## 5 Discussion  
### Analysis of Phase Change of Dissolved Minerals  
The phase change of dissolved minerals results from chemical reactions triggered by the workover process. CBM workover operations necessitate sand flushing, requiring substantial quantities of workover fluids [36; 37]. Regardless of how scientifically formulated the workover fluid ratio is, it will differ from the water composition within the coal reservoir. Additionally, to prevent damage to the coal reservoir, inhibitors and lubricants are often added to the workover fluids, further increasing the compositional difference between the workover fluids and the original water in the coal reservoir [23; 37]. Consequently, the workover process induces a phase change in the dissolved minerals in the CBM well's output fluid. To analyze this phase change, this paper employs the Stiff diagram method [38], a hydrogeological technique for representing water composition, demonstrating that water compositions of samples from different sources--and even within the same source--can vary. Figure 9, which is based on the concentration of anions and cations in the water samples, shows that before and after the workover, the stiff diagram exhibits a triangular or inverted flag-shaped map. This represents the gas-water two-phase flow stage [38; 39], indicating that the water composition is dominated by Na\({}^{+}\), Cl\({}^{-}\), and HCO\({}_{3}\)\({}^{-}\) ions. Table 1 and Figures 4-6 also demonstrate that the dissolved mineral ions in the stable production stage are predominantly Na\({}^{+}\), Cl\({}^{-}\), and HCO\({}_{3}\)\({}^{-}\), with trace ions such as Ca\({}^{2+}\) and Mg\({}^{2+}\) being less prevalent in the disturbance stage, both before and after the workover. During the stable production stage, before and after the workover, the water composition types for the three CBM research wells were GC-06 (Na-Cl-HCO\({}_{3}\) type) as well as GC-09 and GC-10 (Na-HCO\({}_{3}\)-Cl type). As shown in Figure 9, the ion composition in the stable production stage closely resembles that of the background ions in the No. 3 coal seam of the Shanxi Formation (Figure 2) [38; 33]. Furthermore, the water composition type during the stable drainage stage suggests that the water in the No. 3 coal seam of the Shanxi Formation is not connected to the upper and lower water-bearing systems. This is because the surface water composition type is HCO\({}_{3}\)-Ca, with Ca\({}^{2+}\) as the main cation, contrasting with the Na\({}^{+}\) dominant water samples [33]. Therefore, there is minimal connection between surface water and the produced water in this area. In the Panzhuang block, surface river recharge is not the primary groundwater source, indicating a low degree of connectivity between different hydraulic systems. Consequently, the water composition changes before and after workovers can reveal the internal structural characteristics of coal reservoirs. During the disturbance drainage stages before and after the workover, ion concentrations exhibited sharp changes compared to the stable production stage. Figures 3 and 9 illustrate that the water composition type GC-06 shifted from Na-Cl-HCO\({}_{3}\) to Na-Cl. Similarly, GC-09 and GC-10 transitioned from Na-HCO\({}_{3}\)-Cl to Na-Cl-HCO\({}_{3}\), with the stiff diagrams showing a nearly funnel shape. Before the workover, disturbances caused by drainage and production system failures led to pulverized accumulation in the wellbore, significantly increasing the ion concentration in the CBM wellbore water. This is well demonstrated by the rapid rise in TDS shown in Figures 4-6. After the workover, the concentrations of ions K\({}^{+}\), Na\({}^{+}\), Cl\({}^{-}\), and TDS surged, indicating that the workover fluid had contaminated the coal reservoir, drastically altering the ion concentration in the produced water [10,23].