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

Abstract

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 (P1s) and Taiyuan Formation (P1t). 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 (δD, δ18O, and δ13CDIC). The findings indicate that the P1s 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 P1s and P1t can be attributed to interactions between water and rocks. The isotopic compositions of δD, δ18O, and δ13CDIC 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 P1t 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 P1t compared to P1s. Principal component analysis reveals a significantly higher contribution of conventional ions toward TDS content observed within the P1t coal seam compared to that of P1s coal seam, accompanied by alterations in pH control parameters. The water produced from the P1s coalbed is primarily controlled by evaporite and silicate weathering/dissolution coupled with substantial cation exchange. Conversely, the water in the P1t 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 P1s and P1t. These research findings provide a scientific foundation for assessing the development potential of CBM and optimizing extraction systems within similar CBM areas.