The deep eastern edge of the Ordos Basin is rich in coalbed methane, presenting great potential for development. Meanwhile, CO₂ imbibition is an important method to increase production. To study the CO₂-water-rock interactions and microstructural damage characteristics before and after supercritical carbon dioxide immersion in deep coal rocks, CO₂ imbibition experiments were conducted on these rocks by using nuclear magnetic resonance and scanning electron microscopy imaging techniques. The results showed that CO₂ imbibition leads to pore dilatation and reveals the key role of coal rock anisotropy on imbibition efficiency under different physicochemical conditions. Specifically, the immersion of CO₂ produces cracks due to the brittle action of the coal rock, as well as calcite dissolution that exacerbates crack production and expansion. Due to adsorption of CO₂, part of the coal rock becomes swollen, which leads to detachment and changed the physical properties and surface characteristics of the coal rock.

Spontaneous imbibition is an essential mechanism for recovering oil from low-permeability fractured water-driven reservoirs. To accurately capture the migration interface of oil-water two-phase flow under these conditions, this study employs phase field theory coupled with Cahn-Hilliard and Navier-Stokes equations. We conduct a numerical pore-scale investigation on countercurrent imbibition in low-permeability fractured porous media. The results show that pore-scale spontaneous imbibition can be divided into four stages. In the first stage, oil-water film is formed when oil contacts with water, and this contact line moves under the action of capillary force. In the second stage, the oil film at the end of the oil cluster ruptures to form isolated oil droplets. In the third stage, these oil droplets are surrounded by water and gradually transported outward. In the final stage, oil droplets accumulate in the fractures and are collectively expelled from the matrix. In the process of oil droplet migration, the phenomenon of sticking occurs under the influence of water extrusion and the internal structure of the matrix, which leads to the formation of residual oil. The increased complexity of open boundary and fracture development strengthens the imbibition effect by elevating the degree of spontaneous imbibition pore utilization, thus improving the oil utilization efficiency. The above findings can provide a numerical modeling reference for the study of spontaneous imbibition in fractured porous media, and at the same time, has some guiding significance for the development of low-permeability reservoirs.