## 1 Introduction  
As an unconventional oil and gas resource, shale gas is currently a key area for increasing reserves and production in China [1, 2]. As shale gas development gradually advances into deeper layers, whether deep shale gas can be economically exploited on a large scale is the top priority for the sustainable and effective development of shale gas resources [3]. Large-scale, long-term effective hydraulic fracture networks are the key technology to achieve economical exploitation of deep shale gas [4, 5]. At the same time, due to the high-pressure coefficient and reservoir temperature of deep shale gas reservoirs, existing shale gas seepage models are hindered when directly used for deep shale gas flow simulations [6, 7, 8]. Therefore, exploring the seepage mechanism of shale gas in fracture networks under deep conditions of high temperature, high pressure and high closure stress is of great guiding significance for achieving increased and stable production of deep shale gas. In the oil and gas industry, [PERSON]'s law has been widely used to describe fluid flow in porous media [9]. However, the traditional Darcy's law in shale nanopores cannot actually describe various flow regimes other than viscous flow [10]. Slip and diffusion processes are generally considered to be the main flow mechanisms [11]. [PERSON] et al. [12] used the model established by [PERSON] and [PERSON] to describe the seepage and diffusion behavior of shale gas in nanoscale pores, introduced Knudsen diffusion into the study of shale gas flow model and defined the apparent permeability after considering the viscous flow and gas slippage effects [13]. [PERSON] and [PERSON] [14] applied the nonequilibrium mechanism to study the effect of shale matrix heterogeneity on gas transportation, strengthening the interaction and influence between adsorbed gas and free gas. They then proposed the application of a nonlinear nonequilibrium model [15] to study the effect of adsorbed gas desorption on free gas transportation. [PERSON] et al. [16] proposed a new model for gas transport in organic and inorganic nanoporous shales, in which Knudsen diffusion was used to describe the free molecular state, and the slip effect was also considered. During the production process, surface diffusion may occur in the adsorbed gas layer, which is driven by the concentration gradient of the adsorbed gas. [PERSON] et al. [17] proposed an apparent permeability model to explain slip flow, Knudsen diffusion and surface diffusion in shale gas reservoirs and found that surface diffusion has a serious impact on the total flux, accounting for 25% at low pressures. Surface diffusion cannot be ignored in gas transport research. There are large differences in the effects on the deformation of matrix pores, natural fractures and artificial fractures during shale gas production, making the gas flow and solid deformation mechanisms more complicated. Finding out the changing rules of porosity and permeability is the key to establishing a fluid-solid coupling model for shale reservoirs [18; 19]. Both the continuous medium model and the discrete model are typical models for describing the seepage laws of fractured reservoirs. [PERSON] and [PERSON] [20] studied the oil recovery mechanism in fractured reservoirs using the MINC (Multiple Interacting Continua) method based on the dual porosity model and gained an in-depth understanding of the behavior of water/oil flow during the imbibition process, but it is not applicable to poorly connected fracture networks. [PERSON] and [PERSON] [21] developed a new discrete fracture multiphase flow model and found that oil bypassing was obvious at higher injection rates. [PERSON] et al. [22] established a hybrid model combining the advantages of continuous model and discrete model to uncover the delayed phenomenon during gas desorption. After hydraulic fracturing operations, fracture closure has an important impact on the maintenance of diversion channels and shale oil and gas production.