## 6 Conclusions  
In this study, a fully coupled model for describing gas flow in multiscale shale reservoirs by combining the effects of pore size distribution, stress-sensitivity, and adsorption/desorption was proposed. The results of this study are available for quantitatively evaluating the gas flow capacities. There are two main advantages in comparison with other gas flow model for evaluation of shale reservoirs. First, this coupled model includes several individual models that can be used to describe the corresponding flow regimes accurately, and the boundaries of various flow regimes can be obtained for a giving geological condition. Secondly, gas flow capacity of shale reservoirs is dynamic variables because of the coupled effect stated above. Based on the findings of the study, the following conclusions can be drawn:  
1. Pore size distribution has a significant influence on gas flow in shale reservoirs. Multiscale pore structure means that various bulk flow regimes may occur rather than single continuum flow dominant in shale reservoirs. The gas flow capacity will be underestimated without considering the impact of pore size distribution. Because the frequency of pores in a certain pore size scale is a positive correlation with fractal dimension. A large amount of pores in slip flow and Knudson diffusion with respect to a larger fractal dimension enhance the apparent permeability of shale reservoirs. 2. The gas flow capacity will be overestimated without consideration of the effect of stress-sensitivity. Matrix permeability and fracture permeability decrease with decreasing reservoir pressure because of the extrusion resulted from an increase in effective stress. The variation on gas flow within fractures permeability caused by a change of effective stress is more significant than that on gas flow within the matrix with a high compressibility. In addition, Stress-sensitivity has a significant influence on bulk phase gas flow. Because matrix shrinkage with increasing effective stress, resulting in diminution of the effective pore diameter for bulk phase gas flow. 3. Gas flow capacity of bulk phase gas will be overestimated without consideration of the impacts of adsorption/desorption due to the adsorbed gas molecules along the pore walls will reduce the available pore space for bulk phase gas flow. In addition, considerable adsorbed gas contents  
Figure 17: Comparison of the impacts of adsorption/desorption and stress-sensitivity on total muss flux. are stored in the small pores results in surface diffusion, which enhances the transport of gas molecules along molecular concentration gradients. In addition, this coupled model also has some disadvantages: (1) Langmuir adsorption model was applied to describe the effects of gas adsorption, which means that adsorbed gas in shale reservoirs is a monolayer adsorption. Some previously research indicated adsorbed gas in micropores (<2 nm) within shale reservoirs at high pressures does not hold Langmuir isotherm (multilayer adsorption), resulting in more complexity of the change in effective pore diameter; (2) in this study, the adsorption parameters of Langmuir pressure and Langmuir volume were constants. However, the effects of adsorption capacity with different Langmuir pressure and Langmuir volume on gas flow in shale reservoirs were not discussed. In a further step, the effects of adsorption parameters such as Langmuir pressure, Langmuir volume, and adsorbed gas layer thickness that influence effective pore diameter and adsorbed gas concentration, and surface diffusion coefficient need to be evaluated to provide a more realistic view of the gas flow in shale reservoirs. Data curation, X.H. ; Funding acquisition, Y.M. ; Investigation, X.H. ; Methodology, Y.L. ; Software, Y.W. and X.H. ; Writing--manuscript, X.H. ; Writing--review and editing, X.H. and Y.Z. This research was funded by the National Natural Science Foundation of China (No. 41402124) and the National Science and Technology Major Project (No. 2017 ZX05035004-002).