Page 401 - Petrophysics 2E

WETTABILITY 369

be demonstrated in the laboratory by treating the shale with strong
hydrochloric acid to give rise to a “hydrogen”-based clay and then with
a solution containing salts of the di- or trivalent cations.
Electrical properties develop because the associated cations are loosely
held and therefore are mobile and can be displaced by a direct current
electrical potential. Swelling occurs when fresh water is introduced
and the H30+ ion can enter the lattice structure of the clay mineral.
The H30+ ion is large and can enter into the lattice of some smectites
(montmorillonite, etc.) causing them to swell into a gel-like mass which
may be 10 to 40 times the volume of the original clay. Dispersion of the
clays occurs when the H30+ ions loosen the clay particles, especially
those lining the pore walls of the rock.
The sand Si02 molecule can react with hot water and water containing
salts to form silanol groups which are Bronsted acids (weak acids capable
of freeing a proton):

Si02 + 2H20 + Si (0H)z + 20H-
0- 0-
I I
polymeric form is: H-0-Si-0-Si-OH (6.10)
I I
0- 0-
= SiOH + SiO- + HS

Because of their acid surfaces, sandstones react with and adsorb
basic compounds readily, whereas acidic compounds are repelled. The
major polar organic constituents of crude oils are weak acids. These
do not adsorb readily on the Si02 surfaces and, therefore, sandstones
generally exhibit neutral to water-wet characteristics, which have been
observed by many investigators. Block and Simms furnished some direct
experimental proof of this [27]: They showed that octadecyamine, an
organic base, is strongly adsorbed on the surface of glass, whereas stearic
acid is hardly adsorbed at all.
Silica and clay minerals mixed with the sand have negatively charged
surfaces and, consequently, behave like weak acids in contact with water
having a pH less than 7. Although these surfaces will form weak acid-base
chemical bonds with the basic organic compounds present in crude oils,
they are unaffected by the acidic compounds. The resins and asphaltene
fractions of crude oils contain polar, polynuclear organic compounds
that can be acidic or basic. The basic compounds can interact with the
acidic silica and negatively charged clay surfaces, rendering the surface
oil-wet to a degree depending on the amount and types of basic organic
compounds available [8, 28-33].