Effects of different water-based fracturing fluids on mechanical properties and energy evolution of deep coal under true triaxial loading

In order to investigate the impact of fracturing fluid invasion on the mechanical degradation of deep coal, coal samples treated with five promising fracturing fluids were used for true triaxial compression tests in deep conditions. Due to water–rock reactions, lubrication, and the water wedge effect, the compressive strength of coal samples decreased. The decrease was most significant for coal samples treated with hydrofluoric (HF). The total energy and elastic strain energy of the coal samples first increased and then decreased. In contrast, the dissipation energy increased gradually, and the energy dissipation ratio exhibited a U-shape, decreasing initially and then increasing, with a turning point at the peak strain. However, the inflection point was delayed to varying degrees following treatment with viscoelastic surfactant fracturing fluid (VESFF) and acidic fracturing fluid (AFF), with the delay being more pronounced for AFF. The coal samples treated with anionic VESFF and hydrochloric acid exhibited the highest peaks of total energy and elastic strain energy, along with the fastest growth rate of dissipated energy and the most rapid decrease in the energy dissipation ratio, whereas the coal samples treated with HF exhibited the opposite behavior. All coal samples primarily exhibited shear damage, with a small number of tensile cracks appearing near the shear surface in the VESFF-treated coal samples. The coal samples treated with two types of AFF exhibited shear-tensile composite damage, while the HF-treated coal samples displayed X-shaped cracks. In summary, VESFF holds significant potential for future applications in the development of deep ECBM.