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

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 degrees C to 550 degrees C, in intervals of 50 degrees 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 degrees C to 500 degrees C, and CH4 content increased significantly at 550 degrees C. Water ions are affected by leaching at temperatures of 250 degrees C-450 degrees C, with Ca2+ being the dominant cation. Due to sulfate dissolution and desulphurization, SO42- is the prevalent anion at temperatures of 250 degrees C-350 degrees C, while HCO3- becomes dominant at 400 degrees C and 450 degrees C. The stable hydrogen and oxygen isotopes (5D and 518O) of water products have good correlation with Ro respectively. At 250 degrees C-350 degrees C, deionized water dissolves oxygen-containing minerals, and oxygen atoms in water and coal exchange, resulting in delta O-18 drift. The increase in 5D value is due to the exchange of 5D between hydrocarbon in coal and deionized water. At 400 degrees C and 450 degrees C, the growth rate of 518O 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 degrees C to 400 degrees C. However, the contents of Sr, Rb, Ba, V, and Ga in coal products decreased except for the increase of Li content at 450 degrees C, indicating that these trace elements begin to release and migrate into the water. This study explains the evolution mechanism of coal seam water chemistry and isotopes driven by thermal metamorphism, and enables the applicability of coal seam water chemistry in supporting the exploration and development of thermogenic CBM.