The Effect of Unconventional Oil Reservoirs’ Nano Pore Size on the Stability of Asphaltene During Carbon Dioxide Injection

Carbon dioxide (CO₂) has been proven to be an extremely successful enhanced oil recovery method to increase oil recovery from hydrocarbon reservoirs. It has also been proposed as a novel production method for unconventional shale reservoirs with nano pores as well. One of the main drawbacks of CO₂ injection is asphaltene precipitation and deposition, which may result in severe pore plugging, and thus a significant decrease in oil recovery. Even though asphaltene precipitation during CO₂ injection in conventional oil reservoirs has been researched extensively, not much research has been conducted to evaluate asphaltene precipitation and pore plugging in unconventional nano pores. This research investigates the impact of several factors on asphaltene precipitation and deposition, and asphaltene pore plugging in nano pores. Composite nano-filter membranes with 10, and 100 nm pore size were used to conduct all experiments. A specially designed high pressure high temperature filtration vessel was constructed and utilized to accommodate both the filter membrane, and the crude oil. The impact of varying the CO₂ injection pressure, temperature, filter membrane pore size, and the CO₂ soaking time on asphaltene deposition, and pore plugging were investigated. Results showed that higher CO₂ injection pressures resulted in a higher oil recovery, a lower asphaltene concentration in the unproduced, bypassed oil, and a higher asphaltene concentration in the produced oil compared to the lower CO₂ injection pressures. An opposite trend was observed with the temperature however, due to the temperature resulting a severe disturbance in the asphaltene thermodynamic equilibrium with the other crude oil components. Increasing the pore size resulted in a less severe asphaltene pore plugging, whereas increasing the CO₂ soaking time resulted in an increase in the asphaltene deposition and pore plugging. This research performs an experimental study to show the main factors that will impact asphaltene precipitation, deposition, and pore plugging in nano pores during CO₂ injection. This may help in improving oil recovery from CO₂ injection projects in unconventional shale reservoirs, especially those with a high asphaltene percentage in their crude oils.