the physical as well as chemical trends of the individual production wells were evaluated. Chemical trends have indicated reinjection fluid breakthrough in four production wells but this has not been reflected in the physical parameters. Ingress of reinjection fluids as evidenced by the changes in the downhole temperature and pressure data has been noted in only one of the nearby monitor wells. In spite of the apparent dispersion of reinjection fluids, injection breakthrough in the outflow region (thermal areas) have gradually ceased. These responses imply change in reinjection fluid movement within the reservoir resulting from the continuing fall in reservoir pressures in the production sector. The lack of evidence for thermal breakthrough in the production wells suggests that injected fluid is being reheated before being discharged.