Application of Electrokinetics for Enhanced Oil Recovery - Electrokinetics for Petroleum and Environmental Engineers - page 119

of complex hydrocarbons, resulting in simpler (hydrocarbon) daughter
products, which have lower viscosities than the original components, and
increase reservoir energy. Both of these conditions will enhance oil pro-
duction. As shown in figure 3.7, electrochemically enhanced reactions,
field experience has demonstrated that this mechanism only occurs in very
low potential (voltage) gradient situations (Döring 1993 and 1997; Döring
et al., 2000, 2001, and 2003).
Of the five electrokinetic mechanisms, Joule heating requires the largest
amount of energy and, consequently, the highest potential (voltage) gradi-
ent, while the electrochemically enhanced reactions require the lowest. The
other three mechanisms: electromigration, electrophoresis, and electroos-
mosis, all have intermediate energy and voltage gradient requirements.

3.8 Role of the Helmholtz Double Layer

3.8.1 Dissociation of Ionic Salts

Water is a very unique fluid. The covalent hydrogen - oxygen bonds result
in an H-O-H angle of 105°. Because both hydrogen ions are on the same
side of the oxygen ion, the water molecule is strongly polar (dielectric con-
stant of 78.54 at STP). It quickly dissociates electrolyte salts (e.g., NaCl)
into positive ions, or cations (such as Na + ) and negative ions, or anions
(such as Cl - ). he Na + ion is one electron short of electrical neutrality,
whereas the Cl - ion has one excess electron. The net result is the Cl - ion
is about twice the size of the Na + ion and one-half the size of water mol-
ecules. Each of the dissociated (positively charged) cations and (negatively
charged) anions, in aqueous solutions, are hydrated, surrounded by enve-
lopes of water molecules with poles of opposite charge (from that of the
ion) facing the ions, attempting to neutralize the ionic charges (as shown,
schematically in figure 3.8).
In free solutions, the hydrolyzed (in water molecule clouds) ions are free
to move and their relative numbers are comparable to what they were in
the solid (i.e., crystalline) state. However, this all changes near the matrix
of clastic rocks.

3.8.2 Silicates

The most common minerals in clastic rocks are silicates, cations sur-
rounded by silicate radicals. The basic silicate building block is a tetrahe-
dron, with a silicon ion at the center and oxygen ions on the four corners.