REPEAT MICROGRAVITY AND LEVELING SURVEYS AT LEYTE GEOTHERMAL PRODUCTION FIELD, NORTH CENTRAL LEYTE, PHILIPPINES

摘要

Leyte Geothermal Production Field (LGPF) in North Central Leyte is the largest geothermal area in the Philippines. It is comprised of two independent hydrothermal systems: the Tongonan Geothermal Field (TGF) in the north and the Mahanagdong Geothermal Field (MGF) in the south. The commissioning of additional power plants beginning 1996 to harness the full energy potential of the area led to a massive mass extraction from the field's reservoir, which to date has incurred a cumulative net mass loss of 291 Mt in TGF and 65 Mt in MGF. Microgravity measurements, in tandem with precise leveling survey were conducted over the area to determine the reservoir's response to exploitation by detecting the minute exploitation-induced changes in gravity over time. The most recent measurements were conducted in 2003. Based on Gauss's Potential Theorem and mass balance equation, the negative gravity changes from 1997 to 2003 indicate that the system (LGPF) has incurred a net mass loss of about 244 Mt and a natural recharge of about 34 Mt or 12percent of the total mass removed. The measured decreases in gravity were associated with the mass loss from the reservoir. The data was also correlated with the pressure drawdown of about 4-4.5 MPa presently experienced over TGF and MGF.