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The characteristics of hydrogen (H) and oxygen (O) isotopes in produced water of coalbed methane (CBM) wells have abundant geochemical indication information. On the basis of the tests of conventional ions and H and O isotopes of 20 water samples produced from CBM wells in western Guizhou, the characteristics of H and O isotopes and productivity response of the wells in the study area have been analyzed in combination with the geological background. The research shows that, if the H and O isotopic compositions of produced water from a CBM well have obvious D drift (D drift means that, when δ18O takes the same value, the measured value of δD that is higher than the theoretical value of δD is obtained from the regional atmospheric precipitation line equation), the well is likely to obtain high production, and if the H and O isotopic compositions have O drift (O drift means that, when δD takes the same value, the measured value of δ18O that is higher than the theoretical value of δ18O is obtained from the regional atmospheric precipitation line equation), the well is not likely to obtain high production. As drainage continues, the water–rock interaction gradually strengthens and the H and O isotope compositions tend to become heavier, but seasonal rainfall can cause their values to decrease. From shallow to medium-deep formations, δD and δ18O values increase continuously, and the wells with the lowest δD and δ18O values in the shallow formation have the highest average daily gas production. d′, D drift comprehensive index, is proposed, which is of universal significance. With the increase of d′, the average daily gas production decreases, and in combination with the productivity performance, the high production wells and low production wells can be distinguished. d′ of less than 0 indicates high production, which can be used as a geochemical index to evaluate potential productivity. It is proposed that δD = −50‰, d′ = 0, and Cl– = 2000 mg/L can be used as a criterion for judging whether produced water of the CBM well is polluted and δD = −25‰ and Cl– = 4000 mg/L can be used to distinguish the degree of formation of water pollution. Moreover, the identification template of the produced water source based on δD, d′, and Cl– has been established preliminarily, and three ranges, coal seam water, light polluted water, and severe polluted water, can be identified according to the template. Moreover, it is considered that the CBM production will drop as the pollution degree of produced water increases.
Wu et al. (2018) studied this question.