Study on the permeability characteristics of coal containing coalbed methane under different loading paths

Creep behavior is one of the most important mechanical behaviors of coal. In the present study, we systematically carried out a series of creep experiments of briquette samples under triaxial compression and gas pressure. The experimental results showed that the creep deformation characteristics of coal were related to deviatoric stress and gas pressure. The accelerating creep stage initiated when the deviatoric stress reached or exceeded the yield stress until failure took place. In contrast, when the deviatoric stress was less than the yield strength, only transient and steady creep stage occurred. Besides, gas pressure played a positive role in the creep process of coal. The axial strain of coal slightly increased with gas pressure (range from 0.13 to 0.40 MPa) when the effective triaxial stress remained constant. For example, in the steady creep stage (σ 1 = 9 MPa, p = 0.13 MPa), the axial strain of sample S 1 is 2.710% and that of sample S 2 is 3.361%, that is, coal samples with high gas pressure have higher axial strain. Moreover, gas pressure has two main effects on the mechanical behavior of coal. One is that the coal adsorbs gas molecules resulting in swelling strain and swelling stress, and the other is that the gas pressure may compress the coal particles. Considering the swelling stress and gas pressure, we intensively studied the effective stress formula of coal containing gas. Then, the effective stress was taken into consideration in the nonlinear Nishihara creep model. Furthermore, the tertiary creep factor n was introduced in the nonlinear model to control the creep deformation evolution with time so that the nonlinear model could accurately describe the full creep stage of coal. In contrast, due to the lack of such a controlling parameter, the Nishihara model failed to reproduce the accelerating creep stage. K E Y W O R D Scoal, creep model, effective stress, gas pressure, swelling stress, triaxial compression | INTRODUCTIONCreep behavior is one of the fundamental mechanical properties of coal or rock, associated with the stability of many buildings and underground structures in mining and rock engineering, such as roadways, tunnels, and dams. It is important to have a precise understanding of the creep or timedependent behavior of coal or rock, which may prevent coal 502 | ZHANG et Al. Highlights• The coupling effect of triaxial stress and gas pressure on coal was investigated. • The gas pressure played a positive role in the creep process of coal. • The effective stress of gas-filled coal was intensively studied. • The effective stress was considered in the nonlinear Nishihara creep model of coal, which could accurately reproduce the three creep stages, especially the accelerating creep stage.How to cite this article: Zhang L, Li X-C, Gao J-X, An Z-X, Yang X-H, Nie B-S. Creep characteristics and constitutive model of coal under triaxial stress and gas pressure. Energy Sci Eng. 2020;8:501-514. https ://doi.

Section: Discussionmentioning

confidence: 99%

Section: Effective Stress For Coal Containing Gasmentioning

confidence: 99%

Section: Effective Stress For Coal Containing Gasmentioning

confidence: 99%

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Creep characteristics and constitutive model of coal under triaxial stress and gas pressure

Zhang

Gao

et al. 2019

“…20,27,[43][44][45][46][47] Among them, the influence of in situ stress regime on K occurs throughout the whole process of CBM exploration and development. 20,27,[43][44][45][46][47] Among them, the influence of in situ stress regime on K occurs throughout the whole process of CBM exploration and development.…”

Section: In Situ Stress Influence On Coal Permeabilitymentioning

confidence: 99%

“…As one of the most important factors in the determination of CBM productivity, K has been influenced by complex geological factors, for example, structural framework, in situ stress regime, burial depth, coal structure, and development of natural fractures. 20,27,[43][44][45][46][47] Among them, the influence of in situ stress regime on K occurs throughout the whole process of CBM exploration and development. In other words, the pore-fracture system varies with the change of in situ stress regime, along with K of coal reservoirs.…”

Section: In Situ Stress Influence On Coal Permeabilitymentioning

confidence: 99%

Characteristics of in situ stress and its influence on coalbed methane development: A case study in the eastern part of the southern Junggar Basin, NW China

Yan

Yang

et al. 2019

Based on 54 sets of well test data of 29 coalbed methane (CBM) wells, the distribution characteristic of in situ stress in the eastern part of the southern Junggar Basin and its control on permeability (K), reservoir pressure (P o ), and gas content (G) were discussed systematically. The results show that three types of in situ stress regime exist and are converted corresponding to a certain depth, (1) <600 m is the strike-slip fault regime (σ H

Comparative analysis on pore‐scale permeability prediction on micro‐CT images of rock using numerical and empirical approaches

Song

Wang

Liu

et al. 2019

Varieties of pore‐scale numerical and empirical approaches have been proposed to predict the rock permeability when the pore structure is known, for example, microscopic computerized tomography (micro‐CT) technology. A comparative study on these approaches is conducted in this paper. A reference dataset of nine micro‐CT images of porous rocks is generated and processed including artificial sandpacks, tight sandstone, and carbonate. Multiple numerical and empirical approaches are used to compute the absolute permeability of micro‐CT images including the image voxel‐based solver (VBS), pore network model (PNM), Lattice Boltzmann method (LBM), Kozeny‐Carman (K‐C) equation, and Thomeer relation. Computational accuracy and efficiency of different numerical approaches are investigated. The results indicate that good agreements among numerical solvers are achieved for the sample with a homogeneous structure, while the disagreement increases with an increase in heterogeneity and complexity of pore structure. The LBM and VBS solver both have a relative higher computation accuracy, whereas the PNM solver is less accurate due to simplification on the topological structure. The computation efficiency of the different solver is generally computation resources dependent, and the PNM solver is the fastest, followed by VBS and LBM solver. As expected, empirical relation can over‐estimate permeability by a magnification of 50 or more, particularly for those strong heterogeneous structures reported in this study. Nevertheless, empirical relation is still applicable for artificial rocks.