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The injection of CO 2  into coal reservoirs offers a promising approach for reducing CO 2  emissions and enhancing coalbed methane production. An inadequate understanding of the CO 2  permeability variation in coals under the coupled constraints of temperature and effective stress limits the accurate evaluation of the CO 2  injection potential. To address this issue, coal samples of different ranks were collected from the Sihe, Zhangjiamao, and Dahebian coal mines in China (designated as SH-1, ZJM-1, and DHB-1, respectively). These samples were used to investigate the characteristics of permeability variation and sensitivity to coal rank, temperature, and effective stress. The coal samples ZJM-1, DHB-1, and SH-1 had absolute permeability of 0.01355, 0.03953, and 0.00064mD, respectively. The SH-1 sample had the lowest absolute permeability due to its high coal rank, and the DHB-1 sample had the highest absolute permeability due to its tectonically deformed and fractured coal structure. The association of permeability to effective stress can be more accurately described by the permeability model with variable pore compression coefficient than that with constant pore compression coefficient. The CO 2  permeability of the coal samples decreased exponentially with increasing effective stress, while the permeability damage rate increased exponentially and the permeability curvature decreased exponentially. Based on the fitted curve of the permeability damage rate to effective stress, three stages of stress sensitivity were identified: rapidly increasing stage, slowly increasing stage, and approximately stable stage. The permeability and temperature sensitivity coefficient exhibited a decreasing trend with increasing temperature, following a power function relationship. Among the undisturbed coal samples, the SH-1 sample showed higher sensitivity to stress and temperature compared to the ZJM-1 sample. The DHB-1 sample, characterized by a fractured coal structure, exhibited the lowest temperature sensitivity and the highest stress sensitivity. The temperature sensitivity of different coal samples tended to be consistent at temperatures above 70°C. Predictive models for CO 2  permeability under the dual constraints of temperature and effective stress were constructed for the three samples using the surface fitting method. These models can assist in evaluating the feasibility of CO 2  injection into coal reservoirs at various depths in associated areas.

Permeability variation and sensitivity of CO 2 injection into coals under the control of effective stress and temperature