An Experimental Study of the Factors Impacting Asphaltene Stability in Crude Oil during Carbon Dioxide Injection in Nanopores

Carbon dioxide (CO₂) injection has been applied recently in unconventional shale reservoirs to improve oil recovery from the nano-pores. During CO₂ injection, asphaltene may begin to precipitate, and eventually deposit into the nanopores of the shale, which will result in formation damage and pore plugging. This research investigates asphaltene pore plugging in nano-pores of shale reservoirs during CO₂ injection, and its impact on formation damage and oil recovery. A novel high pressure high temperature experimental setup with multiple nano-filter membranes was designed and constructed to study asphaltene impact on oil recovery. The effect of varying CO₂ injection pressure, temperature, oil viscosity, CO₂ soaking time, and pore size was studied. Asphaltene pore plugging was observed in all experiments conducted. Increasing the pore size from 10 to 100 nm resulted in a significant decrease in oil recovery, which reached 11.2% using 10 nm and 23% using the 100 nm. Heterogeneity increased the difficulty of the oil to propagate thorough the nano-pores. This resulted in an extremely low oil recovery, reaching up to 3.44% in the most severe heterogeneity design. The soaking time is the time at which the CO₂ is allowed to interact with the oil. Increasing this interaction time resulted in an early high oil recovery, which was then reduced due to higher asphaltene pore plugging, which reached 42% for the 2 hour soaking time. Increasing CO₂ injection pressure resulted in an increase in oil recovery, with the 700 psi pressure resulting in an oil recovery of 43% of the pores, whereas the 200 psi resulted in 8.21% pore plugging. The temperature increase resulted in a significant increase in oil production rate, however asphaltene pore plugging increased with time. Higher oil viscosity had higher initial asphaltene concentration and also much lower oil mobility which resulted in extremely low oil recovery.