Effect of fracturing fluid immersion on methane adsorption/desorption of coal

Fracturing fluid intrusion into coal seam may cause serious change in the methane adsorption/desorption behaviors. Isothermal adsorption experiments of raw coal and immersed coal samples were carried out to investigate the adsorption/desorption behaviors of methane. The surface free energies of the samples were calculated from the isothermal adsorption results. The functional groups at surface of the samples were monitored by Fourier transform infrared spectroscopy. The pore structure of the coal samples was observed by scanning electron microscopy. Solid-liquid surface interactions were determined by using wetting contact angle and spontaneous imbibition methods. The results showed that the methane adsorption capacity in samples immersed with the fracturing fluids had reduced when compared with the raw coal and formation water immersion treated sample. Due to the reduction of the characteristic pressures after the fracturing fluid immersion, a greater pressure drop was required to enter the effective desorption stage. The changes of the surface free energies during desorption were greater than that of adsorption, which showed that coal had a stronger adsorption affinity to methane during desorption. The surface properties and pore structure of the coal samples had changed after the fracturing fluid immersion, which led to an obvious difference of adsorption/desorption behaviors between the samples. Properties and factors affecting methane adsorption/desorption on immersed coal were useful to understand the methane transport in coal formation after hydraulic stimulation.