Predicting the Effects of Acid Gas on Enhanced Oil Recovery in Hydrocarbon Gas Injection

Abstract

Abstract Hydrocarbon gas injection is one of the major EOR methods. Hydrocarbon gas has some similar properties with the oil in reservoir, which has no damage to the formation. In addition, there is a beneficial mass transfer between hydrocarbon gas and crude oil, when achieving miscibility, the residual oil saturation where the area is affected by hydrocarbon gas is very low. The effects of the components of hydrocarbon gas as well as its impurities on enhanced oil recovery have been researched. However, in some fields, injected gas is from the produced gas with large amounts of acid gas. The corresponding EOR effects of hydrocarbon gas with high acid gas content are rarely mentioned in the literatures. Thus it is necessary to study it when hydrocarbon gas with acid gas is injected. In this paper, acid gas consisting two common components (78.11% of H2S and 21.89% of CO2 respectively) is studied. The oil sample from typical block in K oil reservoir is selected to investigate the EOR effects of hydrocarbon gas with the above acid gas. The slim-tube experiments are performed with different percentages of acid gas in injected gas at different displacement pressures. The acid gas percentages are respectively 0%, 10%, 20%, 30%, 40%, 50%. The experimental procedure is that firstly the slim tube is saturated with oil, then it is displaced by hydrocarbon gas with one certain percentage of acid gas at constant temperature and displacement pressure. The ultimate oil recovery which is also considered to be the displacement efficiency is obtained after injecting 1.2 pore volumes of gas or when there is no oil production. Similarly, the experiments at different pressures are also conducted. The experimental results show that under the same temperature, oil composition and gas composition, the displacement efficiency increases as the displacement pressure increases. However, displacement efficiency is no longer increasing when displacement pressure increases to one certain value. This pressure point, that is to say, is minimum miscibility pressure (MMP). It means, when the pressure is higher than MMP, the effects of displacement pressure on the oil recovery would not be significant. The experimental results also indicate that under the same temperature, oil composition and displacement pressure, displacement efficiency increases as the percentage of acid gas in the injected gas increases. From the work here, MMP can be predicted. The effects of acid gas on enhanced oil recovery can also be estimated. In the oil field, all the results can be for reference in hydrocarbon gas injection with high acid gas content. In a word, this work is very meaningful, which provides a new perspective on enhancing oil recovery as well as cutting the pollution of acid gas. It is a win-win work.