Conflicting\nLong-Term CO<sub>2</sub> Effects on Shale\nOil Formations for Simultaneous CO<sub>2</sub> Sequestration and CO<sub>2</sub>‑EOR

Enhanced oil recovery (EOR) with CO<sub>2</sub> injection\nis particularly\nattractive for its potential to improve shale oil production while\nsimultaneous CO<sub>2</sub> sequestration. The effects of CO<sub>2</sub> injection on shale oil formations are multiple but also conflicting\nand long-term scale. To examine the responses of shale oil formations\nto CO<sub>2</sub> injection, we performed long-term CO<sub>2</sub> exposure experiments on an oil-bearing shale sample containing two\nstages, each stage lasting 45 days. The results showed the continuous\nCO<sub>2</sub> extraction of light and heavy hydrocarbons in the organic\nmatter of the shale sample for the two stages of CO<sub>2</sub> exposure.\nThe inorganic matter showed a decreased carbonate and increased quartz\nand clay minerals after each stage of CO<sub>2</sub> exposure. The\nincreased clay minerals resulted from the formation of smectite and\nillite. However, the formation of illite mainly occurred in the first\nstage. Pores and microfractures in the shale sample were altered by\nboth improvement and impairment effects. The improving effects originated\nfrom carbonate dissolution, while the impairment effects originated\nfrom particle-filling and clay swelling. Under these conflicting effects,\nthe porosity and permeability of the shale sample were enlarged after\neach stage of CO<sub>2</sub> exposure. The permeability enlargement\nin the second stage was extraordinary on account of new visible fractures\non the shale sample. Additionally, the adsorption trapping capacity\nof CO<sub>2</sub> in shale oil formations is considerable and could\nbe enhanced by long-term CO<sub>2</sub> exposure. Our findings contribute\nto a better understanding of long-term CO<sub>2</sub> effects on shale\noil formations for simultaneous CO<sub>2</sub> sequestration and CO<sub>2</sub>-EOR.