pH prediction in concentrated aqueous solutions under high pressure and high temperature.

摘要

pH prediction always represents a crucial step before selecting materials for use in sour oil and gas wells as regards the risk of cracking. Among the numerous parameters which determine the equilibrium pH, some of the most important are CO_2 and H_2S partial pressure (PCO_2 and PH_2S respectively), the total pressure, the ionic strength of the solution, and temperature. Most industrial softwares used by oil and gas operators present a narrow range of validity for these parameters, which makes them inappropriate for HPHT fields or for CO_2 reinjection. For example, the computer code used by the author's company is limited to approximately 50 bar of acid gases and 150°C.This paper presents improvements which allows extending the prediction validity in temperature and pressure to respectively 200-250°C and above 200 bar of acid gases, and for a ionic strength up to 5 mol.kg-1. The new model takes into account the fugacities in gas phase of CO_2 and H_2S as determined by the Soreide and Whitson formalism. The influence of water and CH_4 pressure is also taken into account up to several hundred bar. Activity coefficients in the water phase were calculated using the Pitzer model. The model was validated by comparison with experimental measurements of pH under high pressure of CO_2 and with other published predictive models. More accurate pH calculation is obtained under HPHT conditions.