Effects of supercritical CO2 on coal microstructure in VES fracturing fluid environment: Experiments and mechanisms

Hydraulic fracturing and CO2 injection are two effective methods for increasing coalbed methane (CBM) extraction. Following hydraulic fracturing, permeability of coal seams increases, which facilitates CO2 injection and further extracts CBM. The fracturing fluid residue will impact the interaction of supercritical CO2 (ScCO2) with the coal in subsequent injection of CO2. To explore the impression of ScCO2 on coal in viscoelastic surfactant (VES) fracturing fluid environment, coal samples were immersed in VES fracturing fluid and subsequently treated with ScCO2. The evolution of the microstructure was analyzed through physical adsorption, Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD) and Raman spectra. The results demonstrate that VES fracturing fluid plug the coal pores and do not elicit obvious chemical interaction with coal. ScCO2 is capable of extracting the VES fracturing fluid residue in coal pores, could potentially facilitate the backdrafting of VES fracturing fluids. The effect of ScCO2 is attenuated in the presence of VES fracturing fluid residue. The present research offers a theoretical foundation for optimizing coalbed methane extraction.