Apparent permeability model for real gas transport through shale gas reservoirs considering water distribution characteristic

Abstract

The accurate knowledge of gas transport mechanisms through shale matrix will significantly advance the development of shale gas reservoirs. At present, the different cross-section shapes for organic and inorganic pores have not drawn much attention. In terms of current literatures, the organic pores are considered as hydrophobic and the inorganic pores is hydrophilic, thus the water film can be adsorbed on the surface of inorganic pore. However, the volume occupied by water film is overlooked and its effect on gas transport capacity has not been investigated ever. In this work, the Beskok’s models are employed, which can be applied to characterize the bulk-gas transport mechanisms through circular nanotubes or slit nanopores with arbitrary aspect ratio. In addition, the organic and inorganic nanopores in shale matrix are assumed as nanotubes and slit nanopores respectively. Considering the presence of adsorbed gas phase, the apparent permeability model for organic pores takes bulk-gas transport regimes, surface diffusion and gas desorption into account. Considering the thickness of adsorbed water film, the apparent permeability model for inorganic matter incorporates the bulk-gas transport mechanisms and effect of water film. More features, such as stress dependence, real gas effect, are included in both models. Based on the proposed permeability models, the influences of pore size, formation pressure, and humidity on apparent permeability for organic/inorganic pores are seriously analyzed. Results show that the surface diffusion will dominate the transport capacity when the organic pore size is less than 2nm. For inorganic pores, it can be concluded that the larger pore radius will obtain the stronger transport capacity. The real gas effect has little influence on apparent gas permeability which can be neglected. The stress dependence, humidity and gas desorption influence the effective radius of nanoscale pores, which have significant effects on transport capacity.