Abstract: In order to reveal the mechanism of gas diffusion in micro- and nano-pores of shale, aiming at the problem that it is difficult to quantitatively characterize the gas diffusion capacity of shale reservoirs, a new gas diffusion coefficient model considering shale porosity, tortuosity and flow K_n number (ratio of average free path of gas molecules to flow characteristic scale, dimensionless) is established based on differential form momentum equation considering fluid viscosity. The model was verified with the self-developed shale near equilibrium experiment, and then a chart of influencing factors of shale gas diffusion coefficient was formed. The research results show that: considering the porosity, tortuosity and other porous media parameters and flow K_n number of shale gas, the newly established diffusion coefficient model can more accurately characterize the diffusion ability of shale gas, and the coincidence degree with the near equilibrium diffusion flow rate is more than 90%. Diffusion coefficient is negatively related to pressure and positively related to pore diameter. When the pressure is lower than 20 MPa, the pore diameter is lower than 10 nm or the K_n number is higher than 0.2, the influence of the above parameters on the diffusion coefficient must be considered. This study realizes the quantitative calculation of shale gas diffusion flow rate under reservoir conditions, which can be used to establish shale apparent permeability model, and provide scientific basis for the contribution of shale gas diffusion to production in different production stages, adjustment of production system and improvement of single well production.

Key words: shale reservoir, diffusion flow, diffusion model, momentum equation, flow experiment, gas diffusion characterization