Estimation and Measurement for pH of High Pressure and High Temperature 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 enabled by using this equation.
A Nb-doped TiO₂ semiconductor electrode of n-type was well applied to measure the pH of the environments and underwent no interference with NaCl, H₂S and CO₂. The measured pH values agreed well with those estimated by the newly proposed technique at the temperature 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 physicochemical parameters in the pH estimation was valid.