Asphaltene precipitation and deposition during CO2 injection in nano shale pore structure and its impact on oil recovery

Carbon dioxide (CO2) injection has been shown to improve oil recovery from conventional oil reservoirs, with a relatively high rate of success. Recently, it has also been applied in unconventional shale reservoirs, with hopes that it could improve oil recovery from them as well. The process proved successful in some shale plays, but failed in others. This research investigates the CO2 flow mechanism in nano-pores and its impact on asphaltene precipitation, which could lead to pore plugging and a reduction in oil recovery. Nano-composite filter membranes were used to conduct all experiments. The setup used was a specially designed filtration apparatus that could incorporate the nano filter membranes. The factors studied include the CO2 injection pressure, temperature, oil viscosity, CO2 soaking time, porous media thickness, nano-pore size, and pore size heterogeneity. Asphaltene wt% was quantified for all the experiments, both for the produced and bypassed oil. Increasing the CO2 injection pressure resulted in a higher oil recovery and a shorter CO2 breakthrough time. Also, the percentage of asphaltene in the recovered oil was higher for the higher CO2 injection pressure. Results indicated that increasing the temperature also resulted in a higher oil recovery, however, the asphaltene wt% in the bypassed oil also increased with temperature due to instability of the oil stabilizing agent, resin. It was found that the higher oil viscosity had a larger asphaltene weight percent. Increasing the thickness and heterogeneity resulted in a decrease in oil recovery and also a higher asphaltene weight percent. Increasing the nano-pore size resulted in a significantly higher oil recovery, and less pore plugging. This research investigates the flow mechanism of CO2 injection and asphaltene precipitation due to CO2 injection in nano-pores in order to better understand the main factors that will impact the success of CO2 injection in unconventional shale reservoirs.