Experimental study on the complex electrical dispersion response characteristics of coal during hydraulic fracturing

ObjectivesTo investigate the dispersion response characteristics of the complex electrical properties of coal during hydraulic fracturing，Methodsthe real part R and imaginary part X of the complex electrical resistance of the hydraulically fractured coal were measured. Their changes were analyzed to determine the fracture development process within the coal body， and their dispersion characteristics were examined using excitation polarization theory.ResultsThe results showed that： （1） during the hydraulic fracturing process， the dispersion curves of the real part of the coal at each pressure point exhibited a “three-stage” pattern with frequency changes， while the imaginary part showed a “U-shaped” pattern. The real part of the dispersion curve shifted upward initially， then downward， and experienced a significant downward shift during fracturing. （2） The extreme value of the imaginary part sensitively reflected changes in resistance caused by small-scale elastic deformation and shear failure， while the frequency of the extreme value could identify larger-scale shear failure， serving as an effective indicator of coal rupture. （3） Due to excitation polarization at the solid-gas-solid interface during hydraulic fracturing， the real and imaginary parts of the coal exhibited “three-stage” and “U-shaped” variations with frequency， respectively. （4） As hydraulic fracturing progressed， elastic deformation occurred first， followed by shear failure， and eventually， the water-conducting channels within the coal body became fully connected. This resulted in the dispersion curve of the real part increasing initially， then decreasing， and dropping sharply during fracturing as water pressure changed.ConclusionsThe complex resistivity method was used to monitor the variations in complex electrical parameters with water pressure during the hydraulic fracturing process and to evaluate the fracture development process of the coal body. The excitation polarization theory was applied to analyze the response characteristics of these parameters with respect to water pressure and AC frequency. This study provides an experimental foundation for evaluating coal seam fractures and fracturing effects using the spectral induced polarization method.