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 commonphenomenon in the process of extracting coalbed methane using hydraulic measures. To study the characteristics of the microscopicpore structure of a coal body and spontaneous imbibition of water in the pore structure by simulating the actual depositionenvironment, the T2spectra and spatial distribution of water in the imbibition process of coal samples under different circumferentialpressure conditions were obtained by online testing using the low-field NMR technique. Results show that during the process of coalsample imbibition, the T2spectra shows the characteristics of multi-peak distribution, and the number of adsorption and seepagepores is considerably higher than the fracture. Moreover, the influence of confining pressure on imbibition mainly appears in theseepage pore. The spontaneous imbibition behavior at different stages is accurately described by the Handy equation combined withthe NMR signal; the higher the confining pressure, the worse the imbibition ability of the coal sample. The rate of water migration tothe end of coal sample will be higher than the rate of migration to the surrounding. Finally, the three-dimensional signal distributionindicates that the imbibition is not a piston advance but a wave advance, and the rate of water migration in the longitudinal directionis higher than the rate of diffusion in the lateral direction.