Figure 3. Effect of wettability on zeta potential.

(a) Zeta potential as a function of water wetting index for each of the four crude oils tested (A–D) aged with FMB. Grey symbols represent aging in the absence of water, open symbols represent aging with water and the filled triangle represents the non-aged sample. (b,c,d) Schematics showing a hypothetical triangular pore occupied by (b) water and (c,d) oil and water. Signs denote the polarity of the zeta potential at water-wet or oil-wet mineral surfaces in, (b) oil-wet samples with a positive zeta potential at the mineral-brine and oil-brine interfaces, (c) water-wet samples with a positive zeta potential at the mineral-brine interface, and (d) oil-wet samples with a negative zeta potential at the oil-brine interface and a positive zeta potential at the mineral-brine interface. Note that the configuration of electrical charge in (d) will be stable except for a few nm (approximately one Debye length, corresponding to the local thickness of the diffuse layer) either side of the three-phase (oil-water-rock) contact points where the polarity of the zeta potential changes. The Debye length is only a few nm or tens of nm depending upon the total ionic strength, and the local distribution of electrical charge within the diffuse part of the EDL is shielded from, and therefore not affected by, nearby electrical charge (such as that at a mineral surface or adjacent EDL) over distances greater than one Debye length²⁸.