Macro- and microscopic mechanical behaviour of flow of coal samples experimentally deformed at high temperatures and pressures

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Coal samples from Qinshui basin, Shanxi, China are experimentally deformed at temperatures and confining pressures of 200–500 °C and 200–500 Mpa, strain rate of 0.5 × 10−5/s and total strain of 10%. The vitrinite reflectance of the coal samples varies from 3.04 to 1.79. It is shown that the strengths of the deformed samples change obviously with coeval increasing temperatures and pressures (T/P). At the experimental range of T/P, the effects of increasing temperature predominate over that of increasing pressure. Microstructural analysis indicates a brittle to ductile transition under experimental T/P conditions from 200 to 300 °C, and 200 to 300Mpa. Brittle deformation microstructures include macroscopic fracture zones and penetrative fracture associations. Elongation, undulose or irregular extinction, deformation lamellae and dynamic recrystallization of grains are the main ductile deformation microstructures. The variation of deformation mechanisms of the experimentally deformed coal samples is related to both the components of coals and T/P conditions. At low T/P, fractures occur in both inertinite and vitrinite of the samples. At higher T/P, crystalline plastic deformations are observed in the inertinite only.