Rheology of high-temperature resistant fracturing fluids formed by VES, acids and oxidants for enhancing CO2 injection in deep coal seams

Abstract

To address the low efficiency of CO2 injection into deep coal seams due to coal adsorption-induced swelling and low permeability, this study proposes the use of a CO2 injection enhancement fracturing fluid (IEFF) to increase injection efficiency, which is composed of viscoelastic surfactant (quaternary ammonium Gemini surfactant, 18-3-18), organic acids (acetic acid, C2H4O2) and oxidants (hydrogen peroxide, H2O2). The rheology and micelle morphology of the IEFF were investigated in terms of temperature and substance ratios. In addition, comparative study was conducted with monomeric surfactants (STAB). The results demonstrated that compared to STAB, 18-3-18 exhibited a two orders of magnitude lower critical micelle concentration (CMC), a lower surface tension (γCMC), and formed a system with favorable rheology at lower concentrations. Moreover, the viscosity of the IEFFs increased as the substance ratio of C2H4O2 to H2O2 decreased. Inflection points in the IEFFs were observed at 313.15 K, 323.15 K, and 333.15 K, where the viscosity exceeded 25 mPa·s. At 323.15 K, a shear rate of 170 s−1, and a substance ratio of 1:1.5, the IEFF reached its maximum viscosity of 40.390 mPa·s, with a micelle size of 109.56 nm, and the intertwined wormlike micelles formed a robust network structure. The results can provide a reference for optimizing fracturing fluid and promoting the efficiency of CO2 injection into deep coal seams.