Experimental Investigations on Repair and Permeability Reduction for Single Sandstone Fracture Using a Mixed CaCO3 and Fe(OH)3 Precipitate

In China, groundwater loss caused by underground coal mining is becoming increasingly serious. The key to groundwater restoration is to repair mining-induced water-conducting fractures (WCFs) in the overlying strata. In this study, the adsorption–consolidation sealing characteristics of chemical precipitates were used to conduct permeability reduction (PR) experiments, including adding mixed CaCO3 and Fe(OH)3 to a sandstone specimen with a single fracture at room temperature. An aqueous solution of Na2CO3 was used as the simulated groundwater, and a solution of mixed CaCl2 and FeCl2 was used as the repair reagent to simulate the water seepage conditions of a fractured rock mass. The two aqueous solutions were simultaneously injected into a single-fractured rock specimen at a constant flow rate. The experimental results show that the Fe(OH)3 colloid encapsulated CaCO3 crystals in a mixed precipitate, reducing the overall structural stability of the mixed precipitate and restricting repair and PR efficiency. However, the Fe(OH)3 precipitate had better PR efficiency in the initial stage of the experiment. Therefore, a better scheme was put forward to repair the WCF, utilizing a mixed Fe(OH)3 and CaCO3 precipitate with a molar ratio close to 1:4 in the early stage and a single CaCO3 precipitate in the later stage.