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

Abstract

Determining the dynamic variation in water microoccurrence in the pore−fracture system of low-rank coal at different water saturations (S w ) is of great significance to reveal the water migration law of coal reservoirs during pressurized water injection. In this study, 10 low-rank coal samples were collected from the no. 7 coal seam of the Kongzhuang mine, Jiangsu Province, China, and the simulation experiments of water intrusive were used to study water micro-occurrence and dynamic variation in the coal pore−fracture system using low-field nuclear magnetic resonance technology. Additionally, water distribution heterogeneity in the pore−fracture system was clarified using the single fractal theory, and a mathematical model for the prediction of relative water saturation (S w ′) at different pressures was established. The results show that the T 2 spectra of most samples in the no. 7 coal seam show trimodal distribution and the fractures are relatively developed. When the saturation pressure continued to increase, the coal samples were further close to saturation. When the saturation pressure increases to 40 and 50 MPa, the pore−fracture begins to expand and connect due to the high-pressure saturated water. The water distribution heterogeneity in most coal samples remains stable at different saturation pressures. The water distribution heterogeneity is mainly affected by the content of adsorption pores. The water porosity (P w ) of the no. 7 coal seam is 2.9−8.1%. In addition, there is no certain correlation between the P w and the coal macerals and proximate analysis. There is a good power function relationship between the saturation pressure and S w ′ (y = a*x b , a is 0.227−0.578, b is 0.161−0.538). The values of a and b are affected by the pore structure. The results presented here provide a reference for the migration law of water in coal reservoirs during high-pressure water injection from the microscopic level in the production of coal-bed methane.

Experimental Study on Differential Thermal Response and Pore-Fracture Structure Evolution Characteristics of Coals under Microwave Irradiation: A Case Study of Five Different Rank Coals

“…The transverse relaxation time ( T 2 , ms) can be expressed as − T 2 = r ρ 2 F S where ρ 2 is the surface relaxation rate (μm/ms), r the pore radius (μm), and F S the geometry factor. The relationship between T 2 and r is proportional, so the pore distribution can be characterized by T 2. .…”

“…A T 2 value of 2.5–100 ms represents the seepage pores, corresponding to a pore size of 100–10 000 nm. A T 2 value of >100 ms indicates fractures, corresponding to pore sizes that are >10 000 nm. ,, …”

“…Due to the heterogeneity and singularity of coal pores, fractal theory has been widely applied to characterize the heterogeneity of pore structure. Li et al found a good positive correlation between the single fractal dimension and the content of adsorption pores through LF-NMR experiments. Hong et al indicated the fractal dimension of coal samples decreases and the permeability increases after MI.…”

“…A T 2 value of >100 ms indicates fractures, corresponding to pore sizes that are >10 000 nm. 43,46,47 The porosity curve of the standard sample is obtained by testing the signal intensity of the standard water sample, and then the water…”

Research on Pore-Fracture Characteristics and Adsorption Performance of Main Coal Seams in Lvjiatuo Coal Mine

“…By combining SEM and AFM, Li et al [19] found significant differences in the mechanical properties between the coal matrix and minerals, as well as far greater than average adhesion in pores near minerals. Li et al [26] used LF-NMR technology to accurately characterize the pore-fracture system of coal seam, studied the migration and occurrence law of fluid in coal pores under different saturation pressures, established the relative water saturation model of coal pores, and found that the pore and fissure would be destroyed under high saturation pressure, resulting in the decrease in coal water saturation. Liu et al [22] used µ-CT imaging technology to establish the micro-three-dimensional pore network ball and stick model of two kinds of coal in Qinshui Basin, and visualized the gas flow in the threedimensional structure.…”