Unstable immiscible displacement study in oil-wet rocks
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Displacement of a viscous fluid by a lower viscosity immiscible fluid (such as waterflood of the viscous oil) in a porous medium is unstable. The displacement front generates viscous fingers which lead to less oil recovery efficiency. Another important problem related to instability in immiscible flow is the lack of reservoir modeling to capture viscous fingerings in simulation grid blocks. Few approaches to represent this phenomenon in reservoir modeling have been proposed previously. A dimensionless scaling group (viscous finger number) has been suggested which have a power-law relationship with the breakthrough recovery and cumulative recovery in unstable core floods. The relative permeability used in large grid-block simulations has been modified to so-call “pseudo-relative permeability” on the basis of the dimensionless group, thus incorporating the effect of fingers in waterflood predictions. However, the previous proposed models were constructed from experiments only in water-wet systems. This research extends the recent viscous fingering models to oil-wet systems. Sandstone cores are treated to alter the wettability to oil-wet. Series of experimental studies are performed in both water-wet and oil-wet cores. Viscosity ratio, velocity and diameter are varied. It is shown that the previously developed viscous finger number works for the new water-wet experiments. However, for oil-wet experiments, the correlating dimensionless number is different. A pseudo-relative permeability model has been developed for oil-wet cores. The core flood experiments have been matched by the new pseudo-relative permeability model. This pseudo-relative permeability model can be used for reservoir simulation of viscous oil waterfloods and polymer floods.