Relationship between Dielectric Relaxation Time and Permeability of High-Rank Coal in the Jiaozuo Mining Area

Abstract

To evaluate the permeability of coal by the complex resistivity method, the real part of complex resistivity (R), the imaginary part of complex resistivity (X), capacitance (C), permeability (k), and strain (ε) of coal in different directions under variable pressure were measured. Based on the physicochemical structure characteristics of coal and its conductivity and dielectric mechanism, the experimental phenomenon was analyzed and correlated with the permeability of coal. The results demonstrated that (1) the R–X curve is U-shaped. With increasing frequency, the amplitude first decreases and then increases, forming an obvious trough. C decreases with increasing frequency. (2) The R–X curve is affected by pressure and directivity. The absolute value of the R–X curve amplitude gradually decreases and shifts to the left as the pressure increases; meanwhile, it decreases and shifts to the left in the order of the vertical bedding direction (z direction), the vertical main fracture direction in parallel bedding (y direction), and the main fracture direction in parallel bedding (x direction). There is also a related change law for C. (3) The dielectric relaxation time (τ) was optimized as the electrically sensitive parameter of coal. τ continues to decrease as pressure increases and decreases in the order of the z, y, and x directions. (4) The permeability of coal is strongly correlated with the dielectric relaxation time, and the relationship between them conforms to a logarithmic function. The relationship of strain and the dielectric relaxation time verifies the strong correlation between permeability and the dielectric relaxation time. This study provides an experimental basis and theoretical support for the subsequent accurate prediction and evaluation of coal permeability using the dielectric relaxation time.