Stress sensitivity of permeability in high-permeability sandstone sealed with microbially-induced calcium carbonate precipitation

Microbially induced carbonate precipitation (MICP) catalyzed by S. pasteurii has attracted considerable attention as a bio-cement that can both strengthen and seal geomaterials. We investigate the stress sensitivity of permeability reduction for the initially high-permeability Berea sandstone (initial permeability ∼110 mD) under various durations of MICP-grouting treatment. The results indicate that after 2, 4, 6, 8 and 10 cycles of MICP-grouting, the permeabilities decrease incrementally by 87.9%, 60.9%, 38.8%, 17.3%, and then 5.4% compared to the pre-grouting condition. With increasing the duration of MICP-grouting, the sensitivity of permeability to changes in stress gradually decreases and becomes less hysteretic. This stress sensitivity of permeability is well represented by a power-law relationship with the coefficients representing three contrasting phases: an initial slow reduction, followed by a rapid drop, culminating in an asymptotic response. This variation behavior is closely related to the movement and dislocation of the quartz framework, which is controlled by the intergranular bio-cementation strength. Imaging by scanning electron microscopy (SEM) reveals the evolution of the stress sensitivity to permeability associated with the evolving microstructures after MICP-grouting. The initial precipitates of CaCO₃ are dispersed on the surfaces of the quartz framework and occupy the pore space, which is initially limited in controlling and reducing the displacement between particles. As the precipitates continuously accumulate, the intergranular slot-shaped pore spaces are initially bonded by bio-CaCO₃, with the bonding strength progressively enhanced with the expanding volume of bio-cementation. At this stage, the intergranular movement and dislocation caused by compaction are reduced, and the stress sensitivity of the permeability is significantly reduced. As these slot-shaped pore spaces are progressively filled by the bio-cement, the movement and dislocation caused by compaction become negligible and thus the stress sensitivity of permeability is minimized.