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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.

To explore the influence of confining pressure on the macro-meso mechanical characteristics of gas hydrate bearing coal (GHBC) under different boundary conditions, the biaxial discrete element tests were carried out subjected to the confining pressures of 12，16 and 20 MPa for GHBC with saturation of 80%. Firstly, the biaxial numerical models of GHBC were established for flexible and rigid boundaries, using the linear model of the rolling resistance and the parallel bonding model. These numerical models incorporated the influences of particle shape effect, the hydrate cementation and the heat-shrinkable pipe. Then, the reliability of the numerical model was verified, by comparing with the indoor test results (stress-strain curves, bulk strain curves, internal friction angle cohesion and specimen failure modes). It is found that the flexible boundary can better reflect the deviatoric, stress-axial strain, the shear expansion and the strength characteristics of the sample. Based on the established numerical model, the roles of the confining pressure and the boundary condition on the macro-meso mechanical properties of GHBC were clarified from the perspectives of the internal displacement field, the mean mechanical coordination number, the mean porosity, the contact force chain and the hydrate bond failure. The results show that: ① with the increase of confining pressure, the numerical sample with the rigid boundary mostly exhibits the single inclined plane shear failure, while that with the flexible boundary varies from the single fork shear failure to the single inclined plane shear failure. ② With the increase of confining pressure, for the two boundaries, the mean mechanical coordination numbers increase and the mean porosity decrease, leading to a denser and higher strength of the sample. ③ With the increase of confining pressure, the normal contact force between particles continues to increase, and the sample strength increases. The normal contact force distributed near the axial direction increases, while that near the horizontal direction varies weakly. The higher the confining pressure is, the more difference between the vertical and the horizontal normal contact forces have, the more prominent the anisotropy is. The normal contact force increases by 54.50%, at the flexible boundary, and increases by 45.70%, at the peak strength point, with the confining pressure increasing from 12 MPa to 20 MPa. ④ Under different confining pressures and boundary conditions, the samples fail with two different failure modes, tensile and shear. The samples mainly crack from the shear between the hydrate and coal. The research results reveal the mechanism of the influence of confining pressure on the strength deformation and failure of GHBC on the mesoscale.

In order to explore the effect of hydrate formation and loading-unloading condition on the strain and permeability of gas hydrate bearing coal, the authors first measured the permeability of coal before and after gas hydrate formation, by using the triaxial testing machine for coupling action of stress, seepage, and chemical effect, as well as measured the strain and permeability of coal (three particle sizes: 0.425−0.850 mm (20−40 mesh), 0.250−0.425 mm (40−60 mesh), 0.180−0.250 mm (60−80 mesh) and three saturations: 40%, 60%, 80%) under the axial loading and confining unloading conditions. Then, the stress-strain curve was used to determine the deformation characteristic of coal during under loading and unloading conditions, the effect of hydrate formation, hydrate saturation and deviator stress on the coal permeability, the extend of coal permeability and volume expansion effect were quantitatively characterized by permeability loss rate formula and deformation angle formula. Furthermore, the influence mechanism of hydrate distribution mode was preliminarily discussed on the coal permeability with the theoretical permeability models. The results show that ① the influence of saturation on the variation law of coal permeability is more complicated. In general, with the increase of hydrate saturation, the greater the percentage of permeability reduction, the more significant the degree of blockage. The coal permeability obviously reduces after the formation of gas hydrate, with the range of reductions being 58.3%−83.3% (20−40 mesh), 61.5%−95.0% (40−60 mesh) and 81.8%−90.9% (60−80 mesh). The coal permeability decreases with the increase of hydrate saturation with a reduction between 55.6% and 86.1%. ② The axial strains of the coal exhibit three-stage process, including stability increasing, slowly increasing, and rapidly increasing with the increase of time. There is a certain correlation between the permeability and the strain of coal, and the permeability varies with the increase of deviator stress, in terms of quadratically increasing, decreasing and first increasing and then decreasing, the quadratic function can better predict the permeability variation law of gas hydrate bearing coal under mining stress disturbance. ③ Introducing the permeability loss rate, the permeability loss rate increases with the increase of the deviator stress applied on the coal under the condition of the same hydrate saturation. ④ Introducing the expansion deformation angle, the coal expansion deformation angle decreases from 19.0°−63.9° to 0.2°−38.2° with the increase of hydrate saturation under the condition of the same deviator stress, which indicates that the lower the hydrate saturation, the more significant effect of the coal expansion.

