Nano-CT measurement of pore-fracture evolution and diffusion transport induced by fracturing in medium-high rank coal

Abstract

Fracturing, as a common fracture-making technique, can improve the permeability of coal seams to enhance fluid transport efficiency. To quantitatively evaluate the microscopic characteristics of medium-high rank coal, the loaded pore-fracture system was characterized by computerized tomography (CT) scanning under triaxial loading, followed by the analysis of stress-strain evolution, stress sensitivity and three-dimensional (3D) fractal dimension. Combined with snow algorithm and incompressible steady laminar flow simulation, the heterogeneous distribution of fluid pressure is investigated, focusing on the diffusion effect of gas transport. The results show that the strain of the high-rank coal Chengzhuang (CZ) in the linear elastic stage increases from 0.25% to 1.25%, greater than that of the medium-rank coal Qiyi (QY) from 0.75% to 1.63%, demonstrating a slight lag of the high-rank coal from the linear elastic stage into the yielding stage. The porosity of CZ changes from 1.66% to 13.58% and that of QY varies from 1.74% to 22.28% after fracturing, reflecting that the primary and secondary pores of the medium- and high-rank coals form a complex network structure for fluid transport through continuous connection-expansion. When the strain is between 0.75% and 1.25%, the stress sensitivity coefficient of CZ decreases from 0.13 to 0.02. Moreover, there are many mutation points in the 3D fractal dimension of coal samples after fracturing, mainly due to the generation of new pore-fractures at different locations of the computational domain. For fluid transport, the pressure of QY after fracturing spreads in a wider range than CZ, accompanied by more distribution of high fluid pressure. The diffusion coefficient of the fractured CZ is 350 times higher than that of the original coal under the gas pressure condition of 0.5 MPa, which provides the possibility for more gas to be converted from Knudsen diffusion to transition diffusion or Fick diffusion in the channel.