Effect of SiO₂ nanofluid concentration on micro-mechanical weakening behavior of coal

Coalbed methane is a crucial component of unconventional natural gas, essential for energy strategies. However, its extraction is often impeded by coal and gas outbursts, which pose significant safety and operational challenges. Coal seam water injection is an effective method for preventing these outbursts, and SiO₂ nanofluids can enhance this method by improving coal seam wettability. Nevertheless, the microscopic mechanical properties of SiO₂ nanofluid-wetted coal samples remain unclear, complicating the understanding of the enhancement mechanism against outbursts. This study employed nanoindentation tests, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) to analyze the effects of SiO₂ nanofluids at concentrations of 0.5 wt%, 1.0 wt%, and 1.5 wt%, comparing them to untreated coal samples. The results reveal that SiO₂ nanofluid-modified coal samples exhibit an increased degree of softening, with hardness reduced by up to 21.51% and elastic modulus decreased by up to 27.97%. SEM images and EDX analysis show that lower concentration SiO₂ nanofluids weaken coal samples by enhancing their water absorption capacity and reducing cohesion, while higher concentration nanofluids strengthen coal samples by filling and covering microcracks, thereby limiting moisture entry. These findings provide a quantitative reference for the application of SiO₂ nanofluids and guide the selection of optimal nanofluid concentrations.