Experimental Investigations on the Effect of CaCO 3 Precipitation on Permeability Reduction in Single Brown Sandstone Fracture

The key to groundwater restoration is to repair the mining-induced water-conducting fractures (WCF) in the overlying strata. In view of the difficulties faced by the traditional grouting sealing in solving this problem, the adsorption-consolidation sealing characteristics of chemical precipitation were used to conduct the permeability reduction experiments of CaCO 3 on brown sandstone specimens with single fracture at room temperature. The aqueous solution of Na 2 CO 3 was used as the simulated groundwater and that of CaCl 2 was used as the repair reagent to simulate the water seepage conditions of fractured rock mass. Finally, the characteristics of permeability reduction in four rock specimens with different fracture development patterns were obtained under different ion concentrations. The experimental results show that CaCO 3 could achieve significant repair effect in all the four rock specimens; no permeability rebound occurs even when the pressure difference between the inlet and outlet ends reaches up to 5 MPa and the permeability reduction amplitude is two orders of magnitude; in either experiment scheme, the permeability reduction rate displays a “slow → fast → slow” change process, which reflects the repair or blockage process of CaCO 3 crystals. To be specific, this process consists of three stages: the adsorption and rooting, the growth and consolidation; the densification and scale formation. The more tortuous the seepage path on the fracture surface becomes and the greater the roughness/unevenness is and the smaller the fracture aperture is, the higher efficiency and better effect the permeability reduction will achieve; similarly, the slower the percentage of permeability reduction per unit time declines, the higher efficiency and better effect the permeability reduction will achieve. Once the key ion concentration in the precipitation reaction exceeds a critical value, the permeability reduction efficiency will not change significantly. the significant permeability reduction effect caused by CaCO 3 precipitation usually appears in a certain pressure range, and its critical pressure value increases with the decrease of fracture aperture, but is basically not affected by ion concentration. The results can provide a reference for the restoration of aquifer ecological function by chemical precipitation for repairing mining-induced fractures in overburden.