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.

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