Zonal Seepage in Coal Seams Generated by Hydraulic Fracturing Under Gas Pressure Attenuation: Characteristics and Affecting Factors

Hydraulic fracturing causes the fracture zone phenomenon in a coal seam. The effectiveness of coalbed methane (CBM) extraction is determined by the seepage characteristics of various fracture zones under gas pressure attenuation by gas extraction in coal seams. Based on this, this paper developed a zonal seepage test device for hydraulic fracturing coal, designed a series and parallel test method for intact coal sample, microfracture coal sample and penetration fracture coal sample. This method can simulate the migration process of CBM in different disturbance areas near the wellbore in the initial stage of hydraulic fracturing (transverse seepage) and the migration process of CBM from the matrix to the wellbore in the later stage (longitudinal seepage). On this basis, the seepage characteristics of the complex zonal phenomenon produced by hydraulic fracturing under the condition of gas pressure attenuation were studied. A series and parallel permeability model combining Klinkenberg effect, expansion effect and effective stress was established. This model can well describe the series and parallel permeability variation law with gas pressure. It indicated that the adsorption expansion effect and effective stress have an impact on the coal matrix during the longitudinal seepage process, which restricts the seepage of both natural and artificial fractures. Artificial fractures are less impacted by the expansion effect and effective stress during the transverse seepage process, which makes them the primary seepage channel. Combined with the extraction data and the permeability model, the gas production trend in the Qinshui Basin is divided into two stages: transverse seepage dominant stage and longitudinal seepage dominant stage.