Estimating Reservoir Steam Fraction from Co2, H2s, and H2 Concentrations in the Southern Negros Geothermal Field, Philippines

Abstract

A method to estimate reservoir steam fraction using CO2 H2S, and H2 concentration in steam has been applied in the Southern Negros Geothermal Field, Philippines. The concentration of these gases in the unexploited reservoir is best approximated by thermodynamics-derived temperature functions assuming gas-mineral equilibrium as mainly controlling the gas concentrations (Arnorsson and Gunnlaugsson, 1985). The average steam fraction of the unexploited reservoir is ñ0.05%, or close to zero, which is realistic in a liquid-dominated system like the Southern Negros Geothermal Field. With prolonged steam production, pressure drawdown in the field has resulted to boiling in the reservoir. This has caused steam fraction to increase to an average of 0.23% and 0.56%, estimated from YHS and YHC respectively, by 1998. These Yestimates, independently derived from H2S-H2 and CO2-H2 pairs, especially differ in wells where oxidation of H2S is occurring due to steam condensation by meteoric waters in shallow zones, resulting to positive YHS and negative YHC. Wells that were heavily affected by RI return (thus remained liquid saturated) continue to have slightly negative steam fraction with good agreement between YHS and YHC. Other wells with excess steam manifest higher YHC than YHS, likely because of the increased influenced of gas-gas reactions taking place in the vapor-dominated zone feeding these wells.