Micro-nano-scale pore stimulation of coalbed methane reservoirs caused by hydraulic fracturing experiments

The size, shape, and connection of pores in a coal seam after hydraulic fracturing determines coalbed methane (CBM) production. To determine the variation characteristics of micro-nano-scale pores in a CBM reservoir, hydraulic fracturing simulation experiments were conducted using Sihe and Chengzhuang coal samples. The pore structures of the coal before and after hydraulic fracturing were studied using scanning electron microscopy (SEM), liquid nitrogen absorption (LNA), and mercury intrusion porosity (MIP) measurements. After hydraulic fracturing, the number of mesopores increased significantly, which consequently caused increases in their pore volume (PV: 20.40–479.85%) and pore specific surface area (PSSA: 58.08–2490.69%). The change characteristics of macropores were controlled by the coal's mechanical properties and in-situ stress. Owing to the different mechanical properties of coal, there were two forms of pore modification by hydraulic fracturing: pore brittle fragmentation and deformation. Coal with a larger elastic modulus and smaller Poisson's ratio was found to be prone to brittle failure, and the connectivity of pores increased significantly after hydraulic fracturing. The mercury withdrawal efficiency of coal samples increased from 14.42% to 27.01–36.94% after hydraulic fracturing. Larger in-situ stress will inhibit pore expansion during hydraulic fracturing, causing further pore compression.