Identifying the key factor of medium-rank coalbed methane productivity with gray relational analysis: a case study in Liulin area, Ordos basin, China

Productivity and controlling factors analysis are critical for the medium-rank coalbed methane (CBM) development. It is of great importance to seek out the key factors affecting the productivity of CBM wells from complex factors. In this paper, the gray relational analysis (GRA) method was used to identify the key factor affecting the productivity of medium-rank CBM wells in Liulin area, a famous and typical medium-rank CBM area in China. Production data and parameters of 29 vertical CBM wells were analyzed. The parameters of the controlling factors were classified into geological and fracturing engineering parameters. The results of statistical analysis show that the high-yield wells in the Liulin area normally have following parameters: resource index above 40 m⁴/t, permeability above 0.25 mD, reservoir pressure below 5.3 MPa, RCI above 0.8, VSPM between 7.7 and 11 m², and VFPM between 90 and 130 m². The results of GRA indicate that the factors affecting productivity of the CBM wells are ranked sequentially decreasing as reservoir pressure, permeability, RCI, resource index, VFPM, and VSPM. It reveals that the reservoir pressure is a key factor of CBM productivity in Liulin area. Relatively low reservoir pressure and successful hydraulic fracturing are critical for CBM development in Liulin area. The results will be helpful for productivity analysis in other medium-rank CBM areas.