Permeability Prediction Method for Dipping Coal Seams at Varying Depths and Production Stages in Northeastern Ordos Basin

Understanding stress and/or temperature sensitivity on permeability helps in predicting permeability distribution in space and its change with production. While permeability evolution with pressure depletion has been well studied, its variation with temperature is usually ignored. In this study, permeability experiments were first conducted on two coal samples from the Northeastern Ordos Basin. Both stress and temperature loadings were implemented according to the in situ conditions at varying coal seam depths and production stages. To reduce experimental uncertainty, all factors that may affect the results were collected for dimensional analysis. The empirical permeability expressions were obtained by relating dimensionless variables to experimental results, followed by the verification with well test data. The results show that coal permeability decreases exponentially with increasing effective horizontal stress. Coal permeability decreases with increasing temperature at higher stress but increases with increasing temperature at lower stress. This suggests that the permeability change with temperature may reverse at specific stress, where the coal deformation due to temperature difference is offset by pressure depletion induced deformation. The stress reversal points for the Shanxi formation sample are located between 1.2 and 1.9 MPa, while those for the Taiyuan formation sample range from 1.8 to 2.5 MPa. In general, coal permeability evolution is essentially the joint effects of reservoir compaction, matrix shrinkage, and thermal expansion. The results also show that the permeability prediction method presented has high precision. The average relative error between the permeability expression and well test data for Shanxi formation is 28.53%.