Interlayer interference analysis based on trace elements in water produced from coalbed methane wells: a case study of the Upper Permian coal-bearing strata, Bide-Santang Basin, western Guizhou, China

Commingling coalbed methane (CBM) production will cause interlayer interference and severely impede the efficient drainage and pressure drop in coal seams. Effective recognition of interlayer interference is important during commingling CBM production. On the basis of water samples and trace element data from CBM wells in the Bide-Santang Basin, western Guizhou, China, the trace element dynamic characteristics and their significance for recognition of interlayer interference are discussed. As drainage proceeds, the trace element concentrations in the water tend to increase. Li, Ga, Rb, Sr, and Ba were selected as characteristic trace elements and were used to detect the water source apportionment of commingling production. The trace element concentrations in water derived from deep coal seams are relatively higher than those in water from shallow coal seams. The characteristic trace element concentrations in produced water exhibit good correlation with gas production and the interlayer span (the maximum vertical distance between produced coal seams in a well) of the CBM wells. Low trace element concentrations are indicative of interlayer interference. Wells with low gas production are characterized by a longer interlayer span, higher water production, and lower trace element concentrations. Wells with high gas production characteristically display a short interlayer span, lower water production, and higher trace element concentrations in produced water. Preliminary diagrams for recognition of interlayer interference are established based on characteristic trace elements in the produced water. The upper CBM system has poor commingling compatibility with the middle and lower CBM systems; thus, the middle and lower CBM systems should be a priority for future development.