Effect of high-pressure fluid injection on deformation and fracture characteristics of coal

The coupled effect of pore pressure, in-situ stress, and coal structure in coal reservoir has a significant effect on the initiation and propagation of cracks and the failure mode of coal. In this study, the coal was used as the research object, and the experiment of high-pressure fluid fracturing coal under true triaxial stress conditions was performed to investigate the crack propagation law and failure mode under the coupled action of different horizontal stress differences and fluids with different viscosities. The peak value of fluid pressure ppeak decreases with increasing intermediate principal stress σ2, and the ppeak produced by pressurized water injection is larger than that of liquid CO2 and N2. As the σ2 decreases, the degree of coal fragmentation increases. Under the lower σ2 level, the tensile cracks extending along the bedding plane and near the bedding plane are mainly formed. Under the higher σ2 level, the main fracture in the coal shows the shear failure characteristics of diagonally crossing the bedding structure, forming a larger coal mass. Under increasing pore pressure, the fracture propagation behavior includes: ① Reversal of coal particles; ② Translation of bedding caused by tensile fractures under high-pressure fluid; ③ Formation of macroscopic shear slip plane caused by shear fractures under deviator stress; and ④ arrest of shear fracture propagation at tensile fractures. The coals show obvious effective stress anisotropy during injecting fluid, and the maximum deviator stress increases accordingly, resulting in the failure of the coals. The modified crack-sliding model is used to get the evolution of the crack density parameter before the coal failure. The crack density parameter increases with increasing fluid pressure under the condition of fixed far-field stress, which is consistent with the strain evolution of coal. Because of the higher viscosity of water, the crack density parameter induced by pressurized water injection is smaller than that in the case of liquid CO2 and N2 injection. Besides, the crack density parameter decreases with increasing horizontal stress difference. The modified crack-sliding model can also better characterize the stress-strain nonlinear behavior of rocks in the accelerated dilatation stage. This indicates that the fluid with low viscosity can activate more pores and fractures. © 2022, Editorial Office of Journal of China Coal Society. All right reserved.