Water-coal reaction and hydrochemical evolution in the process of coal metamorphism: Based on water-coal thermal simulation experiment

[Display omitted] •Hydrogeochemistry was studied by hydrous thermal simulation experiment.•Analysis of main ion sources and the relationship between ions in water products.•The δ18O and δD values vary from −5.0 ‰ to −2.8 ‰, and from −34.3 ‰ to −6.7 ‰.•The water-coal reaction in the thermogenesis can lead to 18O and D drift.•450 °C is a demarcation point between the release and migration of trace elements. The hydrochemical composition and stable isotope characteristics of coal seam water play a crucial role in the exploration and development of coalbed methane (CBM). However, most current studies are based on the produced water samples from CBM wells, lacking an in-depth understanding of the evolution mechanism of hydrochemical composition and stable isotope in coal seam water during the coal-to-hydrocarbon process through hydrous thermal simulation experiments. In this study, a large-scale closed-system hydrous thermal experiment was conducted at temperatures ranging from 250 °C to 550 °C, in intervals of 50 °C, using low-rank bituminous coal from the Hedong coalfield on the eastern margin of Ordos Basin. The evolution of conventional ions, trace elements, and stable isotopes in water products during coal-to-hydrocarbon was investigated. The results show that CO2 yields are higher than CH4 yields at temperatures ranging from 250 °C to 500 °C, and CH4 content increased significantly at 550 °C. Water ions are affected by leaching at temperatures of 250 °C–450 °C, with Ca2+ being the dominant cation. Due to sulfate dissolution and desulphurization, SO42− is the prevalent anion at temperatures of 250 °C–350 °C, while HCO3− becomes dominant at 400 °C and 450 °C. The stable hydrogen and oxygen isotopes (δD and δ18O) of water products have good correlation with Ro respectively. At 250 °C–350 °C, deionized water dissolves oxygen-containing minerals, and oxygen atoms in water and coal exchange, resulting in δ18O drift. The increase in δD value is due to the exchange of δD between hydrocarbon in coal and deionized water. At 400 °C and 450 °C, the growth rate of δ18O value slows, and a large amount of gaseous products (H2S and CH4) react with water, resulting in D drift. The contents of Li, Sr, Rb, Ba, V, and Ga do not decrease significantly from 250 °C to 400 °C. However, the contents of Sr, Rb, Ba, V, and Ga in coal products decreased except for the increase of Li content at 450 °C, indicating that these trace elements begin to release and migrate into the water.