Capillary-dominated imbibition in heterogeneous-wet reservoirs: Mechanisms, characterizations, and critical insights

The heterogeneous-wet state within oil and gas reservoirs introduces complex fluid distribution and flow dynamics, which significantly impacts enhanced oil recovery and geo-sequestration of carbon dioxide and hydrogen. Capillary behavior in heterogeneous-wet reservoirs is governed by the interplay between wettability and pore geometry, which collectively dictates macroscopic imbibition processes, including forced, negative-pressure and spontaneous flow of the wetting phase. Despite extensive research, the mechanisms governing imbibition in heterogeneous-wet reservoirs remain insufficiently understood. This review critically examines studies on pore-filling mechanisms and imbibition behaviors, integrating both experimental and theoretical approaches, while emphasizing key scientific questions and challenges. Existing literature suggests that theoretical models of pore-scale displacement often rely on polygonal capillaries for computational simplicity. However, the practical applicability of these models is constrained by rigid assumptions about wettability parameters, limiting their relevance to real-world scenarios. Conversely, circular capillaries offer a more accurate representation of two-phase flow dominated by heterogeneous-wet states, effectively accommodating the entire range of wettability parameters. This is particularly important for capturing primary terminal meniscus characterized by dual curvatures during piston-like flow. Furthermore, while capillary pressure curves have helped identify the influential factors of forced and negative-pressure imbibition, they still struggle to provide a clear scientific explanation for the observed similarities in pressure characteristics under low oil-wet conditions. Prediction models of capillary pressure that rely on polygonal capillaries add computational complexities, emphasizing the importance of using circular capillaries in practical applications. Additionally, the complex microscale mechanisms behind induction time, multi-scale percolation transitions, counter-current imbibition, non-wetting phase trapping, ganglion dynamics, and interface roughening challenge the development of theoretical models for spontaneous imbibition.