Mechanisms of CO2 huff and puff enhanced oil recovery and storage within shale nanopores

S Wang and MQ Zhang and YL Zhang and ZD Lei and QH Feng and SQ Xu and JY Zhang, CHEMICAL ENGINEERING JOURNAL, 506, 160098 (2025).

DOI: 10.1016/j.cej.2025.160098

CO2 huff and puff technology plays a vital role in enhancing shale oil recovery and achieving carbon neutrality. However, the interactions between CO2 and multi-component hydrocarbons, and their effects on oil recovery and CO2 storage, remain ambiguous. Utilizing molecular dynamics simulations, CO2 huff and puff mechanisms within shale calcite nanopores were studied accounting for a mixture of methane, n-octane, and asphalt. Before CO2 injection, adsorption layers formed near pore walls with decreasing density towards the center. Asphalt exhibited the highest interaction energy with calcite, resulting in preferential adsorption. During CO2 injection, the higher diffusion coefficient of CO2 (1.86 times that of methane) enabled rapid entry into pores. CO2 primarily relied on diffusion to displace methane and n-octane in central pores. In the soaking stage, strong CO2-calcite interactions led to CO2 preferentially adsorbing on the walls, displacing hydrocarbons through competitive adsorption. Asphalt was challenging to extract from the pores due to the stronger intermolecular forces and lower diffusion coefficient. Oil and CO2 were driven by pressure gradient and diffusion during production. Using CO2 huff and puff technology, 21.06 % of shale oil can be recovered, with recovery rates following the order of methane > n-octane > asphalt. CO2 was stored in the pore through competitive adsorption and molecular diffusion. Increasing CO2 injection pressure, reducing outlet pressure, and enlarging pore diameter improve oil recovery. This study offers deeper insights into multi-component oil recovery mechanisms within calcite nanopores and sheds light on the CO2 storage in shale reservoirs.

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