Spontaneous imbibition in coal with in-situ dynamic micro-CT imaging

The gas/fluid flowing behavior and fluid distribution under capillary forces in porous media are essential for evaluating the pore-fracture characteristics of coal. Capillary force is crucial for flow-back efficiency after hydraulic fracturing during enhanced coalbed methane (CBM) production. In this work, in situ dynamic X-ray micro computed tomography (X-ray μ-CT), field emission scanning electron microscopy (FE15 SEM) combined with mercury intrusion porosimetry (MIP) were used on two coal samples of different wettability to explore the behavior of spontaneous imbibition. The results show that both pore-fracture and mineral content could affect the imbibition behavior of coal, which the former has played a major role. The spontaneous imbibition process can be divided into 4 stages to reveal fluid flow characteristics by the fluid distribution at different time steps. The gas relative permeability of coal attenuates exponentially with imbibition time result for capillary force based on fractal theory (the gas relative permeability reduces its value from 0.72 mD to 0.01 mD for WD, 0.5 mD to 0.08 mD for WZX). A low flow-back rate of fracturing fluid would lead to severe permeability damage. The longer the return drainage time, the more severe the permeability damage.