Experiment, Model, and Mechanism of Multiscale Dynamic Diffusion–Permeability in Coal under Different Fluids

Abstract

The permeability of coal exhibits multiscale characteristics in space and time, which is caused by the presence of micro and nanopores in coal. Water, free gas, and adsorbed gas are common engineering fluids in coal seams during gas extraction. Thus, it is of significance to study the multiscale characteristics and mechanisms of seepage−diffusion of different fluids in coal for gas extraction engineering. Experiments of seepage−diffusion for He, CH 4 , and water are carried out using ϕ50 × 100 mm cylindrical coal. It is found that the apparent diffusion coefficient for He, CH 4 , and water are not a constant but a variable that decays dynamically with time. The phenomenon is independent of fluid properties and determined only by the multiscale characteristics of pores in coal, and then a novel model of multiscale dynamic apparent diffusion that can accurately describe the full-time process of flow in various fluids is proposed. The mechanism of dynamic seepage−diffusion for different fluids in coal is elucidated based on a new proposed geometrical model of multiscale pores. At the early stage of flow, fluids first flow in or out of the largest pores outside coal, and at the later stage of flow, fluids flow in the micro and nanopores inside the coal matrix. The pore sizes through which the fluid flows decrease with time, which leads to a dynamic attenuation of the apparent diffusion−permeability with time. The initial apparent permeability K 0 for He/CH 4 shows a "U"-shaped pattern of decreasing and then increasing with the decrease of gas pressure. When the gas pressure is lower than the turning point, the slip effect dominates, making the initial apparent permeability K 0 decrease with the increase of gas pressure. When the gas pressure is higher than the turning point, the effect of the pressure difference of the gas dominates, and K 0 increases with the increase of gas pressure. When water flows in multiscale pores, it occupies the space of large pores, making the homogeneity of pores increase, and the decay coefficient of apparent permeability becomes smaller. The multiscale permeability can explain the reason for the rapid attenuation of coalbed methane production in the late stage. It is of great significance for coalbed methane productivity prediction.

Study on the Solution and Variation Law of Diffusion Coefficient Based on the Numerical Simulation Optimization Method

“…Meanwhile, due to the assumption of a constant diffusion coefficient, the single-pore diffusion model has a large deviation in describing desorption curve over a long period of time. For this reason, the time-dependent diffusion coefficient model and the fractal diffusion coefficient model have been proposed, 52−55 yet the principle and physical implication of diffusion coefficient variation have not been explained scientifically. Therefore, the diffusion coefficient solution and the diffusion law need to be further studied.…”

Experimental Study on Methane Diffusion Characteristics of Different Metamorphic Deformed Coals Based on the Counter Diffusion Method

“…Therefore, when other conditions, including pore (gas) pressure, remain unchanged, as the confining pressure increases, the effective stress of the coal body continues to in-crease, causing the coal body deformation to increase continuously. The pores and throats in coal contract and deform under the action of effective stress, ultimately leading to decreases in coal-body porosity and D. D, similar to permeability [12,54], negatively affects the effective stress.…”

“…The pores and fractures in coal are storage spaces for coalbed methane and channels for methane migration [10,11]. Due to the differences in the causes, forms, and scales of pores and fractures in coal, the different migration mechanisms of methane in the pore and fracture channels can be divided into pore diffusion and fracture seepage [12,13]. Seepage can occur through the cleavage and fracture system in the coal body, which is driven by pressure differences, following Darcy's law [14,15]; diffusion can occur within the coal matrix pores and micro-fractures, which is driven by concentration differences, following Fick's law [16,17].…”

Effect of Coal Particle Breakage on Gas Desorption Rate during Coal and Gas Outburst

“…This concept is extended to larger-scale coal cores, encompassing the entire gas diffusion process [39,40]. Additionally, a microscopic analysis is conducted to examine the multi-scale dynamic diffusion mechanism of gas in coal [41]. Sophisticated gas diffusion models will help researchers calculate the gas diffusing rate accurately in coal particles of different sizes [42].…”