THERMODYNAMIC ISSUES IN DOWNHOLE SOUR GAS PURIFICATION BY WATER SCRUBBING

Natural gas, which is the cleanest of all fossil fuels, allows to satisfy the growing global demand for energy while reducing greenhouse gas emissions. It is well known that raw natural gas in reservoirs contains sour components such as carbon dioxide (CO2) and hydrogen sulfide (H2S). Because of its sour nature, natural gas is subjected to sweetening processes where carbon dioxide and hydrogen sulfide are removed to meet pipeline quality standard specifications as a fuel and to avoid equipment corrosion. Traditionally, the separation of these undesired components from natural gas has been carried out in natural gas processing plants built at surface locations. More recently, methods and systems have been proposed for removing CO2 from natural gas downhole. These techniques offer two main advantages. Firstly, they avoid the transportation of the extracted raw natural gas to processing facilities. Secondly, they allow direct re-injection of the waste gas into the downhole formation for geological sequestration. At downhole conditions water scrubbing may result to be convenient for acid gas removal because the high pressure in particular enhances the solubility of the acidic components in water. A feasibility study of such a downhole gas purification process requires a deep understanding of the phase behavior of mixtures involving water and acid gases. The aim of the work is to assess the reliability of existing thermodynamic models available in commercial simulation software in predicting vapor-liquid equilibrium conditions at high pressures for the carbon dioxide-water and hydrogen sulfide-water non-hydrocarbon binaries. The Predictive-Soave-Redlich-Kwong (PSRK) Equation of State as implemented in Aspen Plus® and AQUAlibrium have been used to describe the complex phase behavior of these systems. Results have been compared with vapor-liquid equilibrium experimental data to evaluate the performances of the investigated models. Moreover, since hydrogen sulfide and carbon dioxide are known to be hydrate formers, several computer methods for hydrate calculations have been also compared to assess their accuracy in predicting the hydrate forming conditions in acid gas mixtures. The discrepancies found among the different investigated methods suggest the need of an ad-hoc prediction tool.