A novel ECBM extraction technology based on the integration of hydraulic slotting and hydraulic fracturing

• Hydraulic slotting integrated with hydraulic fracturing is proposed.
• The circular slots control the extension direction of the fracture cracks.
• The simulation results are similar to those of the field test.
• The coal-seam disturbance range of the single drilling increases.
• The CBM extraction efficiency is significantly improved.

Because most coal mines in China have high gas content with low permeability, it is necessary to increase the efficiency of the CBM extracting technology. Enhanced coalbed methane (ECBM) recovery is a suitable option in this regard. This paper demonstrates a novel ECBM extraction technology, which involves the integration of hydraulic slotting (HS) and hydraulic fracturing (HF). The efficiency of the method is evaluated by field test. The slots produced by the HS method are used to control the extension direction of fracture cracks, and the efficiency of CBM extraction from the single drilling is improved by performing slotting drilling in coordination with fracturing drilling. The numerical software Coupled Analysis of Flow and Solid Mechanics in Rock Failure Process Analysis (RFPA2D-Flow) was used to analyse the direction of the crack propagation in the HF process. The coal-crack-propagation process, hydraulic gradient evolution, acoustic emission phenomena, and the change in the stress of coal body with the propagation of the fracturing crack were studied. The results of the study show that because the water pressure around the fracture bore is affected by the slots created, the hydraulic gradient in the horizontal direction is larger than that in the vertical direction, and the cracks more easily extend along the direction of the larger hydraulic gradient following the initiation of the cracks. After HF, a fully unloading area, a transition pressure-relief area, and the original stress area around the fracturing boreholes are formed. Results of the field test show that once the ECBM extraction is completed, the coal-seam disturbance range of the single drilling increases, and the CBM extraction efficiency is significantly improved. This novel ECBM extraction technology is a very promising method for improving the effect of the CBM extraction in the field.