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Frontiers of Earth Science

ISSN 2095-0195

ISSN 2095-0209(Online)

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Front. Earth Sci.    2023, Vol. 17 Issue (3) : 776-787    https://doi.org/10.1007/s11707-022-1036-8
RESEARCH ARTICLE
Effect of damage zone around borehole on carbon dioxide injection promoted gas extraction in soft and low-permeability coal seam
Lijun ZHOU1,2(), Xihua ZHOU1,2, Gang BAI1,2, Xianlin LI1,2, Mingkun LUO1
1. College of Safety Science and Engineering, Liaoning Technical University, Fuxin 123000, China
2. Key Laboratory of Mine Thermodynamic Disasters and Control (Ministry of Education), Fuxin 123000, China
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Abstract

Injecting external CO2 into soft and low-permeability coal seams can improve CH4 extraction efficiency, and also benefit in CO2 sequestration. However, the distribution law of damage zone around borehole in soft coal seam and its effect on the efficiency of CO2 injection promoted CH4 extraction are not clear. In this paper, a multi-physics coupling mathematical model considering damage effect is established for simulating the process of CO2 injection promoted CH4 extraction in soft and low-permeability coal seam. The distribution of damage zone and permeability around boreholes under different diameters and coal strengths are analyzed. The gas pressure and gas content in coal seam during CO2 injection promoted CH4 extraction when the model considered damage effect are compared with that of ignored. The results show that small borehole diameter corresponds to narrow damage zone around the borehole in coal seam. The damage zone expands with the increase of the borehole diameter. The damage zone increases exponentially with the borehole diameter, while decreases exponentially with the compressive strength of coal seam. The highest permeability in the damage zone has increased by nearly 300 times under the condition of simulated case. CH4 pressure around the extraction borehole reduces, and the reduction area expands with the increase of time. Compared with the result of considering the damage effect, the reduction area of ignoring it is smaller, and the reducing speed is slower. The integrated effect of CO2 injection and CH4 extraction leads to rapid decrease of CH4 content in coal seam near the boreholes. The CO2 pressure and content increase around the injection borehole, and the increasing area gradually extends to the whole coal seam. In soft coal seams, failure to consider the damage effect will underestimate the efficiency of CH4 extraction and CO2 sequestration, resulting conservative layout of boreholes.
Keywords soft and low-permeability coal seam      carbon dioxide injection      gas extraction      damage effect      permeability evolution     
Corresponding Author(s): Lijun ZHOU   
Online First Date: 30 June 2023    Issue Date: 12 December 2023
 Cite this article:   
Lijun ZHOU,Xihua ZHOU,Gang BAI, et al. Effect of damage zone around borehole on carbon dioxide injection promoted gas extraction in soft and low-permeability coal seam[J]. Front. Earth Sci., 2023, 17(3): 776-787.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-1036-8
https://academic.hep.com.cn/fesci/EN/Y2023/V17/I3/776
Fig.1  Stress-strain relation curve of coal mass (Zhu et al., 2018).
Fig.2  Geometry and boundary conditions for CO2 injection promoted CH4 extraction.
Fig.3  The heterogeneous distribution of elastic modulus in coal seam.
ParameterValueUnitParameterValueUnit
Young?s modulus of coal seam (E)4815MPaLangmuir volume constant of CH4 (VL1)0.0196m3/kg
Young?s modulus of skeleton (Es)8469MPaLangmuir volume constant of CO2 (VL2)0.0304m3/kg
Initial permeability (kf0)4.7 × 10?17m2Heterogeneous coefficient of coal (m)5–
Coal density (ρc)1470kg/m3Langmuir pressure constant of CH4 (PL1)1.32MPa
Porosity of matrix (φm)0.035–Langmuir pressure constant of CO2 (PL2)0.83MPa
Porosity of fracture (φf)0.011–Adsorption time of CH4 (τ1)4.0d
Poisson?s ratio of coal (ν)0.32–Adsorption time of CO2 (τ2)3.5d
Fracture stiffness (Kn)2.8GPa/mLangmuir strain constant of CH4 (εL1)0.0128–
Extraction pressure (pc)0.08MPaLangmuir strain constant of CO2 (εL2)0.0362–
Dynamic viscosity of CH4 (μ1)1.03 × 10?5Pa·sTemperature in coal seam (T)298.5K
Dynamic viscosity of CO2 (μ2)1.37 × 10?5Pa·sInitial CH4 pressure (pmg10)1.24MPa
Compressive strength of coal (σc)6.5MPaTensile strength of coal (σt)0.23MPa
Tab.1  Key parameters for numerical simulations
Fig.4  Distribution of elastic modulus in coal seam with different borehole diameters.
Fig.5  Change of damage zone in coal seam with borehole diameters.
Fig.6  Distribution of coal permeability under different borehole diameters. (a) d=100 mm; (b) d=150 mm; (c) d=200 mm; (d) d=250 mm.
Fig.7  Distribution of elastic modulus in coal seam with different coal strengths.
Fig.8  Change of damage zone in coal seam with coal strengths.
Fig.9  Distribution of coal permeability under different coal compressive strengths.
Fig.10  CH4 pressure contour in coal seam. (a) Without damage effect; (b) damage effect.
Fig.11  Change curve of CH4 pressure in coal seam. (a) CH4 pressure on reference line A-B; (b) CH4 pressure on reference point C.
Fig.12  CH4 content contour in coal seam. (a) Without damage effect; (b) Damage effect.
Fig.13  Change curves of CH4 content in coal seam. (a) CH4 content on line A-B; (b) CH4 content on point C.
Fig.14  Contour of CO2 pressure in coal seam. (a) Without damage effect; (b) damage effect.
Fig.15  Change curve of CO2 pressure in coal seam. (a) CO2 pressure on reference line A-B; (b) CO2 pressure on reference point C.
Fig.16  Contour of CO2 content in coal seam. (a) Without damage effect; (b) damage effect.
Fig.17  Change curves of CO2 content in coal seam. (a) CO2 content on line A-B; (b) CO2 content on point C.
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