Impacts of complex boundary conditions on spontaneous imbibition in gas-water systems

Abstract

Boundary conditions play a critical role in regulating the mechanical factors and flow patterns during spontaneous imbibition. While numerous studies have examined effects of boundary conditions using cylindrical cores, the anisotropic flow behaviors induced by geometric asymmetry and the limited boundary condition types in such geometries pose challenges for quantitative analysis. Moreover, prior research has primarily focused on static boundary conditions, neglecting the impact of time-varying scenarios. To address these gaps, this study employed four cubic rock samples to investigate imbibition process of gas-water systems under 12 static and 4 time-varying boundary conditions. The work systematically explores imbibition characteristics, gas drainage mechanisms, and the combined effects of boundary conditions and permeability. The results show that imbibition process under static boundary conditions exhibits distinct initial, transition, and late stages, whereas only the initial stage is observed under time-varying conditions. However, in low-permeability rock samples, a high water injection rate results in decreasing imbibition velocity during the end-stage imbibition, due to pronounced counter-current flow. Under static boundary conditions, gas is rapidly displaced from the core despite capillary back pressure. In contrast, under time-varying conditions, gas drainage remains unaffected by capillary back pressure, because gas expulsion occurs primarily through rock surfaces exposed to air. In addition, the number of open boundaries influences imbibition recovery and velocity, following different nonlinear trends. The transition stage contributes most significantly to the total imbibition recovery, yet the relative contributions of different imbibition stages remain independent of the number of open boundaries. Furthermore, increasing water injection rate enhances imbibition velocity under time-varying boundary conditions, but this effect becomes less pronounced once the injection rate exceeds a critical threshold. Interestingly, imbibition recoveries remain consistent across all time-varying and static boundary conditions, owing to strong hydrophilic interactions and efficient gas displacement. Additionally, imbibition capacity is significantly improved when more open boundaries or higher water injection rates are coupled with greater permeability. These observations provide new insights into the distinct imbibition mechanisms under complex boundary conditions.