Abstract
The aim of this paper was to develop a model that can characterize the actual micropore structures in coal and gain an in-depth insight into water’s seepage rules in coal pores under different pressure gradients from a microscopic perspective. To achieve this goal, long-flame coals were first scanned by an X-ray 3D microscope; then, through a representative elementary volume (REV) analysis, the optimal side length was determined to be 60 μm; subsequently, by using Avizo software, the coal’s micropore structures were acquired. Considering that the porosity varies in the same coal sample, this study selected four regions in the sample for an in-depth analysis. Moreover, numerical simulations on water’s seepage behaviors in coal under 30 different pressure gradients were performed. The results show that (1) the variation of the simulated seepage velocity and pressure gradient accorded with Forchheimer’s high-velocity nonlinear seepage rules; (2) the permeability did not necessarily increase with the increase of the effective porosity; (3) in the same model, under different pressure gradients, the average seepage pressure decreased gradually, while the average seepage velocity and average mass flow varied greatly with the increase of the seepage length; and (4) under the same pressure gradient, the increase of the average mass flow from the inlet to the outlet became more significant under a higher inlet pressure.
Highlights
In recent years, rock bursts, coal-gas outbursts, and dust explosion accidents have occurred frequently in underground coal mines [1,2,3]
Coal is a type of porous medium with a loose structure, whose density, specific surface area, and porosity all show a wide range of variations. ese microstructural properties can significantly affect the characteristics of the flow of water in coal pores [8,9,10]. e pressure water seepage in coal pores refers to the flow of water into coal that is driven by the water injection pressure. erefore, an investigation into the flow characteristics of water pressure in coal is important and of practical significance to ascertaining the essentials of water injections at a microlevel and improving their efficiency [11,12,13]
An ore core with a diameter of approximately 2 mm and a length of approximately 5 mm was drilled, which was sealed by wax. is was done to prevent the evaporation of water in coal during scanning, which would affect the experimental results due to the changes in shape. rough measurements, it was determined that the coal core after sealing was 2.29 mm in diameter, as shown in Figure 1. e sealed core was fixed at the tip of a toothpick, placed on the sample stage and fixed in place by screws
Summary
Rock bursts, coal-gas outbursts, and dust explosion accidents have occurred frequently in underground coal mines [1,2,3]. Gong et al proposed an imagedescription-based multiscale precise description method of core rock’s CT images, which was developed based on the multiscale structural characteristics of coal rock’s pore system [21]. On the basis of nondestructive, precise, and quantitative characterization, Yao et al proposed a novel characterization method in combination with the microfocus CT scanning technique, which can provide more precise quantitative characterizations [22] In these studies, CT was only proposed as a characterization method and has not yet been broadly applied in engineering practices. Cai et al studied the material distribution, heterogeneity, pore development, porosity, and permeability of coal at multiscales through the use of multiscale X-ray CT, scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP) [25]. Golab et al applied micro-CT to investigate the occurrence states of minerals in coals [28]
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