## 4 Discussion  
The Encapsulation of CaCO\({}_{3}\) Crystals by Fe(OH)\({}_{3}\) Colloid Is the Main Reason for the Decrease in Fracture Repair Efficiency Caused by the Mixed Precipitate  
The experimental results show that no matter how large the mixed amount of Fe(OH)\({}_{3}\) is, its final repair efficiency is always lower than that of the single CaCO\({}_{3}\) precipitate. To explore the underlying mechanism, the mixed precipitate on the fracture surface corresponding to Schemes 2, 3, and 4 was sampled and tested via SEM. As shown in Figure 6(a) (Scheme 2), through the energy dispersion spectrum (EDS) analysis of Ca and Fe elements, it can be seen that the crystalline substance is the CaCO\({}_{3}\) precipitate, while the debris or flake substance is the Fe(OH)\({}_{3}\) precipitate. The microstructures of the mixed precipitates corresponding to the three schemes are compared. The number of CaCO\({}_{3}\) crystals in the mixed precipitate of Scheme 2 is large and closely distributed, and Fe(OH)\({}_{3}\) only sporadically covers the CaCO\({}_{3}\) crystals or fills the spaces between the crystals. By contrast, in Scheme 3, CaCO\({}_{3}\) crystals are rare, and most are wrapped in Fe(OH)\({}_{3}\) precipitate, resulting in sparse crystal distribution. Although the number of CaCO\({}_{3}\) crystals in Scheme 4 is increased compared with Scheme 3, the volume of a single crystal is obviously smaller (the volume ratio of the two is approximately 1:6). Even if many small crystals can be tightly bonded, they cannot nucleate as a whole due to the encapsulation of Fe(OH)\({}_{3}\) precipitate. Accordingly, when the proportion of Fe(OH)\({}_{3}\) in the mixed precipitate exceeds a certain degree, its \"separation\" effect on CaCO\({}_{3}\) crystals can be highlighted. Then, the overall structural stability of the precipitate formed by mixing this scattered crystal into the colloid can ultimately affect the repair effect on the fractures. According to [23; 24], under the same conditions, the effect of a single CaCO\({}_{3}\) precipitate on fracture repair and PR is significantly better than that of the Fe(OH)\({}_{3}\) precipitate. The main reason for this is the strong adsorption-consolidation nucleation ability between CaCO\({}_{3}\) crystals. Comparatively speaking, the Fe(OH)\({}_{3}\) precipitate is mostly colloidal [25], and its ability to finally form a stable consolidated body is weak, despite its large volume. Figure 6: SEM test photos (magnified 2500 times) of mixed precipitate on the fracture surface corresponding to Schemes 2, 3, and 4: (**a**) Scheme 2, mixed precipitate, M = 1:4; (**b**) Scheme 3, mixed precipitate, M = 1:1.9; (**c**) Scheme 4, mixed precipitate, M = 1:1. When the water injection pressure increases to a certain extent, the hydraulic erosion will directly disperse its colloidal structure, ultimately restricting the effect of PR. This is the main reason why Scheme 5 could not meet the end conditions of the experiment. Therefore, when the colloidal precipitate of Fe(OH)\({}_{3}\) is mixed with the crystalline precipitate of CaCO\({}_{3}\), the colloid will give full play to its volume advantage and form a package on the crystal, directly hindering the adsorption and nucleation of the crystal and other new crystals. At the same time, cations such as Na\({}^{+}\) adsorbed on the colloid further repel the adsorption of Ca\({}^{2+}\) and crystals. In this case, the overall stability of the mixed precipitate is naturally lower than that of the single CaCO\({}_{3}\) precipitate, which will eventually reduce the efficiency of fracture repair and PR. On the other hand, by comparing Schemes 3 and 4, it is not difficult to see that a further increase in Fe(OH)\({}_{3}\) in the mixed precipitate actually improves the repair and PR efficiency. This indicates that as the mixed proportion of the Fe(OH)\({}_{3}\) precipitate increases, the overall stability of the mixed precipitate also improves. According to the statistical results in Figure 5, the final repair time of Scheme 4 is shorter than that of Scheme 3.