Abstract
Under high pressures of CO2 only, the pH of the waters produced by oil and gas wells was shown to depend as much upon the physical chemistry of water as upon the partial pressure of the acidifying gas. Especially, the content in buffering species, such as bicarbonate and acetate ions, and the content in saturated species, such as calcite, are of prime importance for the evolution of the pH as a fonction of the CO2 partial pressure.
In presence of CO2 and H2S, additional species have to be considered: a second acid H2S, a second buffer HS− and a second saturated salt FeS.
The various laws for the pH variations are developped for each interesting case and they are experimentally checked.
Beyond the quantitative prediction of the pH in any field conditions, the main conclusions are:
when the acidity of an aqueous solution is expressed directly as a function of the partial pressures of CO2 and H2S in the gas in equilibrium with this solution, the acid strengthes of the gases CO2 and H2S appear nearly equivalent.
in the presence of H2S, the corrosion products are liable to buffer strongly the solutions used in laboratory tests, even when the volume/surface ratio would be commonly considered as "satisfactory".
when corrosion tests are supposed to represent the corrosion in sweet or sour fields, the acidity of corrosive medium and the precipitation of corrosion products cannot be reproduced simultaneously… but under the exact pressure of CO2 and H2S !
