Impact of Separated Brine Injection at the Malitbog Sector Tongonan, Philippines

Abstract

Large-scale brine injection at the Malitbog sector, Tongonan, Philippines began in mid 1996 during the commissioning of the Malitbog Fluid Collection and Disposal System (FCDS). With the subsequent operation of the 231 MWe Visayas Geothermal Power Company main plant and the 14.56 MWe ORMAT bottoming plant, brine injection has been continuous except on some occasions when the FCDS was on shutdown for its preventive maintenance and servicing (PMS). The continuing mass extraction has caused the reservoir pressures to fall, with most production wells showing uniform drawdown. The declining pressures in effect increase the potential for reinjection fluid incursion into the production sector, as the pressure difference between the production and reinjection sectors increases. This paper examines the changes in the reservoir characteristic of the nearby production sector resulting from brine injection and establishes the possible movement of the injected fluids across the field. Over the past years, changes in fluid movement across the Malitbog have been noted. Peripheral fluids in the northeast now recharge the northern Malitbog production wells initially showing evidence of injection returns. During the first quarter of 1998, thermal activity in the Bao-Banat-I thermal area waned and eventually ceased, implying possible shift in the natural flow pattern in the reservoir. Production wells close to the reinjection sink began exhibiting effects of reinjection returns in 1999. The presence of returning reinjection fluids in Malitbog however, have provided recharge to the sector. The reservoir pressures after initially showing steep decline have tapered off since 1999. Since the same period, the available brine for the second flash remained relatively stable. This proved to be beneficial to the bottoming plant as the supply of low-pressure steam stabilized. On the other hand, the decline in high-pressure steam supply is relatively minimal and manageable.