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Polymer Injection Process Description
Polymer injection is directed toward improving the volumetric sweep efficiency of a waterflood. Water-soluble polymers are added to the injection water to increase its viscosity and thereby improve (lower) the mobility ratio for displacing oil.
Figure 2 shows displacement fronts up until breakthrough for mobility ratios from 0.151 to 71.5 within a quarter of a five-spot pattern. Two points should be noted here:
- The lower the mobility ratio the higher the areal sweep efficiency at breakthrough of the injected fluid.
- For situations involving adverse mobility ratios, viscous fingers develop that causes severe bypassing of the reservoir oil by the injected fluid.
Polymer solution viscosity must therefore be carefully tailored to mitigate any tendency for the injected fluids to finger through the reservoir.
Economic considerations dictate that polymer floods be conducted using only a slug of polymer solution (3.5% to 45% reservoir pore volume have been used) displaced with drive water.
It is common practice not to inject the polymer slug solution at a constant concentration. Rather a "graded concentration profile" is employed. The leading edge of the polymer slug is designed with sufficient effective viscosity to improve the oil displacement mobility ratio as required with allowance made for adsorption losses (most polymers adsorb to the reservoir rock to some degree). The polymer concentration is then reduced in multiple stages towards the trailing edge of the slug, which
Reduces the overall cost of the slug, and Prevents an adverse mobility ratio from existing between the slug and the drive water.