Experimental study of spontaneous imbibition from coal based on nuclear magnetic resonance relaxation spectroscopy

Understanding the spontaneous water imbibition mechanism in coal of varying ranks has substantial implications for hydraulic operations and the safe, efficient extraction of coalbed methane. During spontaneous imbibition, low-field nuclear magnetic resonance techniques were used to test the T₂ spectra and imaging (MRI) of coal samples, enabling the determination of water signal distribution in the samples at different time intervals. By combining this with the pore structure, we explored the water migration characteristics of the coal samples during spontaneous imbibition from a microscopic perspective. Additionally, we deeply investigated the relationship between the MRI pixel eigenvalues, the capillary absorption coefficients, and the degree of pore development. The results demonstrated a positive correlation between the capillary water absorption coefficient and the degree of pore development in the coal samples, while revealing a negative correlation with both pore diameter and tortuosity. In the process of spontaneous imbibition, micropores exert a dominant influence and achieve saturation first, followed by a gradual increase in the contribution of mesopores and macropores (including microfractures) to spontaneous imbibition. The MRI results demonstrate that water migration primarily occurs within the interior of the coal samples before extending towards the exterior. The pixel values obtained from MRI follow a normal distribution, with the mean value reflecting the extent of pore development and the entropy representing the randomness in pore distribution. The higher the pixel eigenvalue, the greater the pore content of the coal sample, indicating a more random distribution of pores and enhanced connectivity among various types of pores, which results in an increased capillary water absorption coefficient.