Hydrogeochemistry characteristics of produced waters from CBM wells in Southern Qinshui Basin and implications for CBM commingled development

The hydrogeochemistry characteristics of water co-produced with coalbed methane (CBM) can act as effective indicators for exploration and development of CBM. This study investigated the geochemical compositions of the water co-produced with CBM extracted from the Pennsylvanian-Permian Taiyuan Formation and Shanxi Formation in Southern Qinshui Basin (SQB), which is the most active area of CBM development in China. Water samples were collected from 27 CBM producing wells involving twenty single-layer drainage wells and seven commingled drainage wells distributed in six CBM blocks of SQB. The samples were conducted for concentration measurements of major ions and trace elements, and determinations of hydrogen (H) and oxygen (O) isotope ratios. The results show that more than 95% water samples produced from CBM wells in SQB are of Na-HCO3 type, and the remaining approximately 5% are of Na-SO4 type. The stable isotope analyses indicate that the waters co-produced with CBM predominantly have a meteoric origin; the H and O isotopic compositions of most of the produced waters are characterised by O-18 drift, in addition, some individual produced waters exhibit D drift. delta D and delta O-18 values of produced waters in the study area are relatively more negative at shallower depths and have a rising trend with the increase of burial depth of the coal seams. It shows that CBM wells with lighter H and O isotopes in the produced waters tend to produce more water during the same producing time; accordingly, the pressure drop funnels of these wells are relatively hard to form. The produced waters from different target coal seams have unique trace elements signatures, and approximately 65% trace elements in the produced waters from the lower main coal seam (No.15 coal seam) have a higher concentration than that in the produced waters from the upper main coal seam (No.3 coal seam). To identify the main source of the produced waters from commingled drainage wells, five indicative trace elements (Li, Ga, Rb, Sr and Ba) were extracted firstly and then the standard templates of indicative trace elements of produced waters from singlelayer drainage wells were established. Based on the standard templates, the produced water source and the degree of interlayer interference process of commingled drainage wells sampled in this study were analyzed qualitatively. Finally, a case study was dissected to verify the applicability of water chemistry indicators (indicative trace elements) in evaluating the feasibility of CBM commingled drainage.