The Investigation on the Key Hydrocarbons of Crude Oil Swelling via Supercritical CO2

Because of CO2 dissolution, crude oil may expand to various degrees. It is positive for oilfield development, such as enhancing oil saturation and formation pressure energy. According to the study object, previous relevant works can be divided into two types: CO2-crude oil system and hydrocarbon component-CO2 system. However, neither of them answered the following question: which is the key or advantageous hydrocarbon for crude oil swelling due to CO2 dissolution? Based on composition analysis of crude oil from five representative oil regions in China, this paper selected eleven hydrocarbons, which had carbon numbers from six to sixteen and molecular structures of chain, cycloalkane with single or double rings, and benzene ring with single or double rings. The eleven hydrocarbons had combined with different molar fractions of CO2 into oil-gas systems for CCE (constant component expansion) experiments. And then system volume at different temperature and pressure condition had been studied. From the experiment's results, the CO2 dissolution in the hydrocarbons could swell the system volume to various degrees. The swelling factor was defined to describe the degree of hydrocarbon volume swelling. By summarizing how the hydrocarbon affects the volume expansion degree, the concept of hydrocarbon molar density was proposed. It turned out that hydrocarbon molar density had a strict linear relationship with the swelling factor. The larger the hydrocarbon molar density, the larger the swelling factor was, while hydrocarbon molar density depended on the molecular structure. Based on further research and analysis, it was proposed that advantageous hydrocarbons for crude oil-CO2 system swelling were low carbon numbers and ring structure hydrocarbons, which have small percentages in crude oil, rather than those n-alkanes. At the same time, the rule of hydrocarbon swelling as a result of CO2 dissolution was summarized and revealed. It is the first time the concept of hydrocarbon molar density has been proposed and the regulation of hydrocarbon swelling with it explained. On this basis, advantageous hydrocarbons of crude oil-CO2 system swelling are announced. The research results will be helpful to predict swelling degree because of CO2 dissolution for a given composition crude oil. Therefore, it has a theoretical significance and application value for CO2 EOR technology.