Environmental Engineering Reference
In-Depth Information
discerned that the parameters affecting the fluid flow are the potential at
the interface (
y
1
), potential at the wall surface (
y
2
), reciprocal of EDL
thickness for the formation water (
k
2
), oil/water saturation (determined
by the ratio of
r
1
and
r
2
), and the radius of the capillary (
r
2
). Normally, as
the potential at the wall and the interface increases, the EO effect becomes
noticeable which results in larger flow rates.
The capillary size, surface potential and water and oil saturation are
important input parameters for the analytical solution of the coupling
coefficients. The pore throat size of common reservoir rocks varies within
the range of 1 to 30μm (Keighin, 1997; Ahr, 2008; Nelson, 2009). For evalu-
ation of
k
er,ow
coefficient, a constant nominal capillary radius of 10μm was
adapted to represent the mean diameter of the pore throat size of the cores
tested. The zeta potential of common water wet reservoir rock minerals
(quartz, kaolinite, and calcite) in contact with connate water varies within
the range of -50 to +50 mV (Lichaa et al., 1992; Marinova, 1996; Rodriguez
and Araujo, 2006). In this analysis, the representative channel wall surface
potential was set to -25 mV. The interface between water and non-polar
oil could possess substantial negative charge even in the absence of chemi-
cals that reduce surface tension (Haydon and Taylor, 1960; Rodriguez and
Araujo, 2006). Hydroxyl ions, released by the dissociation-association
equilibrium of the water molecules are adsorbed at the oil-water interface
where they create a negatively charged layer whose charge concentration
strongly depend on pH (Haydon and Taylor, 1960; Marinova, 1996). The
magnitude of this interface charge depends largely on the ionic composi-
tion of the aqueous phase, while the nature of the oil phase is of secondary
importance. Typical range of the interface potential is -20±
5 mV (Haydon
and Taylor, 1960). In this analysis, the oil-water interfacial potential was set
at 0 mV and -15 mV, for two separate solution schemes.
Water and oil saturations were found based on their respective volume
fractions in the capillary. The volume fractions of these components were
varied by changing the values of
r
1
and
r
2
. Figure 5.11 shows the veloc-
ity profile for half of the capillary for the case when the charge density
at the oil-water interface is set to zero (ψ
1
=0 mV). As observed in figure
5.11, the oil velocity profile is constant, which results in plug flow and the
water velocity increases within the EDL and remains constant across the
capillary.
The velocity profile for when the interface possess charge (ψ
1
= -15 mV)
is shown in figure 5.12. In this case, the water velocity increases across the
capillary which results in higher water flow. The oil flow remains as plug
flow, but at an increased quantity as expected.
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