Page 190 - Geochemistry of Oil Field Waters

Chapter 6. ORGANIC CONSTITUENTS IN SALINE WATERS

Water is a peculiar solvent and has been considered to possess at ambient
temperature a quasi-crystalline, open structure which will allow solute mole-
cules to fill the space between the lattice points (Eley, 1939). As a nonpolar
molecule dissolves in water at ambient temperature, the structure of the
water in its immediate vicinity becomes more crystalline, or a microscopic
“iceberg” surrounds the solute (Frank and Evans, 1945). Water also has been
considered to be an equilibrium mixture of an icelike and a close-packed
structure, and with a molecule of gas as a solute, it reacts with the icelike
structure filling one of the cavities to form a gas-hydrate and shifting the
equilibrium from the close-packed structure to the icelike structure (Namoit,
1961).
Another theory is that water is composed of clusters of highly hydrogen-
bonded molecules which are surrounded by a closely packed structure of
monomeric water. These flickering clusters form and dissolve perpetually as
a result of local energy fluctuations. Therefore, a water molecule can have a
solute molecule as a neighbor along with its four H-bonded water neighbors.
Interactions between the solute and water molecules will depress the energy
level of the tetrabonded water molecule. However, large numbers of water
molecules surround an unbonded molecule, and if it acquires a solute
neighbor after the latter replaces a water molecule, the energy level is raised.
Changes in the energy levels cause a shift of water molecules between various
levels in accordance with the Boltzmann distribution law, giving an increase
in the “icelikeness” and an increase in the clusters of water molecules near
the surface of the solute molecule (Nemethy and Scheraga, 1962).
When a hydrocarbon molecule transfers from the pure liquid to the solu-
tion hydrocarbon-water, interactions are established while hydrocarbon-
hydrocarbon interactions are broken. The amount, kind, and state
(suspended, dissolved, or colloidal) of organic matter in petroleum-associated
waters is important in determining the origin and migration of petroleum,
and in problems concerning pollution of fresh waters by petroleum-
associated waters. Probably the most plausible theory concerning the origin
of petroleum is that it originated from organic constituents which are
recognized as remnants or degradation products of living organisms of past
ages; these organic source materials entered fine-grained aquatic sediments
where biochemical and chemical conversions and fractionations occurred
(Erdman, 1965). As increased sedimentation took place, the resulting over-
burden pressure and compaction caused the interstitial water, which con-