Page 36 - Geochemistry of Oil Field Waters

REPORTING THE ANALYTICAL RESULTS 25

Reporting the analytical results

A study conducted by the American Petroleum Institute (1968) indicated
that some laboratories reported the results of oilfield water analysis as parts
per million (ppm) or as milligrams per liter (mg/l) without regard to the
specific gravity of the sample. For example, a sample with a specific gravity
of 1.200 containing 12,000 mg/l of calcium does not contain 12,000 ppm of
calcium but contains 12,000/1.200 = 10,000 pprn of calcium. Such an error
obviously is more serious in reporting the analytical results for a brine than
in reporting the results for a fresh water. The unit ppm means parts per
million by weight, while the unit mg/l means milligrams per liter or weight
per unit volume; therefore, they are not interchangeable until the volume is
changed to a unit weight. Table 3.111 indicates the relation between various
units of measurement.
Because the American Petroleum Institute now recommends that oilfield-
water analysis be reported in units of mg/l, other associations will no doubt
recommend the same uniform practice. Such standardization implements
studies concerned with the chemistry and geochemistry of waters.

Sign if ican t figures

The term significant figure (Ballinger et al., 1972) is used rather loosely to
describe some judgment of the number of reportable digits in a result. Often
the judgment is not soundly based and meaningful digits are lost or meaning-
less digits are accepted.
Proper use of significant figures gives an indication of the reliability of the
analytical method used. The following definitions and rules are suggested for
retention of significant figures.
A number is an expression of quantity. A figure or digit is any of the
characters 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, which, alone or in combination, serves
to express a number. A significant figure is a digit that denotes the amount
of the quantity in the place in which it stands.
Reported values should contain only significant figures. A value is made
up of significant figures when it contains all digits known to be true and one
last digit in doubt. For example, if a value is reported as 18.8 mg/l, the “18”
must be a firm value while the “0.8” is somewhat uncertain and may be
“0.7” or “0.9”.
The number zero may or may not be a significant figure:
(a) Final zeros after a decimal point are always significant figures. For
example, 9.8 g to the nearest milligram is reported as 9.800 g.
(b) Zeros before a decimal point with other preceding digits are signifi-
cant. With no other preceding digit, a zero before the decimal point is not
significant.
(c) If there are no digits preceding a decimal point, the zeros after the