Estimation and Measurements of pH in High Temperature and High Pressure Sour Environments

Abstract

A new technique for estimating the pH of concentrated aqueous solutions in equilibrium with high pressure H₂S and CO₂ at elevated temperature was proposed. Based on the thermodynamic investigation for solution properties, the following equation was obtained.pH=-logγ_H+{(K_{1, H,} K_{H, H,} P_H2S/γ²_±H2S)^1/2+(K_{1, C,} K_{H, C,} P_CO2/γ²_±CO2)^1/2}The physico-chemical parameters in the equation were calculated by PBILC (Principle of Balance of Identical-Like Charges) proposed by Cobble et al. or assumed to be identical with those for HCl+NaCl aqueous solutions. The calculation of the pH of high temperature and high pressure sour environments was facilitated by the of this equation.
A Nb-doped TiO2 semiconductor electrode of n-type was found to be suitable for the measurement of the pH of the environments; no interference from NaCl, H₂S and CO₂ occurred. The measured pH values agreed well with those estimated by the newly proposed technique at the temperatures up to 473 K and at partial pressures of H₂S and CO₂ up to 4 MPa. It was ascertained by pH measurements for HCl+NaCl aqueous solutions at elevated temperatures that the assumption made for physico-chemical parameters in the pH estimation was valid.