Permeability constitutes a critical factor controlling the production of coalbed methane (CBM), and the sensitivity of the CBM reservoirs to stress and the degree of coalification strongly influences permeability variations. Elucidating the mechanism underlying the sensitivity of reservoir permeability to stress and degree of coalification is therefore a crucial prerequisite for enhancing CBM production capacity. Helium permeability tests were conducted on raw coal pillar samples to investigate the variation in coal permeability under different effective stresses and degrees of coalification. The effective stress ranged from 1.5 to 7.5 MPa, and the maximum vitrinite reflectance (Ro,max) varied between 0.456% and 3.211%. The results indicate that permeability decreases with increasing effective stress and Ro,max. When internal fractures in the coal samples are poorly developed, this relationship follows a negative exponential trend. To evaluate the permeability sensitivity of the coal samples, a stress sensitivity index (S1) and a coalification degree sensitivity index (S1R) were introduced and constructed. In addition, the permeability damage rate (PDR) and stress sensitivity coefficient (αk) were also employed to assess permeability sensitivity. The results show that the stress sensitivity of coal decreases with the increase in effective stress but increases with the rise in Ro,max; the coal sensitivity of coalification degree decreases with the rise in Ro,max and increases with the increase in effective stress. Furthermore, S1 and S1R exhibit strong positive linear correlations with other sensitivity evaluation parameters, indicating that they can serve as comprehensive indices for evaluating the overall permeability sensitivity of coal samples. A predictive model relating permeability to effective stress and maximum vitrinite reflectance was established for coal reservoirs. Using Pearson’s, Spearman’s, and Kendall’s correlation coefficients, the relationships among effective stress, coalification degree, and permeability were analyzed. The results reveal that coalification degree exerts a stronger control on permeability than effective stress. The permeability control mechanism was thereby clarified, providing theoretical guidance for the efficient development of CBM reservoirs.
Tan et al. (Mon,) studied this question.