Liquid permeability of organic nanopores in shale: Calculation and analysis

- Liquid permeability of organic nanopores is studied by mathematical modelling.
- The effects of boundary slip and physical adsorption are incorporated and discussed.
- The shape of velocity profile is studied quantitatively and related to slip factor.

The contribution of kerogen to oil flow in shale rock is not well understood but is crucial, and understanding the transport behaviors of oil through organic nanopores is important for shale oil reservoir development. Based on relevant MDS (molecular dynamics simulations), experimental and theoretical studies, a mathematical model was derived to calculate the liquid permeability of organic nanopores in shale. The model can incorporate the mechanisms of boundary slip and physical adsorption, and the complicated structural properties are included. The results show the following: (a) Flow enhancement or permeability is linearly related to the pore length. (b) For pore radii under 10 nm, the permeability changes slightly. The corresponding equivalent pore radius is 24 nm. (c) There is no need to consider the threshold pressure gradient or nonlinear flow characteristics for oil flow in kerogen. (d) For pore radii greater than 200 nm, slip is negligible. (e) Physical adsorption cannot obviously influence the permeability of the organic nanopores. (f) For pores larger than 500 nm, the mass flux of the physically adsorbed oil is negligible. By the definition of the normalized velocity, the shape of the velocity profile is studied quantitatively to show the dominance of a plug type velocity profile. The relationship between the slip factor and normalized velocity is also derived. As the normalized velocity approaches 1, the slip factor increases rapidly to infinity. This work can help in the understanding of the flow properties of kerogen and will shed light on the development of shale resources.

63