Impact of fracturing fluid on multi-scale mass transport in coalbed methane reservoirs

Abstract

The effect of fracturing fluid on multi-scale mass transport, including methane desorption, diffusion and percolation, was investigated on an anthracite coal from the Qinshui Basin. Methane sorption and diffusion characteristics were measured using the volumetric method. The Langmuir equation was used to fit the sorption isotherm data and unipore model to estimate the effective diffusion coefficient (De). The impairment of gas permeability, characterized by the residual resistance factor (Frr,g), was determined using the pulse-decay method. In addition, changes in pore structure and surface properties were also analyzed by SEM/EDX, low-temperature N2 adsorption and XPS. The results show that the methane adsorption capacity was reduced by 10.5% after fracturing fluid treatment. The Langmuir volume (VL) decreased while the Langmuir pressure (PL) increased, which is beneficial for gas desorption. The effective diffusion coefficient (De) exhibited a negative correlation with pressure and decreased by 23.0% to 37.6%. The gas permeability was significantly diminished and Frr,g for a macro-fractured sample was much greater than that for a micro-fractured sample. Furthermore, due to the adsorption of fracturing fluid on pore/fracture walls, the mass transport channels were narrowed and hydrophilia was enhanced. This possibly explains the reduction of gas adsorption capacity and mass transport rate in coal.