Evolution of Coal Permeability during Gas Injection—From Initial to Ultimate Equilibrium

The evolution of coal permeability is vitally important for the effective extraction of coal seam gas. A broad variety of permeability models have been developed under the assumption of local equilibrium, i.e., that the fracture pressure is in equilibrium with the matrix pressure. These models have so far failed to explain observations of coal permeability evolution that are available. This study explores the evolution of coal permeability as a non-equilibrium process. A displacement-based model is developed to define the evolution of permeability as a function of fracture aperture. Permeability evolution is tracked for the full spectrum of response from an initial apparent-equilibrium to an ultimate and final equilibrium. This approach is applied to explain why coal permeability changes even under a constant global effective stress, as reported in the literature. Model results clearly demonstrate that coal permeability changes even if conditions of constant effective stress are maintained for the fracture system during the non-equilibrium period, and that the duration of the transient period, from initial apparent-equilibrium to final equilibrium is primarily determined by both the fracture pressure and gas transport in the coal matrix. Based on these findings, it is concluded that the current assumption of local equilibrium in measurements of coal permeability may not be valid.

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Reservoir Permeability Evolution during the Process of CO2-Enhanced Coalbed Methane Recovery

Energies ◽

10.3390/en11112996 ◽

2018 ◽

Vol 11 (11) ◽

pp. 2996 ◽

Cited By ~ 3

Author(s):

Gang Wang ◽

Ke Wang ◽

Yujing Jiang ◽

Shugang Wang

Keyword(s):

Effective Stress ◽

Coalbed Methane ◽

Volumetric Strain ◽

Fracture Aperture ◽

Permeability Evolution ◽

Methane Recovery ◽

Permeability Model ◽

Reservoir Permeability ◽

Stress Changes ◽

Enhanced Coalbed Methane

In this study, we have built a dual porosity/permeability model through accurately expressing the volumetric strain of matrix and fracture from a three-dimensional method which aims to reveal the reservoir permeability evolution during the process of CO2-enhanced coalbed methane (CO2-ECBM) recovery. This model has accommodated the key competing processes of mechanical deformation and adsorption/desorption induced swelling/shrinkage, and it also considered the effect of fracture aperture and effective stress difference between each medium (fracture and matrix). We then numerically solve the permeability model using a group of multi-field coupling equations with the finite element method (FEM) to understand how permeability evolves temporally and spatially. We further conduct multifaceted analyses to reveal that permeability evolution near the wells is the most dramatic. This study shows that the farther away from the well, the gentler the evolution of permeability. The evolution of reservoir permeability near the injection well (IW) and the production well (PW) are very different, due to the combined effects of effective stress changes and gas adsorption and desorption. Furthermore, adsorption is the main controlling factor for the change of permeability for regions near the IW, while the change in effective stress is the main cause for the change in permeability near the PW. Increasing the injection pressure of CO2 will cause the reservoir permeability to evolve more quickly and dynamically.

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COAL PERMEABILITY CHANGE CAUSED BY MINING-INDUCED STRESS

Mining Scince ◽

10.37190/msc192612 ◽

2019 ◽

Vol 26 ◽

Author(s):

Lulu Zhang ◽

Bo Li ◽

Jianping Wei ◽

Zhihui Wen ◽

Yongjie Ren

Keyword(s):

Experimental Data ◽

Effective Stress ◽

Axial Stress ◽

Sensitivity Index ◽

Permeability Change ◽

Permeability Evolution ◽

Confining Stress ◽

Coal Permeability ◽

Calculation Results ◽

Loading And Unloading

To study coal permeability evolution under the influence of mining actions, we conducted a sensitivity index test on permeability to determine the influence of axial and confining stresses on coal permeability. Loading and unloading tests were performed afterward, and the differences between loading and unloading paths in terms of strain and permeability were studied. A permeability evolution model was built in consideration of absorption swelling and effective stress during modeling. An effective stress calculation model was also built using axial and confining stresses. The calculation results of the two models were compared with experimental data. Results showed that permeability were more sensitive to confining stress than axial stress, and effective stress placed a large weight on confining stress. Large axial and radial deformations at peak strength were observed during unloading. In the unloading phase, the permeability of coal began to increase, and the increment was enhanced by large initial axial stress when confining stress was loaded. permeability sensitivity to axial and confining stresses were used to explain these permeability changes. The calculation results of the models fitted the experimental data well. Therefore, the proposed models can be used to calculate effective stress on the basis of axial and confining stresses and describe permeability change in coal under the influence of mining actions.

