Low Production Mechanism of Coalbed Methane in Soft and Low Permeability Coal Seam of Zhaozhuang Block

In order to explore the cause of coalbed methane wells with low production in soft and low permeability coal seam of Zhaozhuang block in southern Qinshui Basin, taking No.3 coal seam as the research object. The pore-fracture development characteristic and clay mineral composition of the target coal seam and so on were analyzed by means of liquid nitrogen adsorption (BET), scanning electron microscopy (SEM) and X-ray diffraction (XRD), to ascertain the physical characteristic of soft and low permeability reservoir. The mechanism of low production of CBM in soft and low permeability reservoir was further analyzed by water sensitive injury experiment. The results showed that Zhaozhuang No.3 coal seam was mostly open-type pores, a small number of closed and impermeable pores were developed. The No.3 coal had dense pore development and the pore size was mainly small and medium. The micro-fracture was wide and long extension distance, but the density was low. The mineral filling of pore and fracture was obvious. The clay mineral composition was ammonium-bearing illite-smectite interlayer with strong water sensitivity. Therefore, Zhaozhuang No.3 coal seam resulted in poor permeability after hydraulic fracturing, and was difficult to achieve ideal permeability enhancement effect.