Page 18 - Geochemistry of Oil Field Waters

Chapter 2. SAMPLING SUBSURFACE OILFIELD WATERS

Subsurface waters associated with petroleum are subjected to forces that
promote mixing and homogeneity, but the assumption cannot be made that
they are so well mixed that no attention to sampling technique is required.
Localized conditions within an aquifer are commonly such that a given
subsurface body of water may not be of uniform composition. The com-
position of subsurface water commonly changes with depth, and also later-
ally in the same aquifer. Changes may be brought about by the intrusion of
other waters, and by discharge from and recharge to the aquifer. It is thus
difficult to obtain a representative sample of a given subsurface body of
water because any one sample is a very small part of the total mass, which
may vary widely in composition. To develop a comprehensive picture of the
composition characteristics of the total mass, it is generally necessary to
obtain and analyze many samples. Also, the samples may change with time
as gases come out of solution and supersaturated solutions approach satura-
tion.
The sampling sites should be selected, if possible, to fit into a comprehen-
sive network to cover an oil-productive geologic basin. Considerations in
selecting sampling sites are as follows:
(1) Which sites will better fit into an overall plan to evaluate the chemistry
of the waters on a broad basis?
(2) Which sites will yield the better information for correlation with data
obtained from other sites?
(3) Which sites are more representative of the total chemistry of brines
from a given area?
The value of the sample is directly proportional to the facts known about
its source; therefore, sites should be selected for which the greater source
knowledge is available.
For surveillance purposes, samples can be collected from the same site at
sufficiently frequent intervals that no important change in quality will occur
between sampling times. Change in composition may result from changes in
rate of water movement, pumpage rates, or infiltration of other water.
Changes that can occur in petroleum-associated water are illustrated in Table
2.1.
Well 1 shows the sort of change that commonly occurs. The water from
well 2 did not change between 1947 and 1957, within the accuracy of the
analytical determination. Water from well 3 changed drastically, suggesting
the intrusion of water from a different source.