Strain and permeability of gas hydrate bearing coal under loading and unloading condition

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.