A sequential model of shale gas transport under the influence of fully coupled multiple processes

• A strain-based apparent shale permeability model is developed.
• Nine sets of experimental data are used to verify this permeability model.
• The permeability model is used to couple multiple processes in shale.
• Results of the fully coupled modelling demonstrate the importance of shale deformation.

Shales have complex microscopic pore structures which significantly affect shale gas production. Effects of microscopic pore structure on flow regimes have been widely investigated. The pressure dependent permeability in shales has been also observed in laboratory and it may cause more significant variation in apparent permeability than flow regimes does. Therefore, numerical models combining flow regimes and pressure dependent permeability are required to describe the gas flow behaviour in shales. In this study, based on literature experimental observations, a numerical simulation model for shale gas transport was built. The model includes the main gas flow characteristics in shale: (1) sequential flow process of different flow regimes for different pores; (2) variation of apparent permeability resulted from both flow regimes and stress variation in shale; (3) permeability change with respect to strain. Nine sets of literature experimental data were used to verify this numerical simulation model, which was shown to be able to accurately describe the data. Using this numerical simulation model, shale gas flow behaviour was analysed and the following conclusions were found: (1) the effect of shale deformation on gas production is significant. Compared with other factors, it is a considerably important factor controlling the apparent permeability evolution during shale reservoir depletion; (2) natural fracture plays a significant role in gas transport inside reservoirs. Although its porosity is much less than those of other pores, it could obviously enhance shale gas recovery rate because of its higher permeability; (3) natural fracture permeability, natural fracture porosity, inorganic pores permeability and Young's modulus have positive correlations with shale gas recovery rate. However, the percentage of adsorbed gas has a negative correlation with shale gas recovery rate.