## 4 Discussion
This is mainly because the frequency and cumulative durations of the \"flat permeability\" stage during the experiment are lower and shorter (the cumulative duration of this stage of Scheme 3 accounts for 65.7%, similar to that for the single Fe(OH)\({}_{3}\) precipitate in Scheme 5). The reason for this can be seen in the microstructure photograph shown in Figure 6. The volume of the nodule formed by a plurality of small crystals in Scheme 4 is larger than that of the single crystal in Scheme 3 (about 2-3-times). Consequently, it is relatively less affected by the barrier adsorption wrapped in the Fe(OH)\({}_{3}\) colloid. Therefore, when the water pressure is relatively high, it can still be adsorbed to the new precipitate to form a whole with it. In this case, it is difficult to prevent the permeability from changing. Accordingly, with the increase in the proportion of Fe(OH)\({}_{3}\) in the mixed precipitate, its influence on the fracture repair efficiency fluctuates and is not linear. Fe(OH)\({}_{3}\) precipitate Can Significantly Improve the Efficiency of PR in the Initial Stage of the Experiment  
The PR and PL curves shown in Figure 4 demonstrate that although the overall time spent on fracture repair by the mixed precipitate is obviously longer, mixing in the Fe(OH)\({}_{3}\) precipitate significantly improves the PR efficiency in the initial stage of the experiment. The partially enlarged view shows that the absolute permeability curves of Schemes 2 to 5 are always below the curve of Scheme 1 during the initial 4 h. This indicates that during this period, mixing in the Fe(OH)\({}_{3}\) precipitate can achieve higher PR efficiency. Specifically, within 1 h of the initial stage, the repair efficiency achieved using a single Fe(OH)\({}_{3}\) precipitate is the highest, directly proportional to the proportion of Fe(OH)\({}_{3}\) in the mixed precipitate. Then, the PR in Schemes 3 and 5 begins to slow down, and the efficiency gradually becomes lower than that of Schemes 2 and 4. As the experiment continues for nearly 2 h, inflection points appear one after another in the PR curves of Schemes 2 to 5, with the curve trend tending to be gentle and the PR rate greatly reduced. When the experiment lasts for nearly 4 h, the repair efficiency of Scheme 1 begins to overtake the others. This phenomenon may be related to the morphology of the two precipitates. Fe(OH)\({}_{3}\) can quickly occupy the fracture channel space by virtue of its large colloid volume. Because the water injection pressure is not significant in the initial stage of the experiment, this advantage is more obvious. As the fracture channel gradually narrows and the water injection pressure gradually increases, the colloidal structure is prone to instability, and the PR is naturally no longer significant. This phenomenon corresponds to the \"fluctuation/oscillation PR trend\". The statistical results in Figure 5 show that, regardless of the proportion of mixed precipitate, the absolute permeability begins to fluctuate when the water pressure reaches about 0.1 MPa. However, in the experimental scheme with the single Fe(OH)\({}_{3}\) precipitate, the absolute permeability fluctuates only when the water pressure reaches about 0.2 MPa. This indicates that under these experiment conditions,the strength of the mixed precipitate's hydraulic erosion resistance is about 0.1 MPa, while that of the Fe(OH)\({}_{3}\) precipitate is 0.2 MPa. Thus, in practical engineering applications, the Fe(OH)\({}_{3}\) precipitate can be prioritized when the repair requirements of WCFs are not high and the water pressure of the aquifer that needs to be repaired is not high. Adopting a Mixed Fe(OH)\({}_{3}\) and CaCO\({}_{3}\) Precipitate with a Molar Ratio Close to 1:4 in the Early Stage and a Single CaCO\({}_{3}\) precipitate in the Later Stage Can Achieve Better Repair Efficiency  
Fe(OH)\({}_{3}\) precipitation significantly improved PR efficiency in the initial stage of the experiment.