Analysis of electrokinetic coupling of fluid flow in porous media using a 3-D pore network

• 3-D pore network extracted from micro CT images of a low-permeability core.
• The pore network model was used to investigate electrokinetic coupling.
• Electrokinetic coupling effect increases flow resistance.
• Electrokinetic coupling effect does not cause non-Darcy flow.

A 3-D pore network model is employed in this paper to investigate the effect of electrokinetic coupling on fluid flow in porous media of low permeability. First, the distribution of electrostatic potential and net charge density resulting from the interplay between the negatively charged wall and ions in formation water in a single capillary is solved using finite element method, then we propose a simple scheme to calculate the mass and charge transport coefficients numerically for a single capillary and the scheme can be readily extended to capillaries of arbitrary cross sections. Next, the results of mass and charge transport in a single capillary are incorporated into a 3-D pore network extracted from micro CT images of a low-permeability core sample to study the influence of electrokinetic coupling on fluid flow. Mass and charge conservation is imposed on each pore to solve the pressure and electrical potential distribution in the pore network, after which the total flow rate can be calculated. Effects of electrokinetic coupling on fluid flow through the pore network at different salinities are investigated. The results show that in low-permeability reservoirs, electrokinetic coupling effect can increase fluid flow resistance and the total flow rate becomes smaller compared with the case in which the electrokinetic coupling is not considered. However, the simulation result also shows that the relationship between flow rate and pressure gradient remains linear when the effect of electrokinetic coupling is considered, suggesting that the nonlinear relationship between flow rate and pressure gradient observed in low-permeability reservoirs should be attributed to some other mechanisms.