Dynamic mechanical response characteristics of coal upon exposure to KCl brine
Geomechanics and Geophysics for Geo-Energy and Geo-Resources
School of Engineering
National Natural Science Foundation of China (51804287)
During hydraulic fracturing, the coal matrix is continually exposed to injected fluids, and the resulting chemical interactions significantly impact the coal seam′s mechanical response. However, there is still a lack of quantitative evaluation and mechanistic understanding of such exposures and the associated interactions. Here we investigate the dynamic mechanical response of coal samples after exposure to different concentrations of KCl brine. To this end, the dynamic compressive strength and elastic modulus of coal are measured as a function of KCl brine concentrations (1 – 3 wt %), exposure times (24 h and 72 h), and impact velocity (4.502 – 9.504 m/s) using split Hopkinson pressure bar (SHPB) tests. Fractal analysis of the experimental observations is also performed. The results indicate that coal's dynamic mechanical properties, including dynamic compressive strength, stress–strain relationship, and typical failure mode, are closely related to the impact loading rate. Importantly, we find that the dynamic elastic modulus of the coal decreases with prolonged exposure to a higher concentration of KCl brine. Also, with the action of low salinity fracturing fluid, the fractal dimension D of the coal sample shows a downward trend. These findings will help understand the change of dynamic mechanical properties in deep coal seams during fracturing fluid injection, which is significant in preventing pre-mature fracture closure, borehole collapse, and gas outbursts.