Conflicting Long-Term CO2 Effects on Shale Oil Formations for Simultaneous CO2 Sequestration and CO2-EOR

Enhanced oil recovery (EOR) with CO₂ injection is particularly attractive for its potential to improve shale oil production while simultaneous CO₂ sequestration. The effects of CO₂ injection on shale oil formations are multiple but also conflicting and long-term scale. To examine the responses of shale oil formations to CO₂ injection, we performed long-term CO₂ exposure experiments on an oil-bearing shale sample containing two stages, each stage lasting 45 days. The results showed the continuous CO₂ extraction of light and heavy hydrocarbons in the organic matter of the shale sample for the two stages of CO₂ exposure. The inorganic matter showed a decreased carbonate and increased quartz and clay minerals after each stage of CO₂ exposure. The increased clay minerals resulted from the formation of smectite and illite. However, the formation of illite mainly occurred in the first stage. Pores and microfractures in the shale sample were altered by both improvement and impairment effects. The improving effects originated from carbonate dissolution, while the impairment effects originated from particle-filling and clay swelling. Under these conflicting effects, the porosity and permeability of the shale sample were enlarged after each stage of CO₂ exposure. The permeability enlargement in the second stage was extraordinary on account of new visible fractures on the shale sample. Additionally, the adsorption trapping capacity of CO₂ in shale oil formations is considerable and could be enhanced by long-term CO₂ exposure. Our findings contribute to a better understanding of long-term CO₂ effects on shale oil formations for simultaneous CO₂ sequestration and CO₂-EOR.