The Hydrochemical Characteristics and Evolution Process of Coal Reservoir Drainage Water in the Fukang Mining Area

Coalbed water participates in the whole process of coalbed gas generation, migration, accumulation, and exploitation, and its hydrogeochemical characteristics play a crucial role in the exploration and development of coalbed gas in mining areas. Taking the Fukang mining area in Xinjiang as the research object, this study tested the conventional cations, anions, trace elements, pH value, and TDS of its aquifer and analyzed the hydrochemical characteristics and evolution process of coalbed gas drainage water. The results show that the drainage water of coalbed gas is generally weakly alkaline, with significant changes in TDS and ion concentrations over time. Among the cations, Na⁺ occupies the absolute position, while among the anions, HCO₃ ^- dominates. Sixteen trace elements were detected in the drainage water of coalbed gas, each with varying concentrations. The concentrations of Al, Ti, V, Mn, Cu, As, Mo, Ba, Sb, and other elements exceeded the standard. In the early stages of the drainage process, the concentrations of Na⁺, Cl^-, and HCO₃ ^-, as well as TDS, were higher near the drainage well but gradually decreased and became more uniformly distributed outward along the wellbore. The distribution trends of Mg²⁺, Ca²⁺, and SO₄ ^2- concentrations were opposite to the former. Vertically, in groundwater within 1000 m, TDS, Na⁺, Ca²⁺, Cl^-, and HCO₃ ^- gradually increased with depth, while SO₄ ^2- and Mg²⁺ showed an increasing trend followed by a decrease with depth. After reaching a burial depth of 1200 m, the TDS, Mg²⁺, and Na⁺ of the reservoir water gradually stabilized, while Ca²⁺ and Cl^- decreased with depth, and HCO₃ ^- increased with depth. The hydrochemical characteristics indicate the influence of external fresh water or upper strata water inflow on the groundwater environment with increasing depth; the drainage activities worsen the closure degree of the groundwater environment, accepting infiltration replenishment and mixing from atmospheric precipitation. The reservoir drainage activities enhance the circulation of groundwater, and during the infiltration process, water-rock interactions such as salt dissolution, cation sorption exchange, and desulfurization occur between groundwater and reservoir. By analyzing the ions, trace elements, and hydrogeochemical characteristics of coalbed gas drainage water, the dynamic characteristics of coalbed reservoir water drainage and the hydrochemical evolution laws were clarified, providing theoretical guidance for coalbed gas exploration and development in the Fukang mining area.