Effects of Active Hydraulic Fracturing Fluid on the Fracture Propagation and Structural Damage of Coal: Phenomena and Mechanisms

Hydraulic fracturing is a common means of reservoir reconstruction in coalbed methane (CBM) wells, but fracturing fluid affects the mechanical properties of weak water-bearing coal seams. To explore the effects of active hydraulic fracturing fluid on coal structural damage, uniaxial compression tests of coal under different fracturing fluid saturations were carried out. Combined with elastic damage mechanics and energy theory, a damage constitutive model of coal under the coupling action of fracturing fluid-stress-strain energy is established, and the rationality and innovation of the model are evaluated. The results show that although micro-cracks have developed in the No. 3 coal seam of the Danshuo Coal Mine, most of them are filled with minerals such as calcite, resulting in poor connectivity between the fractures. The injection of fracturing fluid reduces the cementation strength of the filling body, the bonding force between the coal matrix and the overall mechanical properties of the coal samples. With increase in fracturing fluid saturation and micro-fracture activation, the coal sample brittleness weakens, the compressive strength and elastic modulus decrease linearly, and the shape of the post-peak curve tends to be complex, showing ductile failure characteristics. The principal stress determines the macroscopic fracture direction, the micro-fracture extends along the boundary of calcite–vitrinite and vitrinite–inertinite, and the fracture complexity of inertinite is lower than that of vitrinite. The uniaxial compression damage constitutive model of coal samples containing fracturing fluid based on strain energy and elastic damage mechanics is in good agreement with the experimental data. The model curve is determined by the measured mechanical parameters and the damage constitutive coefficient (n). This can reflect the coal damage constitutive relationship under the coupling action of fracturing fluid-stress-strain energy and is suitable for analyzing the stress-strain problem of coal with fracturing fluid under uniaxial compression.