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 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.

Single and multi‐fractal dimension variation of tight sandstone by using centrifuge T2 spectral curve

“…Zhang et al 3,12 . shows that fractal models can be divided into single fractal and multi‐fractal models, and then a single model can be divided into three fractal dimension parameter, this is, D 1 ( T 2 is less than T 2 cut off , which represents smaller pore size distribution heterogeneity), D 2 ( T 2 is larger than T 2 cut off , which represents larger pore size distribution heterogeneity) and D 3 (represents total pore size distribution heterogeneity).…”

“…Based on the T 2 spectrum in a saturated state, quantitative characterization of pore size distribution heterogeneity by using fractal theory has been studied. 3,10,11 Zhang et al 3,12 shows that fractal models can be divided into single fractal and multi-fractal models, and then a single model can be divided into three fractal dimension parameter, this is, D 1 (T 2 is less than T 2 cut off , which represents smaller pore size distribution heterogeneity), D 2 (T 2 is larger than T 2 cut off , which represents larger pore size distribution heterogeneity) and D 3 (represents total pore size distribution heterogeneity). Meanwhile, the relationship between different fractal parameters and pore structure, porosity-permeability parameters and mineral content are discussed, indicating that D 1 has a positive correlation with smaller pore volume, and the multi-fractal parameters have good applicability in characterizing pore structure heterogeneity of tight sandstone.…”

Dynamic Variation on Water Micro-occurrence in Low-Rank Coals by a Low-Field Nuclear Magnetic Resonance Experiment: A Case Study of the No. 7 Coal Seam in Kongzhuang Coal Mine

“…When the researchers performed LF-NMR tests on watersaturated coal samples, they found that the higher the saturation pressure, the more fully the fluid can fill the pores, and the more NMR signal characteristics can represent the real pore characteristics. 26,27 Zhang et al 28 adjusted the saturation pressure of coal samples to 5, 10, and 15 MPa sequentially. However, when the saturation pressure was above 10 MPa, the variation of NMR signal became less and less obvious.…”

“…used LF-NMR to test the coal samples under confining pressure, and they found that pore volume did not decrease but increased and suggested that the reason may be that during the loading process, the pore volume shrunk and water migrated from micro- and minipores to meso- and macropores/fractures, resulting in an increased T 2 signal, which further caused the phenomenon of pore volume increase. When the researchers performed LF-NMR tests on water-saturated coal samples, they found that the higher the saturation pressure, the more fully the fluid can fill the pores, and the more NMR signal characteristics can represent the real pore characteristics. , Zhang et al adjusted the saturation pressure of coal samples to 5, 10, and 15 MPa sequentially. However, when the saturation pressure was above 10 MPa, the variation of NMR signal became less and less obvious.…”