Microscopic displacement efficiency, effect

The amount of oil that is recoverable from a reservoir by a displacement process depends on (1) the effectiveness with which the injected fluid displaces oil from the pores in the rock (microscopic displacement efficiency) and (2) the volumetric fraction of the reservoir contacted by the injected fluid (macroscopic sweep efficiency). This latter efficiency is governed by the mobility ratio but also in large measure by the geologic heterogeneity of the reservoir rock. Permeabilities vary both areally and vertically, and large changes typically occur in the vertical direction in a single well. As an example, Ffe. 5.72 shows permeability variation with depth for a shallow sandstone reservoir in eastern Kansas. ... 

Fig. 1. The effect of capillary number, N, on the microscopic oil displacement efficiency in porous media of various size distribution (Ref. 1).

The use of this analytical approach is rather limited in field-scale systems since many other factors that cannot be included in a 1-D model are involved. However, it is useful to apply this method to simplified 1-D system properties when considering that the polymer will play a role in improving the microscopic (linear) displacement efficiency. If there already exists a near-unit mobility displacement in the waterfront, then there is no need to apply this procedure. In this case, the only quantities of interest would be the relative velocities of the (almost piston-like) water saturation front and the polymer front, but these may be found quite trivially without applying the full procedure described above. Note that this procedure predicts a water bank made up of the connate water and water that has been stripped of polymer. This is an important effect which may be observed in both 1-D and 2-D experiments even in the absence of polymer adsorption (where the bank consists of connate water only). 

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