Coal Permeability Evolution Under Different Water-Bearing Conditions

Self Cite

The environment of coal reservoirs is complex and variable, with various factors, not independent, but interlocking and coupled. However, their coupling effects have rarely been considered in previous theoretical studies. In addition, when a fractal theory was applied to analyze the permeability of coal, the linkage between fractal dimensions and the pore radius under external factors has usually been ignored. In this study, two parameters, pore compressibility and dynamic water saturation, were introduced to quantitatively represent the internal interactions between stress and water. Based on the results, dynamic fractal dimensions under the coupling actions of stress and water were obtained by considering variation in the pore radius and the assignment status of gas and water, following which an apparent permeability model of coal, with dynamic fractal dimensions, was established. The newly developed model was verified by reference to published experimental data. The results showed that stress dominated changes in the fractal dimension for pore-size distribution under the coupling actions of stress and water; however, changes in the fractal dimension for tortuosity was dominated, in turn, by stress and water. Subsequently, it was found that the fractal apparent permeability model could match the experimental data well. It revealed that the newly developed model could predict the evolution of coal permeability under dynamic changes in stress and water. Further, under the coupling actions of stress and water, the apparent permeability of coal possessed a positive correlation with the fractal dimension for pore-size distribution or water-dominated fractal dimension for tortuosity, while it was negatively correlated with the critical capillary radius or stress-dominated fractal dimension for tortuosity. In addition, the evolution of the apparent permeability was consistent with specific fractal dimensions and dynamic fractal dimensions, but the evolution of the slippage factor revealed an opposite result. Consequently, this initiates a new analytical perspective into investigating change mechanisms relating to coal permeability.

Section: Introductionmentioning

confidence: 99%

Fractal Apparent Permeability Model for Coal under the Coupling Actions of Stress and Water

Duan

Gao

et al. 2023

Self Cite

“…The gas permeability decreased to 0.13 μD when the water saturation reached 91%, and the permeability decline rate was 0.336 μD. The permeability of coal decreased with the increase in water saturation, which was consistent with the results of Li et al 24 3.2. Relative Permeability Measurement Results.…”

Section: Gas Permeability Of Coal Under Different Watermentioning

confidence: 99%

Experiment on the Gas Permeability and Gas–Water Relative Permeability in High-Rank Coal with a Self-Developed Device

Shen

Fang

et al. 2023

Water saturation, displacement pressure, and gas type are essential factors affecting the seepage characteristic of fluid in coal due to the swelling induced by gas adsorption. However, the understanding of the seepage behavior of water, CH 4 , and CO 2 in coal seams is still poor owing to the lack of research. In this work, a series of gas permeability and gas−water relative permeability experiments were conducted by our self-developed device. The results can be summarized as follows: (1) with the increase in water saturation, the gas permeability decreased rapidly at first and then reduced sharply again after a period of slow decline; (2) the two-phase flow span showed no significant difference for He−water and CH 4 −water systems under different displacement pressures, but it increased for CO 2 -water system. The different behaviors of the two-phase flow span suggested that the effect of displacement pressure on relative permeability properties is dependent on the gas type. (3) The CO 2 −water relative permeability showed a sizeable two-phase flow span compared to the He−water or CH 4 −water system under the same displacement pressure, and the gas relative permeability decreased obviously in the following sequence: He > CH 4 > CO 2 . The concept of relative permeability surface was proposed based on the experimental results. A parametric interpolation method was applied to determine the relative permeability surface, which could be used to accurately describe the flow characteristics of fluids in a mixed gas−water system.

“…Therefore, when modeling permeability, the impact of various influential factors is considered on the fracture permeability in coal; for example, classic P&M and S&D models have established fracture permeability models under conditions of uniaxial strain. , Tang et al based on S&D models considered the influence of fracture compressibility by establishing a permeability model of variable fracture under different boundary conditions. When the influence of moisture and gas adsorption on permeability is considered, Li et al established fracture permeability models that consider the excess adsorption in wet coal. However, it was thought unreasonable to ignore the conductivity of matrix pores when the scale of differences between pores and fracture networks in the matrix was small .…”

Section: Introductionmentioning

confidence: 99%

“…32,33 Tang et al 34 based on S&D models considered the influence of fracture compressibility by establishing a permeability model of variable fracture under different boundary conditions. When the influence of moisture and gas adsorption on permeability is considered, Li et al 35 established fracture permeability models that consider the excess adsorption in wet coal. However, it was thought unreasonable to ignore the conductivity of matrix pores when the scale of differences between pores and fracture networks in the matrix was small.…”

Section: Introductionmentioning

confidence: 99%

A Study of the Dynamic Changes in Wet Coal’s Water Film and Permeability under Stressed Conditions

Cheng

et al. 2022

Self Cite

Coal permeability is an important influential factor in the efficient development and utilization of coalbed methane (CBM); however, coal is in an environment of combined gas and water and is exposed to continuous reservoir pressure for considerable periods, which makes permeability changes in its natural state extremely complicated and those changes difficult to evaluate. In this study, we established a dual-porosity permeability model suitable for wet coal that considered the influence of stress and gas adsorption. In the process of modeling, the shapes of the matrix pores and fractures were simplified into regular cylindrical and slit; based on the generalized Hooke’s law, the effective stress–strain relationship of the coal matrix and fracture was represented; meanwhile, the adsorption capacity decay coefficient λ was introduced to describe the influence of moisture on gas adsorption; and then changes in the pore radius and fracture width under the action of stress and gas adsorption were quantified. Moreover, the interaction between the water film and the pore wall and the influence of stress and gas adsorption in the natural reservoir environment were considered, relating to the dynamic water film calculation formula in matrix pores and fractures under the influence of stress and gas adsorption that was derived, revealing the dynamic evolution law of water film thickness under the action of stress and gas adsorption. By combining the above influential factors and based on the relationship between permeability–porosity–pore radius (fracture width), a dual-porosity permeability model that considered the effects of stress, gas adsorption, and dynamic water film combinations was established. Further, we compared the predicted results of this model with published experimental data and discuss the influence of stress and gas adsorption on water film thickness and the contribution of matrix permeability and fracture permeability to resultant permeability under different water saturations. The complex variation of wet coal permeability under stress and gas adsorption is revealed.