Modeling and Experiment on Permeability of Coal with Hydraulic Fracturing by Stimulated Reservoir Volume

Because the mechanisms that cause hydraulic fracturing to increase coal seam permeability are not clear, the effective application of hydraulic fracturing has limitations. This paper presents hydraulic fracturing and gas seepage experiments using raw coal under different stress states. The results showed that fracture propagation in coal was influenced by the properties of natural cleats and their ability to induce complex fractures. Using complex fracture abstraction and a connectivity coefficient β, a permeability model of coal subject to hydraulic fracturing with a stimulated reservoir volume (SRV) was established. The SRV effect was optimal when the pressure coefficient (R_n) was 1.46. The value of the Biot coefficient α remained unchanged at approximately 0.854 as the net pressure coefficient was changed. As R_n was increased, the stress sensitive coefficient C_f decreased and the increase in permeability caused by SRV was higher. The permeability of coal subjected to SRV was about two to three orders of magnitude higher than before fracturing. Conventional fracturing only increased the permeability to twice that before fracturing. Therefore, the influence and benefits of SRV should be considered when predicting coal-bed methane production from the permeability of coal beds.