Experimental Study on the Alteration in Pore Structure of Chang 7 Shale Oil Reservoirs Treated with Carbon Dioxide

Understanding the changes in the pore structure of reservoirs in the presence of CO2 is critical for carbon neutrality, especially for shale oil reservoirs with ultra-low permeability and porosity. However, studies examining the alteration in the pore structure of shale oil reservoirs that have been treated with CO2 remain limited. Thus, in this paper, nuclear magnetic resonance (NMR) and low-temperature nitrogen adsorption (LNA) technologies were employed to address this issue. The results show that the permeability and porosity of shale oil reservoirs increase after exposure to CO2. The permeability improves by 49.03%, and the porosity increases by 29.54%. The NMR results reveal that the pore structure of shale oil reservoirs is altered. Specifically, increases of 11.14%, 74.54%, and 990.02% in the presence of CO2 are observed for micropores, mesopores, and macropores, respectively. CO2 is more sensitive to macropores, followed by mesopores and micropores. Furthermore, the LNA results indicate that some small pores expand into larger pores, leading to a decrease in the number of small pores and an increase in the number of larger pores. Combining the results of NMR and LNA, it is found that the increase in big pores is the reason behind the enhancement in permeability and porosity. This paper sheds light on the change in the pore structure of shale oil reservoirs after exposure to CO2, further guiding the evaluation of CO2 storage capacity.