Enhanced heavy oil recovery by CO2 injection

Abstract

The main objective of this study was to experimentally evaluate the performance of CO2 injection under various conditions of pressure and temperature (variable CO2 density and oil viscosity) as an enhanced oil recovery (EOR) method for heavy oil reservoirs. The parameters that affect the amount of oil recovery by CO2 injection and its underlying mechanisms were investigated. The results of coreflood experiments showed that CO2 injection (i.e. vapour, supercritical, and liquid CO2) can enhance heavy oil recovery. Moreover, the results revealed that the presence of dissolved gas in heavy oil significantly affects the mechanisms of oil recovery. The density of CO2 is found to be a dominant factor determining the impacts of active mechanisms on the process of oil recovery. The dissolution of CO2 in heavy oil and the resultant reduction of oil viscosity is an important mechanism of oil recovery regardless of the properties of CO2. It was observed that the extraction of hydrocarbons plays a crucial role in the process of oil recovery when CO2 is a dense fluid. Additionally, it was demonstrated that the contact of dense CO2 with live heavy oil causes to the instant liberation of methane from the oil. This phenomenon leads to sharp swelling of oil and hence that enhances oil recovery by processes similar to the concept of solution gas drive. The results of compositional analysis of the oil produced in the coreflood experiments revealed that a higher quality oil than the original heavy oil in porous media is recovered by CO2 injection. An important reason for the improvement of the quality of heavy oil is due to the extraction of hydrocarbon compounds by CO2. In the visualisation experiments, it was observed that a significantly mobile oil-rich phase appeared in the CO2-rich phase as a result of oil and CO2 contact in pore spaces. The formation of this new phase is related to the extraction strength of CO2 which causes the accumulation of light and intermediate components of heavy oil in the oil phase, near the interface of oil and CO2. The dissolution of CO2 in oil further facilitates the mobility of this phase by reducing oil viscosity. The results of this study highlight that it is crucial to evaluate the performance of any process of gas injection for EHOR under the actual conditions of the reservoirs.