Hydrogeochemical Disparities and Constraints of Water Produced from Various Coal Seams in the Baode Block, Ordos Basin, China

The hydrogeochemical characteristics of coalbed water play a crucial role in assessing the production level of coalbed methane (CBM) due to its involvement in the entire process of CBM generation, migration, accumulation, and extraction. To investigate variations in hydrochemical characteristics and controlling factors among different coal seams, a representative CBM field (Baode block) within the Ordos basin in China was chosen as a target. We have systematically collected produced water samples from coal seams of the Permian Shanxi Formation (P(1)s) and Taiyuan Formation (P(1)t). Tests and analyses were conducted on conventional cation and anions, trace elements, pH value, total dissolved solids (TDS), stable isotopes of hydrogen and oxygen in water, and inorganic carbon (delta D, delta O-18, and delta C-13(DIC)). The findings indicate that the P(1)s coal seam primarily contains HCO3-Na type water, while the P1t coal seam consists of Cl-Na and HCO3-Na types of water. The disparity in water types between P(1)s and P1t can be attributed to interactions between water and rocks. The isotopic compositions of delta D, delta O-18, and delta C-13(DIC) suggest that the sampled coalbed waters originate from atmospheric precipitation, with subsequent microbial activity. It is suggested that TDS content along with bicarbonate concentration can serve as effective indicators for determining high productivity due to weaker hydraulic conditions and a more enclosed water environment in P(1)t coal seams; threshold values being >1000 mg/L for TDS and >10 mequiv/L for bicarbonate concentration. Additionally, microbial activity is found to be more widespread in P(1)t compared to P(1)s. Principal component analysis reveals a significantly higher contribution of conventional ions toward TDS content observed within the P(1)t coal seam compared to that of P(1)s coal seam, accompanied by alterations in pH control parameters. The water produced from the P(1)s coalbed is primarily controlled by evaporite and silicate weathering/dissolution coupled with substantial cation exchange. Conversely, the water in the P(1)t coalbed is mainly influenced by silicate weathering/dissolution as well as evaporative concentration, with a limited occurrence of cation exchange. Moreover, there are distinct disparities in ion sources between P(1)s and P(1)t. These research findings provide a scientific foundation for assessing the development potential of CBM and optimizing extraction systems within similar CBM areas.