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Coal Permeability Evolution and Gas Migration Under Non-equilibrium State

Transport in Porous Media ◽

10.1007/s11242-017-0862-8 ◽

2017 ◽

Vol 118 (3) ◽

pp. 393-416 ◽

Cited By ~ 31

Author(s):

Ting Liu ◽

Baiquan Lin ◽

Wei Yang ◽

Cheng Zhai ◽

Tong Liu

Keyword(s):

Equilibrium State ◽

Gas Migration ◽

Permeability Evolution ◽

Coal Permeability ◽

Non Equilibrium

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Impact of Sorption-Induced Strain and Effective Stress on the Evolution of Coal Permeability under Different Boundary Conditions

Energy & Fuels ◽

10.1021/acs.energyfuels.1c01900 ◽

2021 ◽

Author(s):

Chunhong Yao ◽

Bobo Li ◽

Zheng Gao ◽

Jianhua Li ◽

Chonghong Ren ◽

...

Keyword(s):

Boundary Conditions ◽

Effective Stress ◽

Coal Permeability ◽

Different Boundary Conditions

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Experimental investigations of CO2 seepage behaviorin naturally fractured coal under hydro-thermal-mechanical conditions

Thermal Science ◽

10.2298/tsci2106651t ◽

2021 ◽

Vol 25 (6 Part B) ◽

pp. 4651-4658

Author(s):

Teng Teng ◽

Xiaoyan Zhu ◽

Yu-Ming Wang ◽

Chao-Yang Ren

Keyword(s):

Gas Flow ◽

Confining Pressure ◽

Gas Pressure ◽

Experimental Investigations ◽

Permeability Evolution ◽

Coal Permeability ◽

Naturally Fractured ◽

Series Of Experiments ◽

Fractured Coal ◽

Increasing Temperature

Gas-flow in coal or rock is hypersensitive to the changes of temperature, confin?ing pressure and gas pressure. This paper implemented a series of experiments to observe the seepage behavior, especially the permeability evolution of CO2 in naturally fractured coal sample under coupled hydro-thermal-mechanical conditions. The experimental results show that coal permeability increases exponentially with the increasing gas pressure, and tends to be linear when the confining pressure is high. Coal permeability decreases exponentially with the increasing confining pressure. Coal permeability decreases with the increasing temperature generally, but it may bounce up when the temperature rises to high. The results provide reference for the projects of coal gas extraction and carbon dioxide geological sequestration.

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Model development and analysis of coal permeability based on the equivalent characteristics of dual-porosity structure

Journal of Geophysics and Engineering ◽

10.1093/jge/gxz108 ◽

2019 ◽

Vol 17 (2) ◽

pp. 313-327

Author(s):

Haijun Guo ◽

Kai Wang ◽

Yuanping Cheng ◽

Liang Yuan ◽

Chao Xu

Keyword(s):

Coalbed Methane ◽

Model Development ◽

Gas Pressure ◽

Dual Porosity ◽

Permeability Evolution ◽

Evolution Law ◽

Fracture Width ◽

Coal Permeability ◽

Porosity Structure ◽

Gas Seepage

Abstract Mining is a dynamic fracture process of coal and/or rock. The structural failure of coal bodies will change the coal matrix-fracture characteristics and then affect the distribution characteristics of the coalbed methane (CBM). Because of the structural complexity of coal, the coal matrices and fractures will be assumed to the geometries with rule shapes when the gas seepage characteristics in coals are analyzed. The size of the simplified geometries is the equivalent scale of dual-porosity coal structures (i.e. the equivalent fracture width and equivalent matrix scale). In this paper, according to the reasonable assumptions with regarding to dual-porosity coal structures, a new coal permeability evolution model based on the equivalent characteristics of dual-porosity structure (ECDP model) was built and the effect of the equivalent characteristics of dual-porosity structure on the coal permeability evolution law was analyzed. It is observed that if the initial fracture porosity is constant and the equivalent matrix scale increases, the range in which the permeability of coal rises with rising gas pressure increases; if the equivalent fracture width decreases and the equivalent matrix scale is constant, the range in which the permeability of coal rises with rising gas pressure decreases. The ECDP model is more suitable for revealing the evolution law of the coal permeability when large deformations occur in the coal bodies and/or the coal structure is damaged irreversibly, especially during enhancing CBM recovery.

