How Hydraulic Properties of Organic Matter Control Effective Liquid Permeability of Mudrocks

Abstract

In organic-rich Mudrocks, porous organic matter embedded within mineral matrix along with inorganic pores forms a dual porosity–permeability (ϕ–k) system. How hydraulic properties of organic matter embedded in the mineral matrix contribute to bulk liquid flow however is not well understood. Using computational methods, Navier–Stokes equations are coupled with Brinkman equations for dual (ϕ–k) mineral matrix–organic matter domains obtained from a focused ion beam–scanning electron microscope (FIB-SEM) image set of an organic-rich Murdock and a series of idealized two-dimensional (2D) pore–organic matrix domains. Results of sensitivity analysis show that variations in organic matter permeability cause variations in effective permeability, which follow ‘S-shaped’ characteristics curve on log–log scale. Hydraulic coupling between the dual (ϕ–k) domains shows magnification of the coupled flow behavior when permeability of organic matter is on the same order as the permeability of the connected inorganic pores. The effect of coupled flow becomes negligible when the permeability of organic matter is 102 higher or lower than the permeability of the connected inorganic pores. The fraction of maximum change in magnitude of effective permeability (Im) is found to be exponentially dependent on organic matter–pore channel fraction (f), i.e., Im ∝ e0.77f, for 0 < f < 5. The maximum contribution of organic matter on bulk liquid flow and a large variation in fluid flow velocities are found to be dependent on both the proportion of organic matter and length scale of investigation, indicating that the µm-scale FIB-SEM domains are far from the scale of representative elementary volume (REV). This study, however, describes how the hydrodynamic coupling of dual ϕ–k porous media contributes to emergent bulk liquid flow in organic-rich Mudrocks, and likely in similar dual ϕ–k porous media encountered within engineered systems to the ones found in nature.