Pore wetting process characterization of Equal-Sized granular coals by using LF-NMR technology

Understanding the pore wetting process is of great significance for determining fluid wetting behavior in porous media such as coal, and for the development and utilization of natural gas reservoirs, however, most studies on the process simplify porous media as glass capillaries, which may lack practical application. To better understand and quantitatively characterize the actual pore wetting process of coals, a new, low-field nuclear magnetic resonance (LF-NMR) experimental characterization method was developed. The experiment used equal grain sizes of glass beads and coals to yield porous media with variable pores of specific size ranges. LF-NMR was then used to characterize the pore wetting process of a given amount of water in the porous media at different times. Six groups of high-rank coals were also characterized by contact angle experiments to analyze the role of the surface contact angle on pore wetting. The results show that the pore wetting process can be divided into three ideal physical phases: a first touch state, a semi-free water state and an inter-granular diffusion or quasi-static state. The wetting pore size distribution (W-PSD) spectrum obtained by measurement of the transverse relaxation time and the transformation coefficient reflected change in pore size during wetting, with the location of the W-PSD quantified by the equivalent wetting pore size. The equivalent wetting pore size obtained at a final time of 24 h was used to characterize the pore wettability, it was significantly correlated with the surface contact angle with a linearity of up to 0.99. However, the W-PSD or the equivalent wetting pore size is dependent on the surface relaxivity. Overall, the experimental characterization method used in this study is an effective way to study the "black box problem" of the wetting process in porous media, as it is simple and has low sample requirements.