Coupling Effect of Temperature, Gas, and Viscoelastic Surfactant Fracturing Fluid on the Microcrystalline Structure and Mineral Composition of Deep Coal

The physical properties of coal following hydraulic fracturing serve as critical indicators for evaluating the effectiveness of reservoir permeability enhancement. Understanding these properties is essential for improving the development efficiency of deep coalbed methane reservoirs. In this study, X-ray diffraction analysis was performed on coal samples subjected to the coupled effects of temperature, gas, and fracturing fluid treatments to investigate the evolution of their microcrystalline structure and mineral composition. The results indicate that the aromatic layer spacing d002 of coal remained essentially unchanged after treatment. In contrast, the diffraction angle of peak 002, microcrystalline stacking height Lc, average number of aromatic layers Nave, and graphitization degree g exhibited significant temperature sensitivity. Higher temperatures led to increased values of these parameters, suggesting that elevated temperature enhances the integrity of the coal’s crystalline structure. The presence of fracturing fluid, however, weakened this temperature effect, whereas gas pressure exerted only a minor influence. Due to its weak acidity, the fracturing fluid dissolved carbonate and clay minerals within the coal, thereby altering the pore structure. Increased gas pressure inhibited this dissolution effect, while elevated temperature enhanced it. Additionally, ion exchange between the fracturing fluid and coal matrix contributed to changes in the pore structure. These findings provide theoretical guidance for the development and application of reservoir stimulation fluids in deep coal reservoirs.