pH calculation for geothermal fluids at reservoir temperature

Abstract

This paper describes a simple method of calculating pH for geothermal fluids at deep reservoir. A complete recovery of both vapor and aqueous phases at the orifice of drilled well is prerequisite for the calculation. The first step in the calculation procedure is to combine H₂O in the vapor with the aqueous phase. The charge balance of the resulting solution is expressed by Eq. (2). Then dissolve CO₂ and H₂S remaining in the vapor phase into the solution and write again an expression for charge balance, which is given by Eq. (3). A quantitative relationship between H ⁺and related species, as formulated by Eq. (4), is obtained by subtracting Eq. (2) from Eq. (3). The terms in Eq. (4) can be calculated by using dissociation constants for pH sensitive species at the presumed reservoir temperature. Solving Eq. (4) for (H ⁺), we get the pH of the geothermal fluid. This calculation procedure has been applied to the geothermal fluid from the Oku-Aizu geothermal field, Nishiyama, Fukushima Prefecture.