3.1. Sample site and collection
A total of 23 CBM co-produced water samples were collected from the CBM wells in Shizhuangnan block in 2019.12, the sample distribution is shown in fig. 4a. All selected CBM wells have been running for more than 5 years constantly and the drainage coal seam is 3# coal. Water samples for ion and isotope analysis and 16S rRNA sequencing were collected directly from the CBM wellhead in 500 mL headspace bottle and 50 mL centrifuge tubes. 0.5 L aluminum foil gas sampling bags were used for gas sampling. During collection, the whole container was filled with the water, placed on the ice, placed in the refrigerator, and immediately sent to the laboratory. All geochemical test including iron test and isotope test were completed in 7 days.
3.2 Ion concentration test
Ion chromatography instrument ICS-1100 (Thermo) was used for the determination of anions and cations. After the water sample was injected into the instrument, it was carried by the eluent and passed through the ion separation column. The conductivity detector was used to measure and record the peak heights of them in turn. From the standard curve drawn under the same conditions, the ion content in water sample could be calculated.
The chromatographic conditions of Cl-、 NO2-、 NO3-、SO42-were as follows: the column was AG19,4×250 mm; the eluent was KOH, 20 mm; the flow rate was 1 mL/min; the column temperature was 30 ℃. The chromatographic conditions of Na+、NH4+、K+、Mg2+、Ca2+were as follows: the chromatographic column was CS12A,4×250 mm; the eluent was MSA, 20 mm; the flow rate was 1 mL/min; the column temperature was 30 ℃.