Aiming at the real problems such as the occurrence of coal and gas outburst and based on the hypothesis of comprehensive action of coal and gas outburst, a method of gas hydration and solidification to prevent coal and gas outburst is proposed. The core of this method is to solidify the gas in coal seam to form gas hydrate, which can not only reduce the gas pressure, but also improve the coal strength, so as to reduce or eliminate the risk of coal and gas outburst. Based on the idea of “coal and gas outburst prevention using hydrate”, the test of gas hydrate formation in coal and the in-situ test of the mechanical property-permeability of gas hydrate bearing coal have been performed, with the numerical modeling technique of the triaxial compression of the gas hydrate bearing coal proposed. The techniques are implemented by comprehensively applying the methods of theoretical analysis, development of testing equipment, indoor test and numerical analysis. In terms of coal and gas outburst prevention, the thermodynamic and kinetic conditions of gas hydrate formation are its theoretical basis, the stable storage of gas hydrate is its technical precondition, and the reduction of gas pressure and the improvement of mechanical properties are its key measures. This paper focuses on the cross mechanics related to the gas hydrate bearing coal. The results show that: ① the theoretical framework of gas solidification technology by the hydrate method for outburst prevention has been initially formed, and the meso-mechanism of improving the mechanical characteristics of coal before and after gas hydration has been preliminarily explored by means of the numerical simulation. ② At present, it has been confirmed that the hydrate formation in coal can not only reduce the gas pressure, but also improve its mechanical properties. High saturation can obviously improve the peak strength of coal. ③ Gas hydrate formation experiences three stages: rapid, slow and stable stage. Additionally, the formation of hydrate will cause the gas seepage channel in the coal to be blocked, resulting in a decrease in its permeability. ④ High gas pressure and high CH4 concentration not only help to increase the saturation but also delay hydrate decomposition, which is conducive to the stable existence of the hydrate. However, a large number of repetitive experiments are still needed to verify the reliability of the method to build up a generalized database. By analyzing current research findings, the limitations and challenges that still exist are discussed, with further research interests pointed out.

Abstract To analyze the relationship between macro and meso parameters of the gas hydrate bearing coal (GHBC) and to calibrate the meso-parameters, the numerical tests were conducted to simulate the laboratory triaxial compression tests by PFC3D, with the parallel bond model employed as the particle contact constitutive model. First, twenty simulation tests were conducted to quantify the relationship between the macro–meso parameters. Then, nine orthogonal simulation tests were performed using four meso-mechanical parameters in a three-level to evaluate the sensitivity of the meso-mechanical parameters. Furthermore, the calibration method of the meso-parameters were then proposed. Finally, the contact force chain, the contact force and the contact number were examined to investigate the saturation effect on the meso-mechanical behavior of GHBC. The results show that: (1) The elastic modulus linearly increases with the bonding stiffness ratio and the friction coefficient while exponentially increasing with the normal bonding strength and the bonding radius coefficient. The failure strength increases exponentially with the increase of the friction coefficient, the normal bonding strength and the bonding radius coefficient, and remains constant with the increase of bond stiffness ratio; (2) The friction coefficient and the bond radius coefficient are most sensitive to the elastic modulus and the failure strength; (3) The number of the force chains, the contact force, and the bond strength between particles will increase with the increase of the hydrate saturation, which leads to the larger failure strength.