Page 269 - Geochemistry of Oil Field Waters

Chapter 8. CLASSIFICATION OF OILFIELD WATERS

Classification of waters provides a basis for grouping closely related
waters. Because the grouping is chemical, it is dependent upon the dissolved
constituents found in the waters. Most of the classification systems devel-
oped to date have considered only the dissolved major inorganic constituents
and have ignored the organic and the minor and trace inorganic constituents.
Waters as related to the earth are meteoric, surface, and subsurface. Sur-
face waters can be fresh or saline if the amounts of dissolved constituents in
the waters are used to classify them. For example, water from melting snow
on a mountain top usually will contain small amounts of dissolved mineral
matter and can be classified as fresh water, while water in an ocean will
contain about 35,000 mg/l dissolved minerals and is classified as saline.
Waters found in rivers connecting the mountain stream to the ocean may
contain varying amounts of dissolved constituents and depending upon the
amounts can be classified as fresh or saline. In a similar manner, subsurface
waters are classified as fresh or saline. Merely classifying a water as either
fresh or saline does not provide a very useful classification. The dissolved
constituents that are used in many classification systems depend upon the
amounts or ratios of sodium, magnesium, calcium, carbonate, bicarbonate,
sulfate, and chloride found in the water. The reason for this is that these are
the ions that usually are determined or calculated in a water. (Sodium often
is calculated from the difference found in the stoichiometric balance of the
determined anions and cations.)
The amounts and ratios of these constituents in subsurface waters are
dependent upon the origin of the water and what has occurred to the water
since entering the subsurface environment. For example, some subsurface
waters found in deep sediments were trapped during sedimentation, while
other subsurface waters have been diluted by infiltration of surface waters
through outcrops. Some waters have been replaced by infiltration water.
Also, rocks containing the waters often contain soluble constituents, which
dissolve in the waters or contain chemicals which will exchange with chemi-
cals dissolved in the waters causing alterations of the dissolved constituents.
The amounts of dissolved constituents found in subsurface waters can
range from a few milligrams per liter to more than 350,000 mg/L This
salinity distribution is dependent upon several factors, including hydraulic
gradients, depth of occurrence, distance from outcrops, mobility of the
dissolved chemical elements, soluble material in the associated rocks, and the
exchange reactions.