Water Mixing During Waterflood Oil Recovery: The Effect of Initial Water Saturation

Abstract

Summary This work studies the mixing of injected water and in-situ water during waterfloods and demonstrates that the mixing process is sensitive to the initial water saturation. The results illustrate differences between a waterflooded zone and a preflooded zone during, for example, water-based EOR displacement processes. The mixing of in-situ, or connate, water and injected water during laboratory waterfloods in a strongly water-wet chalk core sample was determined at different initial water saturations. Dynamic 1D fluid-saturation profiles were determined with nuclear-tracer imaging (NTI) during waterfloods, distinguishing between the oil phase, connate water, and injected water. The mixing of connate and injected water during waterfloods, with the presence of an oil phase, resulted in a displacement of all connate water from the core plug. During displacement, connate water banked in front of the injecting water, separating (or partially separating) the injected water from the mobile oil phase. This may impact the ability of chemicals dissolved in the injected water to contact the oil during secondary recovery and EOR processes. The effect of the connate-water-bank separation was sensitive to the initial water saturation (Swi). The time difference between breakthrough of connate water and breakthrough of injected water at the outlet showed a linear correlation to the initial water saturation Swi. The results obtained in chalk confirmed earlier findings in sandpacks (Brown 1957) and thus demonstrated the generality in the results.

Enhanced oil recovery

“…The time difference between breakthrough of connate water and breakthrough of injected water at the outlet showed a linear correlation to the initial water saturation [52].…”

“…The mixing of injected water and in situ water during waterfloods was studied [52]. It was demonstrated that the mixing process is sensitive to the initial water saturation.…”

“…The mixing of in situ, or connate, water and injected water during laboratory waterfloods in a strongly water-wet chalk core sample was determined at different initial water saturations [52]. Dynamic one-dimensional fluid-saturation profiles were determined with nuclear-tracer imaging during waterfloods, distinguishing between the oil phase, connate water, and injected water.…”

Doctrines and Realities in Reservoir Engineering

“…The saturation of the connate water bank can be calculated from the fractional flow curve of the connate water/oil. The recent studies of Graue et al (2012Graue et al ( , 2014 are remarkable for their clarity in demonstrating this effect. Such a displaced connate water bank is intrinsic to the mathematical models used currently for polymer flooding and low salinity waterflooding (Jerauld et al, 2006).…”

“…The "connate bank" doctrine has been derived by focusing on artificial systems and ignoring more realistic conditions. For the oil phase, the elegant connate water bank experiments of Graue (2012) used exclusively the chemically inactive paraffin n-decane; but many years earlier, other studies used a variety of oil phases: mineral oils (paraffins), kerosene, and viscous oils produced from commercial petroleum reservoirs. The experiments of Brown (1966) used both kerosene and a viscous oil (140 cp); the experiments of Mungan and Smith (1966) used a mineral oil (12 cp) and a viscous oil (11 cp).…”