Application of Electrokinetics for Enhanced Oil Recovery - Electrokinetics for Petroleum and Environmental Engineers - page 122

have differing amounts of excess negative surface charge. This variation
in net negative surface charge affects the number of dissociated cations,
which will attach themselves to the clay minerals.
This discussion covers silicate minerals and clastic (sand/shale) res-
ervoirs, which tend to be preferentially water wet. This condition results
in the continuous electrolytically conductive current pathways along the
walls of the reservoir pore throats.
Some of the world's most significant hydrocarbon reservoirs are car-
bonates, however, which tend to be preferentially non- water wet. Except
for very high S w , these reservoirs would not expect to have continuous
electrolytic current pathways, because of the hydrocarbons covering the
pore-throat walls. If, however, there are significant clay minerals coating
the pore throats of the carbonate reservoirs, they should begin to behave
much like the clastic reservoirs.

3.8.4 Cation Exchange Capacity

In addition to the net surface charge, phillosilicates and clay mineral crys-
tal structures are composites of layered silicates. The space between the
composite layers can be filled with water and/or hydrated cations, as shown
in figure 3.11. A measure of the interlayer cation attraction is called cat-
ion exchange capacity (CEC) and is measured by determining the number
of cations, which can be exchanged on the clay minerals (Mitchell, 1993).
CEC is commonly measured in meq/gm. A more useful measure of CEC,
for determining the electrochemical and flow behavior of shaly-sands silts,
and, perhaps, even shaly carbonates is the CEC/unit pore volume, Q v , of
Waxman and Smitts (1968):

CEC
V

, (3.4)

Q =

v

p

where: Q v is CEC per unit Pore volume, in meq/ml.
CEC is cation exchange capacity, in meq/gm.
V p is the pore volume, in ml/gm.
he Q v measure of CEC converts it from a mass measure to a specific
volume measure. It also provides a direct link between CEC and:

• The membrane spontaneous potential
• Resistivity behavior of shaly-sands
• Streaming potential