Precision Gravity Monitoring for Reservoir Management at the Tiwi Geothermal Field, Philippines 1979-1998

Abstract

The 15 precision gravity surveys run at the Tiwi Geothermal Field since production began in 7979 have provided important insights for the resource conceptual model and numerical reservoir simulations used for field management and evaluation of development options. Production with only partial reinjection at Tiwi has caused matrix steam saturation to increase. Resulting gravity decreases, of up to 600 microgals in the western Kap-Mat area, were interpreted by comparing them to synthetic gravity changes computed from the fluid density predictions of a 30 reservoir simulation. Approaches to improving the gravity match were evaluated with respect to geology, hydrology and engineering data using gravity time series for individual benchmarks and 2.50 gravity models. For example, gravity showed that an earlier simulation underestimated the increase in water saturation in the eastern part of the Tiwi field due to meteoric recharge. Geologically reasonable adjustments to shallow reservoir permeability improved the gravity match. Successive conceptual model improvements will continue as new gravity data are acquired and simulations are updated to address development issues.