Pore-scale study of spontaneous imbibition: Effects of boundary conditions and pore structure

Spontaneous imbibition is a fundamental process in numerous engineering and industrial applications, where both imbibition efficiency and the pore-scale distribution of fluids play a critical role. In this study, pore-scale simulations of spontaneous imbibition were performed on four types of digital rock models using the color-gradient model of the lattice Boltzmann method. The aim was to systematically investigate how imbibition boundary conditions and microscopic pore structure characteristics affect imbibition behavior. Results reveal that, compared to global metrics, such as porosity and connectivity, descriptors that capture the spatial distribution of microscopic pore structures provide a more direct and quantitative correlation between pore structure and imbibition efficiency. Specifically, digital rocks with greater pore size variability and stronger structural heterogeneity exhibit enhanced imbibition performance. Boundary conditions influence imbibition primarily through the number and spatial configuration of open faces. Increasing the number of open faces expands the initial two-phase interface area and accelerates the rupture of the non-wetting phase, thereby markedly improving imbibition efficiency across different models. In contrast, variations in the spatial location of open faces have a relatively minor impact, as imbibition remains predominantly governed by the intrinsic pore structure. This study sheds light on the pore-scale mechanisms controlling imbibition efficiency and offers theoretical insight for optimizing spontaneous imbibition in engineering applications.