The chemical composition of formation water reflects the characteristics of groundwater alternation and runoff, which has a certain indicative effect on the enrichment conditions of coalbed methane. The ion characteristics of the water produced by coalbed methane wells in the study area showed that the formation water was mainly composed of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO 4 2 − and HCO 3 − ions, and contained a very small part of CO 3 2 − ions. Deep 8^# coal seam water salinity degree 35606 mg/L - 373981 mg/L, the average is 123647.842 mg/L, the water type is CaCl₂ type, belongs to the pressurized water area, the hydrogeological conditions are simple, the hydrodynamic conditions are weak, the average PH value is 5.6, and the acidity is weak.

The solubility of Na⁺ and K⁺ ions is better than that of Ca²⁺ and Mg²⁺ ions, and they are easy to enter the dissolved state, so their migration capacity is stronger than that of Ca²⁺ and Mg²⁺, and Mg²⁺ enrichment means that it is close to the water source recharge area. The enrichment of Na⁺ and Cl⁻, Ca²⁺ and Mg²⁺ are reduced as signs of good gas reservoir closure conditions. SO 4 2 − is a sign of an oxidizing environment, and if it is present in large quantities in the original coal seam water, it indicates that the sealing conditions are not good. HCO 3 − in coal seam water is buried, when organic matter exists, Na₂SO₄ is reduced in the reduction environment, and the presence of H₂S means that the storage conditions of gas reservoirs are better [9] [10] [11] . It can be seen from the relationship between total mineralization and each ion in the study area (Figure 2) that the relationship between salinity and Cl⁻ and SO 4 2 − is good, while the correlation of HCO 3 − is general, and it is speculated that there are areas with poor sealing conditions in the study area.

The distribution of minerality on the plane of the study area (Figure 3) shows high salinity in the west and north, and low mineralization in the south and east. The low salinity in the east may be related to the weak runoff zone in the east. The runoff conditions in the weak runoff area are weak, the salinity is relatively high, and the gas content is medium. The stagnation zone has the weakest runoff conditions, the highest degree of mineralization and the best gas content [12] . The results show that the salinity distribution is basically consistent with the distribution of coalbed methane enrichment, the higher the salinity, the higher the gas content, the high salinity in the western part of the work area, the relatively high gas content, the low salinity in the east, the lower the gas content, the

higher the salinity reflects the better the preservation conditions of coalbed methane and the relatively high gas content.

The hydrogeochemical characteristics of coal reservoir have important indicative significance for the enrichment and development of coalbed methane, and the high formation water metamorphic coefficient, low sodium chloride coefficient, low desulfurization coefficient, low magnesium calcium coefficient and high magnesium chloride coefficient are usually found in coalbed methane high gas content areas.

The metamorphism coefficient can reflect the degree of metamorphism of the produced water of the coal seam and indicate the enrichment characteristics of coalbed methane. The metamorphic coefficient in the study area was between 9.8 - 93.1, and the average was 27.4 (Figure 4). The larger the metamorphic coefficient, the greater the degree of water-rock action and ion exchange, the better the closure of the CBM reservoir, and the more conducive to the preservation of the CBM reservoir [9] . The coalbed methane capping conditions in the high-value area with a water metamorphism coefficient greater than 40 are good, which is conducive to the formation of coalbed methane enrichment area.

The rNa⁺/rCl⁻ value is the sodium and chlorine coefficient, which is not directly related to the coalbed methane enrichment reservoir, but can reflect the metamorphism degree of formation water and the hydrogeochemical environment of the reservoir [13] . According to the sodium and chlorine coefficient

value, the degree of metamorphism of formation water can be divided, and the smaller the value, the lower the degree of metamorphism, which is conducive to the preservation of coalbed methane reservoirs. The overall sodium chloride coefficient in the study area was less than 0.7 (Figure 5), and the data showed that the water metamorphism degree in the deep strata in this area was low, the water type was generally CaCl₂ type with low degree of metamorphism, and there were a small number of NaHCO₃ type with medium degree of metamorphism.

