Page 102 - Geology and Geochemistry of Oil and Gas

CLASSIFICATIONS OF OILFIELD WATERS 75
liquid and gaseous non-electrolytes declines when charged ions appear in the
solution, which facilitates their precipitation out of solution.
Hydrogen and oxygen in water may have different isotopic composition. Three
hydrogen isotopes (protium H, deuterium D, and tritium T) are encountered in
water. The first one predominates. The D/H ratio ranges from 1/8,400 to 1/3,800.
Only tritium is radioactive; it is very scarce (T/H ratio is usually negligible — about
10 18 ).
Six oxygen isotopes are known: 14 O, 15 O, 16 O, 17 O, 18 O, and 19 O. The 14 O, 15 O,
and 19 O isotopes are radioactive, but with a short half-life. The most common
isotope is 16 O.
6
Water density r depends on the isotope composition of water (r ¼ 10 d, where d
3
is the water density at 3.98 1C in g/cm ). Surface water and the water within the free
circulation zone are of normal density. Water density increases within the zones of
stagnation (limited circulation).
The amount of dissolved and colloidal substances in the water is represented by its
salinity. The salts are chlorides, sulfates, and bicarbonates of alkaline and alkaline-
earth metals. It is assumed that these salts totally dissociate in water into the equal
number of cations and anions. This assumption forms the basis for the ion-
equivalent presentation format of the water composition. This is true for weak
solutions, but is not justified when the concentration of salt is high. This is important
to remember when interpreting electric log; there is no straight-line relationship
between the water salinity and the electric conductivity of water.
Mineral composition of ground water may be expressed in terms of weight (mass) (as
g/l, g/kg, g/100 g), or in terms of equivalents. To obtain the latter, the weight is divided
by the ion-equivalent value. The sums of anion and cation equivalents are considered to
be equal to 100%, and the ion percent-equivalent is calculated on this basis.

4.2. CLASSIFICATIONS OF OILFIELD WATERS
Waters can be classified in a number of ways. Most commonly they are grouped
according to the following criteria:
(1) water origin — meteoritic, connate, or juvenile waters,
(2) water chemistry, e.g., bicarbonate, sulfate, or chloride waters, and
(3) total water salinity, i.e., fresh water, saline water, or brine water.
Many chemical classifications have been proposed or discussed by Tolstikhin
(1932), De Sitter (1947), Durov (1948), Sulin (1948), Vassoyevich (1954), Chebotarev
(1955), Krejchi-Graf et al. (1957), Gorrell (1958), Rainwater and White (1958),
Chave (1960), and Eremenko (1960), to mention just a few investigators. For
example, see Table 4.1 (in: Eremenko and Chilingar, 1996). The water classifications
have been reviewed in Chilingar (1957, 1958), Chilingar and Degens (1964), and
Samedov and Buryakovsky (1966).
As an example, the classification scheme of N. I. Tolstikhin (in: Vassoyevich, 1954,
p. 112) is presented in Fig. 4.3. It is based principally on the distribution of the most

2
+

abundant cations (Na , Mg 2+ , Ca 2+ ) and anions (HCO 3 , Cl , SO 4 ). In following