Page 19 - Geochemistry of Oil Field Waters

8 SAMPLING SUBSURFACE OILFIELD WATERS

TABLE 2.1
Changes in the composition of petroleum-associated waters (mg/l)

Constituent Well 1 Well 2 Well 3
1947 1957 1947 1957 1956 1959

Sodium and
potassium 29,062 25,000 46,038 45,924 1,491 856
Magnesium 1,100 1,200 2,011 2,200 30 2
Calcium 5,900 5,500 14,200 14,400 60 10
Bicarbonate 34 12 24 12 600 1,800
Sulfate 14 50 3 52 200 0
Chloride 58,500 51,800 102,100 102,800 2,000 300

Total dissolved
solids 94,610 83,562 164,376 165,388 4,381 2,968
~- _. -

There is a tendency for some petroleum-associated waters to become more
dilute as the oil reservoir is produced. Such dilution may result from the
movement of dilute water from adjacent compacting clay beds into the
petroleum reservoir as pressure declines with the continued removal of oil
and brine (Wallace, 1969).
The composition of petroleum-associated water varies with the position
within the geologic structure from which it is obtained. For example, if the
water table is tilted, the more dilute water probably will be on the structural-
ly high side. In some cases the salinity will increase upstructure to a
maximum at the point of oil-water contact. Usually this is caused by in-
filtrating meteoric waters.
Few of the samples collected by drill-stem test are truly representative
formation-water samples. During drilling, the pressure in the well bore is
intentionally maintained higher than that in the formations. Filtrate from
the drilling mud seeps into the permeable strata, and this filtrate is the first
liquid to enter the test tool.
The most truly representative formation-water sample usually is obtained
after the oil well has produced for a period of time and all extraneous fluids
adjacent to the wellbore have been flushed out. Samples taken immediately
after the well is completed may be contaminated with drilling muds, with
drilling fluids, and/or with well completion fluids, such as filtrate from
cement, tracing fluids, and acids, which contain many different chemicals.

Drill-stem test

The drill-stem test, if properly made, can provide a reliable formation
water sample. Mud filtrate will be the first fluid to enter the drill-stem test
tool, and it will be found at the top of the fluid column immediately below