Confinement-controlled infiltration–fracturing transition in two-phase flow through deformable porous media

Stable fluid displacement in porous media under confining stress underpins processes from gas extraction to geological storage. While the onset of hydraulic fracturing is well characterized, the confinement-controlled transition to stable infiltration—governed by evolving permeability and matrix stiffness—remains poorly resolved. Laboratory studies rarely combine precise stress control with pore-scale visualization of invasion patterns, and many numerical approaches neglect boundary compliance (fixed vs movable confinement) and grain-scale force chains that are critical for predicting regime transitions. Here we use a fully coupled hydromechanical pore-network–discrete element method model to capture two-way feedbacks among pore-pressure buildup, permeability evolution, and granular mechanics under fixed and movable confinement boundaries. Increasing confining stress suppresses grain deformation and shifts displacement from fracturing- to infiltration-dominated regimes. Decreasing the viscosity ratio weakens viscous forcing and further favors infiltration, whereas boundary mobility and stress anisotropy promote fracture growth and directional channeling. We introduce a composite, morphology-based classification factor with an empirical transition band (0.19–0.33) that, together with dimensionless velocity and force ratios, delineates nonlinear transition boundaries across variations in fluid viscosity, inflow rate, matrix stiffness, and permeability. Energy analysis shows that confinement and boundary mobility jointly control displacement efficiency and reservoir integrity and that fixed-boundary Hele–Shaw idealizations can underestimate displacement efficiency and fracture propensity under low confinement. The framework links pore-scale mechanisms to field-relevant efficiency–damage tradeoffs, providing guidance for injection strategies that improve displacement while minimizing reservoir damage.