Role of VES-based fracturing fluid on gas sorption and diffusion of coal: An experimental study of Illinois basin coal

As a kind of environmentally friendly and non-polluting fracturing fluid, the VES-based fracturing fluid is gradually promoted in the field coalbed methane (CBM) stimulation in recent year. However, the fracturing fluid residue trapped in the reservoir may have a potential negative impact on gas flow in CBM reservoirs. In this study, the fracturing fluid based on surfactant was selected to treat the coal sample. The lab experiments have been conducted including the isothermal methane sorption, the low-temperature nitrogen sorption and the ¹³C nuclear magnetic resonance (NMR) measurements. The results showed that the methane sorption capacity was modified with cleaning fluid treatment and residuals. It was found that the sorption capacity can be recovered through water washing. It is important to point out that the diffusivity of coal has been significantly damaged indicated by the paramount decrease of diffusion coefficient even after the water washing. Additionally, no significant chemical coal molecular structure changes were observed with VES-based fracturing fluid treatment based on the NMR data. The fracturing fluid changes the coal pore structure mainly because the residue of the fracturing fluid enters the coal pore system, thereby adhering on the pore wall or blocking the pore throat. Most residues entered into the coal seam will be trapped in mesopores and macropores, cause serious damage to the CBM diffusivity in the coal matrix, thereby posing an adverse effect to the CBM production.