Nuclear Magnetic Resonance Experiment on the Influence of Confining Pressure on Spontaneous Imbibition of Water in Coal

Residual water after hydration measures can wet a coal sample to lead to spontaneous imbibition; it is a common phenomenon in the process of extracting coalbed methane using hydraulic measures. To study the characteristics of the microscopic pore structure of a coal body and spontaneous imbibition of water in the pore structure by simulating the actual deposition environment, the T2 spectra and spatial distribution of water in the imbibition process of coal samples under different circumferential pressure conditions were obtained by online testing using the low-field NMR technique. Results show that during the process of coal sample imbibition, the T2 spectra shows the characteristics of multi-peak distribution, and the number of adsorption and seepage pores is considerably higher than the fracture. Moreover, the influence of confining pressure on imbibition mainly appears in the seepage pore. The spontaneous imbibition behavior at different stages is accurately described by the Handy equation combined with the NMR signal; the higher the confining pressure, the worse the imbibition ability of the coal sample. The rate of water migration to the end of coal sample will be higher than the rate of migration to the surrounding. Finally, the three-dimensional signal distribution indicates that the imbibition is not a piston advance but a wave advance, and the rate of water migration in the longitudinal direction is higher than the rate of diffusion in the lateral direction.