Water Saturation and Distribution Variation in Coal Reservoirs: Intrusion and Drainage Experiments Using One- and Two-Dimensional NMR Techniques

Determining water occurrence in pore-fracture systems under specific water saturation is of great significance to reveal the correlation between the water content and porosity/permeability of coal reservoirs. In this work, simulation experiments of water intrusion and drainage are used to study the micro-occurrence and migration of water using NMR T-2 and T-1-T-2 techniques and discuss the influence of pore-fracture system structure parameters on water micro-occurrence. Meanwhile, water distribution heterogeneity in the pore-fracture system is clarified by single- and multifractal theories. The results show that (1) the vacuum saturation method without pressure is unsuitable for high-rank coal samples with micropore development, and water saturation variation leads to a change in significant permeability when water saturation is greater than the critical value, which is related to the coal rank and degree of fracture development; (2) the single-fractal theory can characterize the heterogeneity of water and pore size distribution under static conditions; however, multifractal analyses have a stronger applicability in characterizing water distribution heterogeneity under dynamic conditions; and (3) multifractal parameters have a good correlation with coal sample characteristics such as the water volume in pores and fractures. In the process of centrifugation, both D-10-D-0 and D-10-D-10 parameters from fractal analyses decrease linearly with a decrease in water content in coal samples, indicating that water distribution heterogeneity in pore-fracture systems decreases with an increase in centrifugal force; and (4) T-2 and two-dimensional spectra in the same coal sample should be comprehensively analyzed as they can quantitatively identify the amount of water migration at different saturation stages.