Dynamic Mechanical Properties And Microscopic Damage Mechanism Of Coal Subjected To Active Water Fracturing Fluid

Accurate assessment of the dynamic mechanical response of coal seams is critical to the success of hydraulic fracturing and safe mining.When mining coal seams after gas pre-pumping,especially after active hydraulic fracturing fluid treatment,coal seams are subject to high and strong stresses,the mechanical properties of coal rock response will change,and coal seams are highly susceptible to destabilization and damage,which will directly or indirectly affect coal mine safety and coal mining methods,so it is important to study the influence of active hydraulic fracturing fluid on the dynamic mechanical properties of coal rock.In this paper,experimental research,theoretical analysis and numerical simulation are used to study coal samples from Liangbaosi mining area in Shandong province,the effect of active hydraulic fracturing fluid with different concentration,time and mode of action on the dynamic mechanical properties of coal and its micro-damage mechanism were investigated by using the improved split Hopkinson pressure bar(SHPB),and ANSYS/LS-DYNA software to investigate the improved coal The main findings of the thesis are as follows.(1)Combining the characteristics of porous and fractured coal samples and considering the composition of fracturing fluid,the weakening mechanism of the effect of active water fracturing fluid on the dynamic mechanical properties of coal samples is analyzed in terms of physicochemical corrosion and liquid bridge force effect,and the enhancement mechanism of the effect of active water fracturing fluid on the dynamic mechanical properties of coal samples is studied in terms of inertia effect,free water at the fracture tip and viscous stress.(2)The dynamic mechanical characteristics of coal after the action of active water fracturing fluid with different concentrations,action time and action mode are studied,and the experimental results show that: with the increase of loaded gas pressure and bullet impact velocity,the strain wave amplitude,dynamic strength and stress-strain values of coal in the incident rod and reflected rod become larger,and the fractal dimension of broken coal products increases nearly 44%;with the increase of active water fracturing fluid concentration,the fractal dimension of broken coal shows a decreasing The dynamic strength and elastic modulus of coal decreased with the increase of active water fracturing fluid action time and concentration and the change of action mode,but the effect was not obvious under the relatively high impact load,and the experimental results indicated that the fracture fluid should be back-drained immediately after hydraulic fracturing to reduce the retention time of fracture fluid.The FTIR changes and pore structure SEM images of coal after different impact loads were qualitatively analyzed to reveal the microscopic influence mechanism of the effect of activated water fracturing fluid on the physical properties of coal.(3)Numerical simulations of the impact damage process of the coal samples were carried out.The experimental and simulation results show that at low impact velocity(0.2MPa provided)loading,the coal specimens exhibit radial cracking gradually starting to appear in the middle and the internal stress distribution state of the samples is gradually dominated by compressive stresses;at high impact velocity(0.7MPa provided)loading,the specimens exhibit significant shear damage before complete damage,and the damage time is shortened and The effect was more pronounced.The experimentally measured stress wave curves are consistent with those plotted from the simulation results,but the fluctuations in the curves are slightly different.The improved HJC instantiation is applicable to the SHPB impact damage process in this test.