Application of Electrokinetics for Enhanced Oil Recovery - Electrokinetics for Petroleum and Environmental Engineers - page 124

Matrix grain

Net negative

Capillary barrier

Bound layer

Helmholtz layer

Pore Throat
Darcy Flow

HC

Outer Helmholtz
layer

Matrix grain

EXPLANATION

Cl -

H 2 O molecule
Hydrocarbon

Bound layer

Na +

Figure 3.12 Pore throat schematic showing double layers forming adjacent to clay
mineral surfaces and restricting darcy flow through the center of the pore throat (after
Hill et al., 1999)

Figure 3.12 is a schematic diagram of a pore throat, illustrating the
Helmholtz double layer lining. It is formed, in preferentially water-saturated
rocks, by the attractive forces between the electrically negative surfaces of
minerals and the positive poles of water molecules and positive (cat) ions,
in aqueous solution (connate waters). There are no negative anions within
the Helmholtz double layer.
This electrical double layer is divided into two electrochemical regions:

• A region of fixed water molecules and cations (Bound, Stern,

or Inner Helmholtz layer), held by strong electrostatic forces
to negatively charged mineral surfaces.

• A more mobile region of loosely bound cations and water

molecules (Outer Helmholtz, or Gouy layer), which are par-
tially shielded from the negatively charged mineral surfaces
by the water molecules and cations of the Stern layer. These
loosely bound cations and water molecules can move in the
presence of an electrical external field.

If the pore throats are large enough, a third electrochemical region (free
fluid) exists, containing water molecules, as well as both anions and cations.