Study of Changes in Apparent Permeability in Shale’s Complex Microfracture Networks

Abstract

The microfracture structure in the reservoirs of shale has a strong influence on gas transport. To depict the transport characteristics of shale gas in widely developed microfractures, this paper has constructed a shale apparent permeability model based on fractal theory. Shale's microfracture structure was first identified as a bifurcation network, with the effects of effective stress and gas adsorption considered to obtain the dynamic fractal dimension definition equation. Effective viscosity was corrected by employing weighting factors relating to continuum flow viscosity and Knudsen diffusion viscosity. Finally, an apparent permeability model for shale was established by combining the three transport mechanisms of continuum flow, Knudsen's diffusion, and surface diffusion. The model was validated. The results showed that the viscosity flow rate of continuum flow and Knudsen diffusion increased under increasing pore pressure, revealing that the effect of a microfracture bifurcation network on shale's permeability was significant when shale gas was transported.

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