Acid Gas Removal from Natural Gas by Water Washing

Abstract

Projections in the future energy scenario outline an important role played by fossil fuels to meet the increasing global energy demand. A “golden age” has been recently outlined for natural gas, in particular, as the fastest growing and the cleanest of all fossil fuels. Although natural gas is mostly considered to be a “clean” fuel with respect to the emission of pollutants from its combustion, the raw natural gas found in reservoir deposits is not free of contaminants. Among the others, hydrogen sulphide and carbon dioxide are two undesired compounds, which are responsible for the sour or acidic nature of natural gas and must be removed for operational and safety reasons. Acid gas treating is typically performed in facilities built at surface locations, mainly by means of chemical absorption into aqueous amine solutions. However, subsurface technologies may allow to possibly separate the gas undesired compounds directly downhole. The high pressure encountered in this environment makes the use of water as liquid absorbent worth considering. This work investigates the possibility of acid gas removal from natural gas by downhole water washing and presents a preliminary evaluation of the performances of the process, which is assumed to be carried out in the gas production casing that can be represented as a bubble column. A previously proposed correlation for the gas holdup in this type of contacting device operated counter-currently has been used to determine the volumetric mass transfer coefficient for design purposes, considering different raw gas flow rates and inlet acid gas concentrations. By solving a simplified model of a bubble column and by using water flow rates compatible with reinjection into the reservoir, it has been found that it is possible to reduce the H2S content from the inlet concentration to the commonly accepted value to meet pipeline specifications and, depending on the inlet CO2 concentration, to perform a bulk removal of it.