LAUSR

Water adsorption on coal plays a significant role in the process of CO2 sequestration and enhanced coalbed methane recovery. Direct evidence of how coal adsorbs water and swells, affecting permeability, is still limited. Here, we studied the impact of waterflooding on fracture networks in coal by means of in situ synchrotron X‐ray microtomography combined with permeability measurements. We demonstrated that swelling‐induced fracture closure was responsible for an order of magnitude permeability reduction after waterflooding for over 4 days. Permeability loss was found to be time dependent, following a logistic function, and about 80% permeability reduction happened in the first 24 hr. There were probably three driven forces for water uptake, including hydrodynamic forces in fractures and macropores, capillary forces in micropores, and diffusion from micropores into deeper coal matrix. Swelling of coal matrix narrowed down and even closed the fractures and as a result weakened fracture connectivity. Residual fractures were mainly mineral‐supported fractures, which have strong resistance to swelling‐induced stresses.