Effects of Lithological Layering and Fluid Diffusivity on the Nucleation of Coal Dynamic Failure.

Lithological layering makes coal mechanically heterogeneous and strongly controls the pore fluid diffusivity. Localized elastic deformation and high pore pressure favor the dynamic failure of coal. The effects of lithological layering and fluid diffusivity on the nucleation of coal dynamic failure were investigated by performing undrained triaxial cyclic loading tests on fully saturated coal of lithological layers. The porous layer in coal provides the dominant sites for fluid storage. The pore fluid in the regional porous layer of coal is strongly compacted, forming high pore pressure due to undrained fluid caused by the barrier effect of the neighboring tight layer. Excited by the mechanical disturbance of periodic mining, the regional pore pressure decomposes mineral grains of the porous layer, resulting in significant radial and volumetric dilation. Moreover, asynchronous deformation occurs among different lithological layers and results in resistance at the layer interface, causing tensile cracks. Different fluid enrichment zones coalesced into a large pore overpressure zone by these tensile cracks. On the microscale, with the maximum stress level (σmax) increase from 0.7σST to 0.9σST, the primary pores' porosity of after loading increased from − 3.74 to 19.61% relative to the porosity before cyclic loading, while the porosity of secondary pores increased from 53.78 to 1573.23%, indicating that the pore fluid in the large pores is compacted more significantly. On the one hand, the high pore pressure formation in the secondary pores weakens the coal strength. On the other hand, such a porosity increase in coal enlarges the fluid storage volume and enhances the regional fluid diffusivity for more gas energy accumulation. The enlarged pore overpressure domain and the corresponding reduction in coal strength contribute to the nucleation of coal dynamic failure. Highlights: The regional pore pressure decomposes mineral grains of the porous layer. The pore overpressure causes significant radial and volumetric dilation of coal. Incongruous radial strain in different layers produces tensile crack. Tensile cracks coalesce fluid enrichment zones into a large pore overpressure zone. Pore overpressure in secondary pores weakens coal strength and enlarges porosity. [ABSTRACT FROM AUTHOR]