Page 38 - Geochemistry of Oil Field Waters

SYNTHETIC BRINE 27

smallest number of places. However, the operation is completed with all
decimal places intact, and rounding off is done afterward. As an example,
11.1 + 11.12 + 11.13 = 33.35, and the sum is rounded off to 33.4.
(b) Subtraction : when subtracting one number from another, rounding off
should be completed before the subtraction operation, to avoid invalidation
of the whole operation.
(c) Multiplication: when two numbers of unequal digits are to be
multiplied, all digits are carried through the operation; then the product is
rounded off to the number of significant digits of the less accurate number.
(d) Division: when two numbers of unequal digits are to be divided, the
division is carried out on the two numbers using all digits. Then the quotient
is rounded off to the number of digits of the less accurate of the divisor or
dividend.
(e) Powers and roots: when a number contains n significant digits, its root
can be relied on-for n digits, but its power can rarely be relied on for n digits.

Synthetic brine

Synthetic brine solutions are used in many of the analytical procedures
for analyzing oilfield waters (American Petroleum Institute, 1968). Such
solutions are a necessity in the development of analytical methods to study
the effects of possible interfering ions. Often these synthetic solutions are
used as an integral part of the analytical technique (Collins, 1967). Prepara-
tion of a fairly stable synthetic brine involves saturating distilled water with
carbon dioxide by bubbling carbon dioxide through it, followed by adding
the bicarbonate and sulfate compounds to one portion of the C02 -saturated
water, adding the alkali chlorides to one portion, and adding the alkaline
earth chlorides to one portion. The alkali chloride solution is mixed with the
bicarbonate-sulfate solution, and to this mixture the alkaline earth chloride
solution is added. Carbon dioxide is bubbled through the synthetic brine to
mix it, and the synthetic brine container is sealed immediately after re-
moving the carbon dioxide source.

Determination of pH

The pH of the water can be determined with a pH meter which utilizes the
principle of measuring the electrical potential between an indicator electrode
and a reference electrode (Potter, 1956, p.56). pH meters measure the elec-
trical potential between two suitable electrodes immersed in the solution to
be tested. The reference electrode assumes a constant potential, and the
indicating electrode assumes a potential dependent on the pH of the solu-
tion. Electrode potential is the difference in poteptial between the electrode
and the solution in which it is immersed. The calomel electrode, which is a
widely used reference electrode in water analysis, consists of a mercury-
calomel rod immersed in a saturated solution of potassium chloride; this