CO2 storage potential during CO2 enhanced oil recovery in sandstone reservoirs

摘要

CO2 can be sequestered in hydrocarbon reservoirs when it is injected for enhanced oil recovery (EOR) purpose. This study aims to experimentally evaluate the CO2 sequestration potential in sandstone reservoirs during CO2-EOR projects with a focus on the effect of active forces in the reservoir. To do so, a series of core flooding experiments at reservoir conditions were conducted using horizontal and vertical systems. A real light crude oil sample together with two native sandstone cores used here that were taken from a Malaysian oil field. The experiments were performed at near-miscible condition. Four secondary CO2 flooding tests were conducted through a composite core arrangement at various flow rates in a horizontal system to investigate the effect of viscous forces on the oil recovery and also the CO2 storage. Additionally, the gravity force effect was studied by conducting a gas (CO2) assisted gravity drainage (GAGD) test in a vertical system. The results showed that the highest oil recovery at all stages of CO2 flooding was obtained during the GAGD test. Considering flooding in the horizontal system, the ultimate recovery factor (RF) was significantly improved by increasing the CO2 injection rate. Furthermore, by investigating the interplay between the governing forces during CO2 injection, it was found that a higher oil RF was obtained at a lower capillary number at early stages of the test, while the CO2 injection rate (viscous forces) should be increased after breakthrough to overcome the capillary forces and recover more oil. In terms of CO2 storage, it was useful to quantify CO2 storage efficiency and CO2 trapping mechanisms which was mainly solubility trapping. Therefore, CO2 solubility in crude oil was also measured at various pressures. The gravitational effect in the GAGD test resulted in the highest CO2 storage percentage. While in the horizontal system, the storage efficiency of CO2 decreased with increasing the injection rate. Furthermore, a dimensionless number was proposed to investigate the combined effect of active forces in addition to CO2 diffusion during CO2 flooding operations. Finally, two correlations based on the combined dimensionless number were introduced to predict oil RF and CO2 storage efficiency for near-miscible CO2 flooding processes.