The Langmuir isotherm has been applied to characterize monolayer adsorption in analytical modeling of methane transport in shale matrix. However, the production predictions based on the Langmuir model commonly deviate from the field data since the Langmuir model fails to capture the oscillating gas density profile in nanopores. This work aims to develop a new model considering multilayer adsorption to compensate the incompetency of traditional monolayer adsorption and improve the accuracy of gas transport calculations. A new analytical model is developed for the first time by integrating the governing diffusivity equation with a generalized Brunauer-Emmett-Teller multilayer adsorption model and a Fickian surface diffusion model. In addition, a modified Peng-Robinson EoS is well-coupled to characterize the Z-factor in nanopores. Subsequently, the new analytical model is validated against molecular dynamic simulation and core experiments with the assistance of an efficient iterative ensemble smoother algorithm.<br/><br/>The deviation between the calculated and the realistic apparent permeability can be significantly decreased from 11.90% to 4.85% while the multilayer adsorption is considered instead of the monolayer adsorption. The multilayer adsorption thickness is inversely correlated to the free gas transport space. A new empirical correlation of adsorption thickness with pressure is developed based on Halsey equation using experimental adsorption data on nanoporous adsorbents, which is adopted to calculate theoretical maximum layer numbers in multilayer adsorption. In addition, the apparent permeability in the new model shows the larger decline compared with that based on the Langmuir model. Consequently, the apparent permeability increases significantly as the number of adsorption layer and the application of the Langmuir model at high pressures in existing gas transport models may extensively underestimate the apparent permeability.<br/>

Gas transport modeling with multilayer adsorption in nanopores

2019 Fall National Meeting

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