Page 22 - Geochemistry of Oil Field Waters

DRILL-STEM TEST 11

the oil. At some point down the column a representative formation-water
sample can be found. The point is variable and will be influenced by rock
characteristics, mud pressure, type of mud, and duration of the test. It is
best to sample the water after each stand of pipe is removed. Normally, the
total dissolved solids content will increase downwards and become constant
when pure formation water is obtained, if the concentration continues to
increase all the way to the bottom, no representative sample can be ob-
tained. A test that flows water will give even higher assurance of an un-
contaminated sample. If only one drill-stem test water sample is taken for
analysis, it should be taken just above the tool, as this is the last water to
enter the tool and is least likely to show contamination.
Fig. 2.1 and 2.2 illustrate two drill-stem test tools with their various
components. Fig. 2.1 illfistrates a Halliburton Company tool; Fig. 2.2
illustrates a Schlumberger Well Services Company tool. Other companies
supply equally adequate tools, and reference to specific brands throughout
this test is made for identification only and does not imply endorsement by
the US. Bureau of Mines. The drill-stem test can provide pressure head and
head decline and buildup data useful in permeability calculation (Brede-
hoeft, 1965) and other information for the determination of additional
reservoir conditions, such as the gas/oil ratio and reservoir depletion
(McAlister et al., 1965). A stratigraphic interval of interest is isolated in the
drilled hole by use of packers attached to the drill string. Opening the tester
valve in the test string allows the formation fluid to enter the drill pipe.
Pressures are recorded by gages in the bottom of the test tool.
To insure that a representative sample is obtained, the pH, resistivity, and
chloride content of samples taken at intervals down the drill pipe can be
determined. Usually a transition zone will be found below which apparently
uncontaminated formation water will be located. The pH, resistivity, and
chloride content will vary above the transition zone, and they will become
constant below it. The sample taken for analysis in the laboratory can yield
positive evidence of contamination, if present. The two most indicative tests
are pH and the color of a filtered sample. If the filtered sample remains tan
or brown and the color cannot be removed even with pressure filtration, it
probably is contaminated with drilling-mud filtrate. A sample can be placed
on a white-spot plate for color evaluation. For positive identification of the
presence of mud filtrate, a sample of the drilling mud used in drilling the
well can be obtained and allowed to react with distilled water, the reacted
water .is analyzed to determine the mud-contributed ions, and the suspected
contaminated sample is analyzed to determine if it contains these ions.
Analyses of water obtained from a drill-stem test of Smackover Limestone
water in Rains County, Texas, show how errors can be caused by improper
sampling of drill-stem test water. Analyses of top, middle, and bottom
samples taken from a 15-m fluid recovery are shown in Table 2.11. These data
show an increase in salinity with depth in the drill pipe, indicating that the
first water was contaminated by mud filtrate (Noad, 1962). The middle