Geochemical characteristics and evolution model of deep-layer No. 8 coal rock water in Ordos Basin

Deep coal-rock gas wells typically characterized by high initial water production that declines or ceases in later stages, along with high salinity of the produced water. The geochemical composition of produced water carries significant geological information, reflecting gas origin, preservation conditions, water sources, and the evolutionary history of coal-rock gas accumulation and dissipation, thereby providing critical insights for the exploration and development of deep coal rock gas. Taking 25 deep coal-rock gas wells in the Ordos Basin as the research objects, experimental tests on major ions and pH values of produced water were carried out. Combined with the spatiotemporal variations of Cl^－ concentration, the original total dissolved solids (TDS) of coal-rock water is predicted, the controlling factors of high TDS were analyzed, and hydrochemical evolution model was established. The results showed that Cl^－ dominated the produced water during the stable production period, with an average content of 80 g/L, followed by Na⁺+K⁺ averaging 30 g/L. The TDS range from 68 to 303 g/L, classifying the water as a CaCl₂ type brine. The average pH is 6.26, indicating weakly acidic conditions. The TDS of the basin displays a “low−high−low−high” zoning pattern from east to west. Based on hydrogeological characteristics, the area can be divided into recharge zone, weak runoff zone, and stagnant zone. In the stagnant zone, which includes areas such as Shenmu East, Yulin East, Mizhi, Hengshan, Jingbian East, and Yonghe South, TDS reaches 200 g/L. Coal-rock water has undergone complex water-rock interactions, including halite dissolution, cation exchange, and dolomitization. The hydrochemical evolution model is established based on these processes. The high-TDS of the Benxi Formation coal-rock water is joinly controlled by the sedimentary environment, thermal maturity, and hydrochemcial processes. The tidal flat-lagoon environment during the coal accumulation period is the basis for the formation of high-TDS formation water. Subsequent evaporation and concentration, water-rock interaction, and gasification and liquid carrying further contributed to the formation of high-TDS coal-rock water.