Study on the plastic zone in the surrounding rock of hydraulic flushing gas extraction boreholes, considering strain-softening

Previous studies studying hydraulic flushing technology for gas extraction, the surrounding rock is often simplified to an isobaric state, and the failure of coal is also analyzed using the Mohr–Coulomb model, which is inconsistent with the field. To solve this problem, we analyzed the strain-softening model and derived a formula for the distribution of plastic zones in the surrounding rock of boreholes, considering strain-softening. Theoretical analysis was conducted on the influence of the lateral stress coefficient and cohesion in the crushing zone on the plastic zone, and verified it with the numerical model. Furthermore, the distribution law of the plastic zone in the surrounding rock of hydraulic flushing boreholes, considering strain-softening and bidirectional non-axisymmetric stress, was explored by changing the borehole diameter. The research results show that the lateral stress coefficient affects the distribution pattern of the plastic zone, and the cohesion of the crushing zone affects the size of the plastic zone. As the diameter of the borehole increases, the difference between the plastic zone radius calculated by the strain-softening model and the Mohr–Coulomb model gradually increases, indicating that the larger the borehole diameter, the greater the error in the plastic zone obtained by the Mohr–Coulomb model. This indicates that the strain-softening of the surrounding rock should be considered when studying the plastic zone of the surrounding rock in a hydraulic flushing borehole under non-axisymmetric conditions. The relevant research results provide a basis for the subsequent use of numerical simulation methods to study the permeability and gas flow law of the surrounding rock in hydraulic flushing, considering strain softening.