Influence of different viscoelastic surfactant fracturing fluid formulations on coal permeability at different temperatures and pressures

This study systematically evaluates the adaptability of viscoelastic surfactant (VES) fracturing fluids under deep coalbed methane (CBM) reservoir conditions. The rheological properties of three VES fluids—single-chain cationic (B), Gemini cationic (C), and nano-SiO2-modified Gemini (D)—were investigated at 303.15, 323.15, and 343.15 K. Four formulations, including de-ionized water (A), were assessed for their impact on coal permeability under coupled temperature–pressure conditions. Coal samples were soaked for 12 h at each temperature and at pressures of 3, 5, and 7 MPa, followed by porosity, permeability, and x-ray diffraction (XRD) characterization. Results show that coal permeability is strongly influenced by temperature–pressure conditions and fracturing fluid formulation. Group D achieved the greatest permeability enhancement, with an increase in up to 199.18 ± 19.06% at 343.15 K and 7 MPa. Rheological analysis revealed that, unlike Groups B and C—where the consistency index decreased markedly with rising temperature—the nano-SiO2-modified Group D maintained stable consistency (10.00–13.87) and exhibited a higher elastic modulus (G′ = 16.83), indicating superior thermal stability. XRD analysis showed that the synergistic interaction between nanoparticles and micelles in Group D significantly disrupted the vertical stacking of aromatic layers within the coal matrix, resulting in the largest reduction in microcrystalline stacking height (Lc) by 23.69%. This promoted the expansion and interconnection of pores and fractures, enhancing coal permeability. The study establishes a multi-scale mechanistic relationship among fracturing fluid composition, microstructural transformation, and permeability evolution, providing theoretical support for optimizing fracturing strategies in deep CBM reservoirs.