The ( rSO 4 2 − × 10 2 )/rCl⁻ value is the desulfurization coefficient, which can characterize one of the main parameters of groundwater desulfuric acidification, and is also an important index to reflect the openness of coal seam water [13] . The desulfurization of coalbed water generally occurs in a reducing environment. The more thorough the desulfurization effect, the smaller the desulfurization coefficient and the stronger the reduction effect, indicating that the better the closure of the formation, the more conducive to the preservation of coalbed methane (Figure 6). The area with low desulfurization coefficient in the study area has high groundwater salinity. It reflects that the formation is well closed and the water alternating effect is slow, which is conducive to the accumulation and preservation of coalbed methane. The total salinity of stratigraphic water in W4, W5, W15, W24 and W25 wells was generally greater than 11000 mg/L, and the desulfurization effect reached 100%, indicating that the preservation conditions in this area were excellent. The magnesium-calcium coefficient (rMg²⁺/rCa²⁺) reflects the degree of metamorphism of the produced water in the coal seam, and the magnesium-calcium coefficient of the coal seam water in the study area 8^# is between 0.08 - 0.79, and the average is 0.3 (Figure 7). According to research reports, when the magnesium-calcium coefficient is less than 0.30, the formation closure is good, which is conducive to the preservation of coalbed methane and the gas content increases. The low-value areas with magnesium-calcium coefficient less than 0.3 were mainly distributed in the W4 and W24 well areas, and the total salinity of formation water was generally greater than 11000 mg/L, which had good storage conditions and easy enrichment of

coalbed methane. The rCl⁻/rMg²⁺ value in the formation water is the magnesium chloride coefficient. A higher value indicates better formation water containment conditions and a stronger degree of concentration and metamorphism (Figure 8).

The direction of groundwater flow can be determined by the head, and groundwater runoff from areas with high heads to areas with low heads. In general, the calculation of the head can be obtained directly by the injection-pressure drop

experiment, but in the actual production process, the coalbed methane well should be continuously drained without pressure recovery test, this study uses the Bernoulli equation to estimate the head, using the sea level as the reference plane, the formula for finding the head height is as follows:

H = Z + P ρ g + α v 2 ρ g (1)

where: H is the total head, m;

Z is the position head, m;

P ρ g represents the pressure head, m;

α v 2 ρ g represents the average flow rate head, m.

In general, the average flow rate head is negligible, and the total head is equal to the sum of the position head and the pressure head. The pressure head can be estimated from the static night face height of the production well, and the position head is replaced by the coal seam elevation. According to the calculation results of the head, the groundwater flow equipotential map of the study area was drawn [14] . The groundwater head is between −600 m - −1300 m, the eastern head of the study area is high, the groundwater is more active, the flow direction of groundwater flows from the east to the west, and the western groundwater momentum is low, basically in the residual environment. (Figure 9)

The effect of hydrogeological conditions on coalbed methane in area X can be summarized into two aspects: hydraulic damage and hydraulic protection of coalbed methane reservoirs (Figure 10). Hydraulic protection is reflected in hydraulic sealing and hydraulic plugging to form pressurized water areas and stagnant water areas, which are conducive to coalbed methane enrichment. Hydraulic damage mainly occurs in the process of hydraulic transport and dispersion, which leads to the dispersion of coalbed methane [4] . The hydrodynamic force in area X generally showed the characteristics of strong east and weak west, and the two slope zones in the east and west were in the pressure zone and weak runoff area, which was conducive to the preservation of coalbed methane. The

whole study area slopes from east to west, and the groundwater flow direction in the area is basically consistent with the slope direction of the terrain in the area. The recharge of groundwater mainly comes from atmospheric precipitation and lateral recharge of eastern Ordovician limestone, with weak hydrodynamic force and high total mineralization, and the whole is located in the pressure-bearing zone, which is conducive to the development of coalbed methane. The distribution of hydrodynamic field is consistent with the distribution of mineralization degree and coalbed methane enrichment, and the higher the salinity, the better the preservation conditions of coalbed methane and the relatively high gas content.