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Application of Steepest-Entropy-Ascent Quantum Thermodynamics to Predicting Heat and Mass Diffusion From the Atomistic Up to the Macroscopic Level

Volume 6B: Energy ◽

10.1115/imece2015-53581 ◽

2015 ◽

Cited By ~ 2

Author(s):

Guanchen Li ◽

Michael R. von Spakovsky

Keyword(s):

Spatial Scales ◽

Local Equilibrium ◽

Phenomenological Approach ◽

Heat Diffusion ◽

Quantum Thermodynamics ◽

Macroscopic Property ◽

State Evolution ◽

Equilibrium Process ◽

Far From Equilibrium ◽

Non Equilibrium

Conventional first principle approaches for studying non-equilibrium or far-from-equilibrium processes all depend on the mechanics of individual particles or quantum states and as a result, require too many details of the mechanical features of the system to easily or even practically arrive at the value of a macroscopic property. In contrast, thermodynamics, which has been extremely successful in the stable equilibrium realm, provides an approach for determining a macroscopic property without going into the mechanical details. Nonetheless, such a phenomenological approach is not generally applicable to a non-equilibrium process except in the near-equilibrium realm and under the limiting local equilibrium and continuum assumptions, both of which prevent its application across all scales. To address these drawbacks, steepest-entropy-ascent quantum thermodynamics (SEAQT) can be used. It provides an ensemble-based, thermodynamics, first principles approach applicable to the entire non-equilibrium realm even that far-from-equilibrium and does so with a single kinematics and dynamics able to cross all temporal and spatial scales. Based on prior developments by the authors, this paper applies SEAQT to the study of mass and heat diffusion. Specifically, the study focuses on the thermodynamic features of far-from-equilibrium state evolution. Two kinds of size effects on the evolution trajectory, i.e., concentration and volume effects, are discussed.

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Coal permeability stimulation by NaClO oxidation

The APPEA Journal ◽

10.1071/aj18142 ◽

2019 ◽

Vol 59 (2) ◽

pp. 846

Author(s):

Reydick Balucan ◽

Zhenhua Jing ◽

Jim Underschultz ◽

Karen Steel

Keyword(s):

Coal Seam ◽

Structural Changes ◽

Time Pressure ◽

Fracture Aperture ◽

Void Space ◽

Coal Permeability ◽

Core Flood ◽

The Impact ◽

Well Completions ◽

Gas Bearing

Many prospective coal seams have limited permeability and are thus marginal for economic coal seam gas (CSG) extraction. To enhance seam permeability, various CSG stimulation techniques, including hydraulic fracturing, cavity well completions and horizontal wells, have been used commercially, but these techniques are not successful everywhere. Alternately, oxidant stimulation to enhance coal seam permeability may have potential benefit. In this paper, we report the oxidation of a cubic coal sample from the gas-bearing Bandanna Formation with 1% NaClO. Permeability variation were measured over time with core stimulation/flooding tests. Coal structural changes, captured via X-ray microcomputed tomography (µCT), were used to analyse and model the impact of oxidant stimulation on coal permeability. We found that NaClO stimulation was able to widen fracture aperture and generate additional cracks and/or void space. Analyses of the coal after reaction indicated improved pore connectivity. This resulted in a significantly higher permeability, as shown by both core flood tests and flow modelling/simulations using GeoDict’s FlowDict module. Although this paper confirms oxidant stimulation has the potential to improve coal seam permeability, the optimum volume and reaction time, pressure and temperature conditions and suitability for use in various coal types requires further research.

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