INJECTION RETURNS IN WELL SK-2D MINDANAO GEOTHERMAL PRODUCTION FIELD, PHILIPPINES

Abstract

Since the start of large-scale exploitation of the Mindanao Geothermal Production Field in 1997, changes in temperature and chloride concentration in some production wells are observed. Well SK-2D is one of the wells that have exhibited a remarkable increase in chloride and a noticeable decline in temperature. The cause was assumed to be the invasion of injectate fluids from the injection wells.

Reservoir chloride changes in SK-2D were modeled by using a time-dependent production-injection lumped-parameter model. By matching the calculated chloride values with the observed values the fraction of injectate returning in well SK-2D is obtained. The total mass in place in SK-2D production sector is approximately 15.8x109 kg. The rapid increase of injectate fraction returning to SK-2D in the early months of exploitation clearly indicates good connection between SK-2D and injection wells MT-1RD and MT-2RD.

The fraction of injectate returning to the production sector varies depending on the injection load of Pad RA wells. The returning injectate peak to 70-75% from April 1998 to March 2000 when injection load was increased from 200 kg/s to 350 kg/s. In the succeeding months the injectate fraction declined to around 45-50% equivalent to around 145 kg/s of injectate returning to the production sector. Around 35% of the fluids produced by well SK-2D are injection returns.

The decline in reservoir temperature based on quartz geothermometer is then modeled by coupling the lumped parameter model to a one-dimensional fracture flow model. The final model that reasonably matches the thermal decline suggests a fractured zone and production zone porosity of 8-10%.