Influence of aperture field anisotropy on the immiscible displacement regime transition in rough fractures

Two-phase immiscible flow displacement in rock fractures is important for many subsurface engineering applications. When one fluid displaces another more viscous one, the displacement regime transitions from capillary fingering to viscous fingering with increasing flow rate. This displacement regime transition is widely investigated in rough fractures. However, how the variable aperture space of rough fractures influences the regime transition and associated mechanism remains unclear, especially for the anisotropic aperture space. In this work, we investigated the influence of the aperture field anisotropy on the regime transition via flow ratecontrolled drainage experiments and simulations. By analyzing the pore -scale fluid-fluid interface advancements, we found that the frequency of the transverse pore -filling events (TPFEs) determined by the two-way coupling dynamics between flow rate and aperture anisotropy, and thus controlled the regime transition. For transverse -correlated aperture fields, the frequency of TPFEs was enhanced, which coupled with the nonmonotonic effect of flow velocity on TPFEs, promoted the regime transition from capillary to viscous fingering with the increasing flow rate. Conversely, the longitudinal -correlated aperture fields reduced the frequency of TPFEs and suppressed the regime transition. Moreover, we obtained the theoretical models of the critical capillary number (Ca) corresponding to the regime transition as a function of aperture anisotropy. We also modified the phase diagram for fractured media by considering the aperture anisotropy. This work bridges the gap between fracture geometric attributes at the Darcy scale and pore -scale displacement processes, which provides the foundation for upscaling modeling of multiphase flow in the future.