Summary
The movement of connate water spiked with gamma-emitting 22 Na (a radioactive sodium isotope) was studied during
laboratory waterflooding of oil-saturated chalk at connate-water saturation
from a North Sea reservoir. Using a 1D gamma-monitoring technique, it was
observed that connate water is piled up at the front of the injection water and
forms a mixed water bank with almost 100% connate water in the front, behind
which a gradual transition to pure injection water occurs. This result
underpins log interpretations from waterflooded chalk reservoirs. An ad hoc
model was set up by use of the results, and the process was examined
theoretically at a larger scale.
Introduction
The behavior of the in-situ, or connate, water in an oil reservoir under
waterflooding has been investigated only sparsely in the past. A study of the
mobility of connate water in sandpacks during waterflooding showed that the
connate water became mobile and formed a buffer zone between the injection
water and the mobilized oil phase (Brown 1957). Water imbibition in a fractured
chalk plug using D2O (labeled connate water) and nuclear magnetic resonance
(NMR) imaging showed that the connate water was swept up in front of the
imbibing water (Nielsen et al. 2000). If these observations are valid on a
reservoir scale, it means that it is the connate water that actually displaces
the oil during a waterflood.
Laboratory corefloods have demonstrated that the remaining oil saturation
after a waterflood depends on chalk type, chalk porosity, and initial oil
saturation. Waterflooding of oil-saturated chalk cores develops an oil/water
shock front that displaces the mobile oil in a nearly pistonlike manner with
very little oil cut after water breakthrough, in agreement with theoretical
expectations (Dake 1978).
Sharp oil/water fronts have been observed from logging of waterflooded zones
in North Sea chalk reservoirs (Ovens et al. 1998). The actual oil saturation
and its potential variation within the waterflooded zone is, however, often
difficult to assess from standard petrophysical logs of a waterflooded zone
because of a change in resistivity and temperature after injection of cold
seawater.
An a priori model has been proposed by Ovens et al. (1998) from an
inspection of resistivity profiles across waterflooded zones in the Danish
North Sea. The observations indicate that the injection of cold seawater into
an oil-bearing chalk reservoir will generate a bank of reservoir-temperature
formation water between the cold injection water and the displaced oil. The
logs (porosity, water saturation, and deep resistivity) show that the injected
water does not mix thoroughly with the formation water when the oil/water front
progresses through the reservoir.
In an attempt to verify the a priori model, a dedicated laboratory
waterflooding program was developed. Synthetic seawater with a chemical
composition corresponding to diluted Dan field brine was injected into plugs
saturated with oil and connate water of the same chemical composition as the
synthetic seawater. The connate water, however, was spiked with 22Na (gamma ray
emitter), whereby the movement of connate water could be followed in time and
space. Basic parameters have been determined from the experiments, and an ad
hoc model describing the interaction between injection water, oil, and connate
water has been constructed. Finally, this model has been used to predict what
will happen for a deep penetration of injection water into chalk saturated with
oil and connate water.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
12 March 2003
- Revised manuscript received:
8 July 2005
- Manuscript approved:
1 March 2006
- Version of record:
20 June 2006
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