Measuring and modeling the displacement of connate water in chalk core plugs during water injection

The movement of connate water spiked with gamma-emitting ²²Na (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.