pH Measurements in Aqueous CO2 Solutions under High Pressure and Temperature

Abstract In order to precise the actual acidity of the corrosive media prevailing in the oil and gas wells where CO2 corrosion is encountered, a pH meter has been built which enables pH measurements under pressures from 0.1 up to 100 MPa. The usual physical chemistry of calco-carbonic equilibria is also presented under a new form, which emphasizes better the influence of the partial pressure of CO2 as the controlling variable in the system. It turns out from both theoretical and experimental results that the pH of a production water depends as much upon its so called alkalinity as upon the level of the CO2 partial pressure. The effect of calcareous saturation or supersaturation is also emphasized. It is then shown by a survey on actual analyses of production waters that their acidity under well conditions is certainly much weaker than it is often said. This stands for both CO2 corrosion and sulfide stress cracking.