Quantitative Study of the Effects of Pore Structure and Wettability on the Water Spontaneous Imbibition Behaviors in Coal

Abstract

Water injection has been widely used to prevent gas disasters and coal dust in coal mines. Coal would be wetted and softened by water through spontaneous imbibition (SI), which is mainly influenced by the pore structure and wettability. Studying the effects of pore structure and wettability on SI is of great significance for optimizing coal seam water injection parameters. This work aims to establish a quantitative relationship between these parameters and extract critical factors controlling the SI. Mercury intrusion porosimetry (MIP), low-temperature N 2 adsorption (LT-N 2 A), Fourier transform infrared spectroscopy (FTIR), and contact-angle measurements were used to assess the pore structure and wettability of the coal samples. Low-field nuclear magnetic resonance (LF-NMR) was used to monitor water in coal. All parameters were quantified with Pearson correlation analysis, and principal component analysis (PCA) was subsequently employed to pinpoint the key factors. The results show that the SI velocity and SI ability of bituminous coal are higher than those of anthracite; bituminous coal can rapidly elute more water. The time index (n) has a strong relationship with the SI velocity, and the higher the n value is, the faster the water migrates in pore space. Porosity is positively proportional to SI ability, and a larger SI ability means that the sample can store more water. The wettability parameter has a higher impact on SI velocity than on SI ability. Among the wettability parameters, the contact angle is a key factor in evaluating the SI process in coal. The bituminous coal with good pore connectivity, hydrophilicity, and high porosity could have a rapid SI velocity and store more water. Oxygen-containing functional groups promote SI by enhancing water and pore surface interactions. This work can better understand the mechanism of SI and provide the theoretical basis for field application.