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Frontiers of Earth Science

ISSN 2095-0195

ISSN 2095-0209(Online)

CN 11-5982/P

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Front. Earth Sci.    2022, Vol. 16 Issue (4) : 963-974    https://doi.org/10.1007/s11707-022-0983-4
RESEARCH ARTICLE
Laboratory investigation of coal sample permeability under the coupled effect of temperature and stress
Yina YU1, Zhaoping MENG1,2(), Jiangjiang LI3, Yixin LU1, Caixia GAO3
1. College of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
2. State Key Laboratory of Coal and CBM Co-mining, Jincheng 048000, China
3. Exploration and Development Research Institute, PetroChina Huabei Oilfield Company, Renqiu 062552, China
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Abstract

The stress and temperature sensitivities of coal reservoirs are critical geological factors affecting coalbed methane (CBM) well exploitation; in particular it is important to reduce or eliminate their influence on coal reservoir permeability. To investigate coal permeability behavior at various effective stresses and temperatures, CH4 permeability tests were conducted on raw coal samples under a varying effective stress of 2.0–8.0 MPa under five different temperatures (25°C–65°C) in the laboratory. The results show that the permeability of the coal samples exponentially decreases with increasing effective stress or temperature, which indicates obvious stress and temperature sensitivity. Through a dimensionless treatment of coal permeability, effective stress, and temperature, a new stress sensitivity index S and temperature index ST are proposed to evaluate coal stress and temperature sensitivity evaluation parameters. These new parameters exhibit integrality and uniqueness, and, in combination with stress sensitivity coefficient αk, temperature sensitivity coefficient αT, and the permeability damage rate PDR, the sensitivities of coal permeability to stress and temperature are evaluated. The results indicate that coal sample stress sensitivity decreases with increasing effective stress, while it first decreases and then increases with increasing temperature. Additionally, coal sample temperature sensitivity shows a downward trend when temperature increases and fluctuates when effective stress increases. Finally, a coupled coal permeability model considering the impacts of effective stress and temperature is established, and the main factors affecting coal reservoir permeability and their control mechanism are explored. These results can provide some theoretical guidance for the further development of deep CBM.
Keywords deep coal reservoir      permeability variation      stress sensitivity      temperature sensitivity     
Corresponding Author(s): Zhaoping MENG   
Online First Date: 27 September 2022    Issue Date: 11 January 2023
 Cite this article:   
Yina YU,Zhaoping MENG,Jiangjiang LI, et al. Laboratory investigation of coal sample permeability under the coupled effect of temperature and stress[J]. Front. Earth Sci., 2022, 16(4): 963-974.
 URL:  
https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-0983-4
https://academic.hep.com.cn/fesci/EN/Y2022/V16/I4/963
Fig.1  Schematic of coal reservoir seepage field under multifield coupling.
Sample ID Confining stress/MPa Axial stress/MPa Effective stress/MPa Temperature/°C
S1 2.0 2.6 2.0 25/35/45/55/65
S2 4.0 4.6 4.0 25/35/45/55/65
S3 6.0 6.6 6.0 25/35/45/55/65
S4 8.0 8.6 8.0 25/35/45/55/65
Tab.1  Specific temperatures and stress conditions for seepage experiments
Fig.2  Schematic of the experimental apparatus.
Fig.3  (a) Coal permeability evolution with effective stress at different temperatures. (b) Initial coal permeability evolution with temperature.
Temperature/°C k0/(10?3 μm2) a1/(MPa?1) R2
25 1.052 0.613 0.999
35 0.405 0.455 0.999
45 0.322 0.518 0.996
55 0.238 0.538 0.999
65 0.215 0.557 0.999
Average 0.446 0.536 0.998
Maximum value 1.052 0.613 0.999
Minimum value 0.215 0.455 0.996
Tab.2  Regression coefficients between coal sample permeability and effective stress at different temperatures.
Fig.4  (a) Coal permeability evolution with temperature at various effective stresses. (b) Initial coal permeability evolution with effective stress.
Effective stress/MPa k0t/(10?3 μm2) a2/(°C?1) R2
2.0 0.934 0.046 0.96
4.0 0.549 0.066 0.97
6.0 0.054 0.029 0.98
8.0 0.034 0.037 0.99
Avg. 0.393 0.045 0.98
Max. value 0.934 0.066 0.99
Min. value 0.034 0.029 0.96
Tab.3  Regression coefficients between coal sample permeability and temperature at different effective stresses.
Fig.5  Relationship between permeability damage rate (PDR), stress sensitivity coefficient (SSC) αk, and effective stress at different temperatures.
Temperature/°C PDR/% αk/(MPa?1)
Maximum Minimum Average
25 96.771 0.353 0.008 0.1238
35 94.450 0.302 0.015 0.1223
45 93.871 0.322 0.011 0.1223
55 95.072 0.331 0.013 0.1229
65 95.751 0.332 0.014 0.1232
Average 95.183 0.328 0.012 0.1229
Maximum 96.771 0.353 0.015 0.1238
Minimum 93.871 0.302 0.008 0.1223
Tab.4  Stress sensitivity evaluation parameters at different temperatures
Temperature/°C Fitting formula S R2
25 y = 1.0039x?1.976 1.976 ± 0. 175 0.995
35 y = 1.0134x?1.546 1.546 ± 0.267 0.972
45 y = 1.0057x?1.714 1.714 ± 0.156 0.993
55 y = 1.0049x?1.769 1.769 ± 0.150 0.994
65 y = 1.0037x?1.821 1.821 ± 0.131 0.996
Tab.5  Statistic results for the new stress sensitivity index S of coal samples
Fig.6  Relationship between permeability damage rate (PDR), temperature sensitivity coefficient (TSC) αT, and temperature under different effective stresses.
Effective stress/MPa PDR/% αT/°C?1
Maximum Minimum Average
2.0 77.163 0.067 0.001 0.018
4.0 89.873 0.080 0.001 0.020
6.0 66.667 0.050 0.002 0.017
8.0 77.627 0.061 0.003 0.018
Average 77.833 0.065 0.002 0.018
Maximum 89.873 0.080 0.003 0.020
Minimum 66.667 0.050 0.001 0.017
Tab.6  Temperature sensitivity evaluation parameters at different effective stresses
Effective stress/MPa Fitting formula ST R2
2.0 y = 0.9874x?1.686 1.686 ± 0.09 0.991
4.0 y = 0.9897x?2.314 2.314 ± 0.19 0.984
6.0 y = 1.0307x?1.155 1.155 ± 0.12 0.962
8.0 y = 0.9999x?1.457 1.457 ± 0.10 0.986
Tab.7  Statistical results for the new temperature sensitivity index ST of coal samples
Fig.7  (a) Correlation analysis of stress sensitivity evaluation parameters. (b) Correlation analysis of temperature sensitivity evaluation parameters.
Fig.8  (a) Coal permeability evolution under the coupled impact of effective stress and temperature; (b) Pearson correlation analysis.
Fig.9  Comparison between measured and predicted permeability.
Fig.10  Simplified multiple fracture system of two groups of orthogonal fracture systems parallel to the z direction.
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