Gas transport behaviors in shale nanopores based on multiple mechanisms and macroscale modeling

• Published in: International journal of heat and mass transfer 2018-10, Vol.125, p.845-857
• Main Authors: Zhang, Qi, Su, Yuliang, Wang, Wendong, Lu, Mingjing, Sheng, Guanglong
• Format: Article
• Language: English
• Subjects: Apparent gas permeability; Dependence; Diffusion coefficient; Diffusion effects; Finite element method; Fractured reservoirs; Fractured well; Gas transport; Gas transport behaviors; Gases; Heat transfer; Hydraulic fracturing; Low pressure; Mathematical analysis; Multilayers; Numerical analysis; Numerical model; Oil shale; Organic and inorganic nanopores; Permeability; Porosity; Real gases; Sensitivity analysis; Shale gas; Shale reservoirs; Slip flow; Surface diffusion; Viscous flow
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2018.04.129

•New AGP models for the organic and inorganic nanopores were developed.•Effects of pore radius, pressure, and surface diffusion coefficient on gas transport behaviors are analyzed.•Numerical model for a fractured well in shale based on the AGP model and DFM model was developed.•Influences of different parameters on gas transport behaviors of the fractured well were discussed. The combined action of multiple transport mechanisms and reservoir characteristics makes gas transport behaviors in nanoporous shale complicated. Accurate apparent gas permeability (AGP) characterization for gas transport in nanopores is crucially essential for macroscale modeling in shale gas reservoirs development. In this study, a new unified AGP model for gas transport in the organic and inorganic nanoporous shale (OM and IM) is presented, incorporating multiple mechanisms, such as real gas effect, viscous-slip flow, Knudsen diffusion, surface diffusion, stress dependence and especially the organic nanopores content. Besides, the effect of multilayer adsorption on gas transport is included. The model is validated by experimental and linearized Boltzmann results and compared with the published AGP models. After that, sensitivity analysis and the contribution of each mechanism to the total AGP are conducted. Moreover, a numerical model for the fractured well in shale based on the presented AGP model and discrete fracture model (DFM) is derived. The finite element method (FEM) is applied to solve the model and then influence factors of gas transport behaviors are discussed. The results show that different transport mechanisms exist in organic and inorganic nanopores respectively. The larger pore radius or pressure causes a smaller ratio of the AGP over the intrinsic permeability. Moreover, the contribution of surface diffusion to the total AGP is significantly influenced by the OM nanopores radius and surface diffusion coefficient. In addition, gas transport is governed by Knudsen diffusion in nanopores with a small radius and low pressure and is controlled by viscous flow under the large pore radius and high-pressure conditions. Then, the presented AGP model is introduced into the macroscale numerical model for a fractured well in shale. Larger hydraulic fracture half-length, OM nanopores content and matrix pore radius as well as smaller natural fracture spacing cause higher gas production. The study provides a new unified AGP model considering gas transport behaviors in nanopores a