Evaluating the changes of sorption and diffusion behaviors of Illinois coal with various water-based fracturing fluid treatments

The water-based fracturing stimulation is commonly used for low permeability coalbed methane (CBM) reservoirs. The influence of various fracturing fluids on the gas sorption and diffusion behaviors were experimentally studied in the work. The Illinois basin coal was selected for the experimental measurements. We used slickwater, guar gel and viscoelastic surfactant (VES) fracturing fluids to treat the coal. The sorption and diffusion behaviors were measured and quantified by the volumetric sorption experiments. The microscope was employed for the coal sample surface observation to qualitatively characterize the coating and residue behavior of fracturing fluids. It was found that all types of fracturing fluids can significantly influence the gas sorption capacity of coal. Apart from the low concentration of polyacrylamide (0.1% and 0.3%) slickwater, all other fracking fluids damage the gas sorption capacity. For most fracturing fluids, the sorption capacity can be recovered closely back to its virgin coal condition with extensive water washing. For the coal samples treated by the VES fracturing fluid, both the Langmuir volume and Langmuir pressure were dramatically increased. The adverse effect of fracturing fluids on coal is apparent and all diffusion coefficients decrease with the treatment. Even with extensive washing, the diffusion coefficient cannot recover for slickwater and VES fluids. But guar gel fluid has the best diffusion recovery because of the easy removal of the guar gel. This study provides a preliminary data set for fracturing fluid selection for Illinois coal in terms of sorption and diffusion.