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

Abstract

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 T2 and T1–T2 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–D0 and D–10–D10 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) T2 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.