Stress-dependent permeability of chemo- and bio-grout rock fractures: Implications for flow control in underground storage systems

Effective control of fracture permeability in surrounding rocks is critical for reliable performance of underground storage systems. However, fracture grouting, one of the most widely used permeability control methods, remains constrained by limited understanding of the hydromechanical behaviors of grout layers under coupled stress and flow conditions. Here, laboratory permeability experiments were conducted on granite cores that were longitudinally sawcut and rejoined using 3-mm-thick layers of polyurethane or microbially induced carbonate precipitation (MICP) grout, with and without sand infill. Steady-state water flow through the grouted fractures was measured in a triaxial vessel under controlled normal stress and inlet pressure, while circumferential extensometers recorded both elastic and inelastic compaction of the grout layers. These measurements were used to develop a physics-based model describing the permeability evolution of grouted fractures. The four grouted fractures exhibited decreasing permeability with increasing normal stress, reflecting stress-dependent closure of the grout layer. Under relatively small variations in inlet pressure, however, fracture permeability showed non-monotonic trends associated with time-dependent closure rather than stress-controlled behaviors. A stress-dominance index was therefore introduced to quantify the relative importance of stress-dependent and time-dependent mechanisms governing permeability evolution. Polyurethane, particularly when blended with sand, provided rapid sealing but remained mechanically deformable, whereas MICP formed a stiffer skeleton with lower stress sensitivity, although sand infill could reduce sealing efficiency due to interfacial effects. This study clarifies the contrasting hydromechanical responses of polyurethane- and MICP-grouted fractures and provides practical guidance for grout selection in underground leakage control applications.