Macro-micro mechanical behavior research on gas hydrate bearing coal based on parallel bonding model

Discrete element simulation of triaxial compression test was carried out to study the effects of confining pressure and saturation on the macro and meso mechanical properties of gas hydrate bearing coal (GHBC). Firstly, the numerical models of three kinds of GHBC with different saturations were established by particle flow program PFC3D, with the cementation of hydrate simulated by parallel bonding model. Then, the meso mechanical parameters were calibrated by using the results of the physical triaxial test. The stress-strain curves of the physical tests agree with the simulated results, with the failure strength and strength parameter error rate within 10%, which verifies the reliability of the established numerical model. Furthermore, the triaxial tests of GHBC were performed under different confining pressures to analyze the macro and meso-mechanical characteristics such as the stress-strain curve, velocity field, contact force chain, coordination number and porosity. The results show that the discrete element simulation results agree well with the indoor test results, especially simulating the strain hardening characteristics of the sample. The velocity field reveals a higher dilatancy degree under low saturation and a higher contraction degree under the high confining pressure of the samples. There is no obvious difference in the main chain transfer direction at the failure strength under different saturations and confining pressures. The contact force, the number of contact force chains, as well as the coordination number increase with the increase of confining pressure and saturation, while the porosity tends to decrease, which enhances the strength of the samples. The friction is an important factor to maintain the stability of the meso mechanical system of the samples. The influence mechanism of saturation and confining pressure on the strength deformation and failure of GHBC is revealed from the microscale, which provides a theoretical reference for the discrete element simulation of GHBC triaxial test and for the prevention of coal and gas outburst by hydrate technology.