Phase-field simulation of countercurrent spontaneous imbibition in fractured porous media with heterogeneous mixed wettability

Countercurrent spontaneous imbibition governs fracture–matrix fluid exchange in fractured reservoirs. While the influence of wettability on imbibition has been widely recognized, the pore-scale mechanisms with heterogeneous mixed wettability remain inadequately understood. In this study, a pore-scale phase-field simulation of countercurrent imbibition in a two-dimensional fractured porous media is conducted by coupling the Cahn–Hilliard equation with the incompressible Navier–Stokes equations. The porous matrix is a heterogeneous packing of circular grains with different sizes arranged in an equilateral triangular pattern, and an adjacent fracture provides the wetting phase continuously. For the uniformly wettability system, a critical contact angle of π/8 is identified when θ ≤ π/2. Below this threshold, very strong water-wet conditions promote early snap-off and loss of oil-phase connectivity, resulting in the decline of the final oil recovery, whereas above it, both the imbibition rate and oil recovery increase significantly as the contact angle decreases. In heterogeneous mixed wettability systems, water from the fracture preferentially invades adjacent, more water-wet regions. The water front ceases its advance upon encountering oil-wet grains or relatively wider pore throats. The spatial distribution of wettability zones is a crucial factor influencing matrix oil recovery. Even with identical contact angle values, different spatial distributions of wettability lead to pronounced differences in invasion patterns and recovery efficiency. An increasing fraction of oil-wet regions consistently lowers the matrix oil recovery. These findings provide pore-scale insight into fracture–matrix fluid transfer under heterogeneous mixed-wettability conditions and demonstrate the capability of the phase-field method to resolve complex countercurrent imbibition phenomena in fractured porous media.