## 1 Introduction  
With its wide distribution and high resource potential, shale gas has received increasing attention and plays a more important role around the world [1; 2; 3; 4]. Shale reservoirs have very low porosity and matrix permeability as compared to conventional gas reservoirs [2; 3]. Rapid technical advances, such as horizontal drilling and hydraulic fracturing technologies, guarantee continued shale gas production [4]. For most shale gas basins in South China and North America, the initial reservoir pressure (pore pressure) is between 2.76 and 68.95 MPa, and the Knudsen number ranges between 0.0002 and 6.00 [5]. Shale reservoirs have complex pore/fracture structures and include numerous nanopores, micropores,and microcracks, as well as larger-scale hydraulic fractures created after hydraulic fracturing [2]. Due to these factors, gas flow through a shale reservoir is very complex: (1) various flow regimes may occur in pores/fractures of different scales [3]; (2) the high content of organic matter or clay minerals and a large surface area for adsorbed gas occurrence that implies complex gas flow mechanisms in shale reservoirs. When reservoir pressure decreases, the desorption of adsorbed gas occurs and the concentration gradient on the pore walls decreases, which results in surface diffusion of adsorbed gas [6]; and (3) stress-sensitivity and adsorption/desorption have effects on the apparent physical properties of shale reservoirs, thereby affecting the gas flow mechanism [7; 8; 9]. During the gas production process, the stress-sensitivity response to a decrease in reservoir pressure will induce matrix shrinkage, while desorption of adsorbed gas causes the effective pore diameter to increase [7]. In general, gas flow through a shale reservoir is controlled by the coupled effects of multiscale pores/fractures, dynamic changes in reservoir pressure, adsorption/desorption, and stress-sensitivity. The gas flow mechanism plays an essential role in both shale gas accumulation and production. Hydraulic fractures are the critical paths for gas transport into production wells, while shale matrix is the mainly reservoir space for shale gas accumulation. When gas is produced from the reservoir, the free gas in the hydraulic fractures will flow out firstly. Then the gas in the matrix will transfer into these fractures, which supply the natural gas production. Therefore, the matrix also plays a critical role in the gas depletion over the whole lifetime of the reservoir. Different from conventional reservoirs, the apparent permeability of shale reservoirs is dependent on pore sizes, and is greater than the intrinsic permeability, because other flow regimes including slip flow, transition flow, and free molecular flow may occur. Therefore, it is important to study the gas flow mechanism in shale matrixes and fractures. Recently, studies of gas flow within shale reservoirs have mainly focused on gas flow in nano-scale pores by experimental analysis and numerical simulation [7; 8; 9; 10; 11; 12]. However, most of those models ignored the coupled effects of dynamic behaviors and shale reservoir properties, such as multiscale pores/fractures, adsorption/desorption, and stress-sensitivity on both the bulk phase gas flow and adsorbed gas flow. In addition to that, the validity of such correlation models using empirical coefficients has been questionable [13; 14; 15; 16; 17; 18]. [PERSON] et al. [19] and [PERSON] et al. [5] proposed gas transport models for describing gas flow in different scale fractures while considering the effects of the fracture shape; [PERSON] et al. [20] established an apparent permeability model for describing gas flow in multiscale shale reservoirs without considering the effects of adsorption/desorption and stress-sensitivity; [PERSON] et al. [3] proposed an apparent permeability model for evaluating gas flow in nano-scale shale matrix by coupling the effects of adsorption/desorption and stress-sensitivity; [PERSON] et al.