Application of Gas Geochemistry to Evaluate Reservoir Processes in the Mindanao Geothermal Production Field, Philippines

Abstract

The geochemical evaluation focuses on the use of gas discharge compositions and equilibria involving H2, HZS, CH4 and C02 to estimate tempemture and vapor fraction in the msetvoir. Physical processes that occumd during the exploitation of Mindanao Geothermal Production Field (MGPF) from 1997 to 2001 are assessed by trend variations thmgh time using Fischer- Thpsch and the pyrite-magnetite reaction grid diagram. Initial state of gas equilibria at the onset of production indicates a vapor loss of 0.1 to 1% at equilibrium tempemture of 245?C in Marbel. Near zero initial steam fraction at 290- 305?C suggests near-liquid conditions in Sandawa pmfuction sectoc The variations in trends from 1998 to 2001 in AP03D and SK2D show a decrease in ôyö values by as much as 5% and a shift indicating an increase in temperature. Small dflerences in the FT-HSH values in high tempemture wells like SK3D, KN2D and KN3B suggests near equilibrium conditions with an initial steam frsction of 0.5 5%. The shift in FT-HSH trend signifies degassing of high temperature t7uids in Sandawa at 290-300?C resulting to vapor gain in nearly a// wells reaching ManSel sector. Vapor gain implies incmase in liquid contribution coming from the hotter Sandawa sector..