An Experimental Research on the Pore and Fracture Evolution Characteristics and Its Driving Mechanism for Coal Samples Under Water-rock Interaction

To reveal pores and fractures evolution characteristics and its driving mechanism for coal samples under long-term water immersion, an accurate characterization and reconstruction method for CT scan of coal pore structure and mineral composition was proposed based on NMR and XRD means.The evolution characteristics of pore fracture and mineral composition before and after water immersion are compared and analyzed.The results show that:(1) the overall porosity of coal samples increases from 20.3% to 22.6% after water immersion, and the volume proportion of mineral components decreases from 9.6% to 7.9%, which indicates that the porosity increase is mainly due to the dissolution of hydrophilic mineral components in water.(2) The connected pores increased from 19.21% to 21.29% after immersion, while the isolated pores only increased by 0.16%, indicating that the increased porosity are mainly connected pores and fractures.(3) The pore radius of coal sample before immersion are 0.0012~0.5742μm, and reduced to 0.0014~0.4337μm after immersion.The variance of each section's porosity is reduced from 0.0006 to 0.0002, which indicates that water immersion reduces the pore size range and makes its distribution more uniform.The main reason for the decrease of large pores is that some mineral components are swelling and diffusing to the fracture surface by water flow.The research results are helpful to evaluate the bearing seepage capacity of coal pillar dam in underground reservoir under long-term water immersion.