Hydrochemical characteristics and dynamic evolution of hydrochemical field for the produced water of CBM wells

Continuous long-term hydrochemical field detection is carried out to the produced water from coal bed methane (CBM) wells in the southern Qinshui Basin. The chemical characteristics and dynamic evolution of hydrochemical field for produced water of CBM wells are summarized. For the produced water from the 3# and 15# coalbeds, Na⁺+K⁺ are the main cations and and Cl^- are the main anions. The hydrochemical type is mainly of HCO₃-Cl-Na type. The pH value ranges from 7.5 to 8.6 and the salinity, from 1 149.6 mg/L to 3 617.4 mg/L. For the 3# coalbed produced water, the average δD is about -80.7‰, and δ¹⁸O, -10.4‰. For the 15# coal bed produced water, the average δD is about -83.6‰, and δ¹⁸O, -11.2‰. The relation between δD and δ¹⁸O indicate that the groundwater from the two coalbeds is of meteoric origin. The values of δD and δ¹⁸O decrease with the increasing buried depth. We can determine the supplying aquifer of the coalbed through comparing the ion compositions and the values of δD and δ¹⁸O between the coalbed water and its adjacent groundwater. There is a positive correlation between the salinity and gas production via analysising the gas production and water yield. The results are of great significance in the prediction of gas production.