Computation of Reservoir Geochemical Conditions From Excess-Enthalpy Wellhead Samples

Abstract

Geochemical modeling of geothermal fluids as they boil, cool, react with wall rocks, precipitate scale, or react with well casing is useful for understanding the chemistry of these processes, which aids in addressing production problems involving scale, corrosion, fluid reinjection, and mixing of fluids from separate wells. Modeling of excess enthalpy well fluids has been challenging because reservoir fluids are difficult to reconstruct from wellhead samples of liquid and gas. To address this problem, we reconstruct the composition of a reservoir liquid-gas mixture, then apply equilibrium calculations at reservoir conditions of pressure and temperature to compute the partitioning of species between gas and liquid, and thereby enable determination of pH, redox state and mineral saturation qualities of the liquid. The computed liquid and gas compositions become the basis for further modeling the process of interest in the system. These modeling calculations are illustrated here for a neutral-pH well in the Mahanagdong System, Philippines.