Page 274 - Geology and Geochemistry of Oil and Gas

MODELS OF STATIC GEOLOGIC SYSTEMS 237

Fig. 11.15. Relationships between the specific gravity of gas (compared to air) and the contents of CH 4
and CO 2 .

11.2.2.2.4. Formation water. The total salinity of formation waters and their com-
position depend on a number of subsurface processes, which are controlled by var-
ious natural factors. Because of different processes of formation and evolution of salt
content, subsurface waters in different oil and gas fields significantly differ in their
characteristics. That is why, the preparation of reliable formation water classifica-
tions for the formation water identification (and clarification of its origin and sub-
sequent evolution) is of great importance. In spite of significant variation in the salt
composition, it is possible to classify formation waters using an approach based on
mathematical simulation.
The main component of formation water in most oilfields is sodium chloride.
Thus, water salinity depends mainly on the content of chlorine anions or sodium
+
cations. Relationship between the content of chlorine rCl or sodium rNa ions and
water salinity P r can be described as a linear function

X
r ¼ a 1 rCl þ a 2
X
þ ð11:45Þ
r ¼ b 1 rNa þ b 2