Laboratory investigation of coal sample permeability under the coupled effect of temperature and stress

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 S_T 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 α_k, temperature sensitivity coefficient α_T, 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

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Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant No. 42172190) and the Shanxi Province Science and Technology Major Project (Nos. 20201102001, 20191102001, and 20181101013). The authors thank the reviewers and the editor for their constructive comments.

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