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Core Flushing Tracer
Theory
Core Flushing Measured
with Tritiated Water
Cores are subject to flushing by drilling fluid during the
coring process. The preferred method for determining the amount of
flushing by water-base drilling fluids is to add tritiated water to
the drilling fluid and then measure the amount of tritium extracted
from the core in the lab.
The measurement process is
very straightforward. For example, if the drilling fluid contained
an average of 100 pCi/mL of tritium and it is later found that water
removed from the core contains 50 pCi/mL of tritium, it can be
concluded that 50% of the water in the core came from the drilling
fluid while the other 50% is connate brine. In this example, if the
original core analyses showed:
| Oil
...................................... |
50% |
|
| Water
................................. |
50% |
Then the complete
analyses would show:
| Oil....................................... |
50% |
|
| Formation Brine
.................. |
25% |
|
| Drilling Water
...................... |
25% |
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Since an invading
drilling fluid would have displaced hydrocarbons or formation
brine, correction can be made for the invasion. The test
furnishes information on the quality of the drilling fluid
from the standpoint of limiting fluid invasion. (In actual
tests we see invasion varying from negligible to near total
flushing.) The amount added is set to be lower than the amount
allowed in drinking water so that there is no radiological
hazard. In spite of this low level, the amount of tritium can
be accurately measured by scintillation
counting. |
Core flushing
evaluation using tritiated water
The Wallac 1415
scintillation counter makes it possible to analyze for tritium
to very low levels with high accuracy. For large samples (such
as drilling mud), it is possible to analyze to a level of 3.6
Bq/L (97 pCi/L). This is 100,000 times lower than the
permitted level for unrestricted discharge of tritium (MPC).
Cores may be segmented and analyzed by Dean Stark techniques
with total water extracted being as low as a few microliters.
Extreme care is required in processing cores, avoiding air
contact, and in storing and analyzing the fluids. Analytical
apparatus of great sensitivity is needed.
An example
case
The following three
graphs illustrate typical results from our analyses of
drilling fluids and Dean-Stark-extracted water from
cores.
Mud tritium levels
In this analysis of drilling
fluids, a sample of the mud was taken every twenty feet
of drilling or every one hour of standing time. A
representative number of samples was distilled, and the
water was analyzed for radioactive content.
The tritium concentration
started off quite high, but was lower later in the
coring process. This is due to make-up water being added
to the mud to offset fluid loss down hole, or it could
be due to an influx of water from an underground source.
The effect of this dilution can be accounted for by
interpolating the concentration of tracer in the mud at
the time a particular core section was drilled. (The mud
can be disposed of by any means, since it is not
considered radioactive at these
levels.)
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Tritium
Activity
This graph
shows the actual level of tracer in the
Dean-Stark-extracted waters from the core. We can get
usable analyses from sample volumes as small as 20
µL.
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