The addition of large molecular weight molecules called polymers to an injected water can often increase the effectiveness of a conventional waterflood. Polymers are usually added to the water in concentrations ranging from 250 to 2000 parts per million (ppm). A polymer solution is more viscous than brine without polymer. In a flooding application, the increased viscosity will alter the mobility ratio between the injected fluid and the reservoir oil. The improved mobility ratio will lead to better vertical and areal sweep efficiencies and thus higher oil recoveries. Polymers have also been used to alter gross permeability variations in some reservoirs. In this application, polymers form a gel-like material by cross-linking with other chemical species. The polymer gel sets up in large permeability streaks and diverts the flow of any injected fluid to a different location.
Two general types of polymers have been used. These are synthetically produced polyacrylamides and biologically produced polysaccharides. Polyacrylamides are long molecules with a small effective diameter. Thus they are susceptible to mechanical shear. High rates of flow through valves will sometimes break the polymer into smaller entities and reduce the viscosity of the solution. A reduction in viscosity can also occur as the polymer solution tries to squeeze through the pore openings on the sand face of the injection well. A carefully designed injection scheme is necessary. Polyacrylamides are also sensitive to salt. Large salt concentrations (i.e., greater than 1–2 wt%) tend to make the polymer molecules curl up and lose their viscosity-building effect.
Polysaccharides are less susceptible to both mechanical shear and salt. Since they are produced biologically, care must be taken to prevent biological degradation in the reservoir. As a rule, polysaccharides are more expensive than polyacrylamides.
Polymer flooding has not been successful in high-temperature reservoirs. Neither polymer type has exhibited sufficiently long-term stability above 160°F in moderate-salinity or heavy-salinity brines.
Polymer flooding has the best application in moderately heterogeneous reservoirs and reservoirs containing oils with viscosities less than 100 centipoise (cp). Polymer projects may find some success in reservoirs having widely differing properties—that is, permeabilities ranging from 20–2000 millidarcies (md), in situ oil viscosities of up to 100 cp, and reservoir temperatures of up to 200°F.
Since the use of polymers does not affect the microscopic displacement efficiency, the improvement in oil recovery will be due to improved sweep efficiency over what is obtained during a conventional waterflood. Typical oil recoveries from polymer flooding applications are in the range of 1% to 5% of the initial oil in place. Operators are more likely to have a successful polymer flood if they start the process early in the producing life of the reservoir.
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