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Polymer flow through the porous medium
1. Retention of the polymer. When polymer flows through the pores of the rock, that is the amount of retained measurable polymer. Retention is mainly caused by adsorption on the surface of the porous material and the mechanical entrapment of pores, that are relatively small compared to the polymer molecule in solution. This phenomenon, in enhanced oil recovery processes, it is instantaneous and irreversible. However it is not entirely true, as small amounts of polymer can be removed from the porous medium. The retention values measured in the field are between 20 to 400 lbm polymer / acre-ft of gross volume, being less desirable retention level 50 lbm / Acre-ft. Lost cause retention of the polymer solution, which may cause the efficiency in controlling mobility is destroyed. Retention can also cause delay in the rate of polymer propagation.
2. PV inaccessible. Polymer molecules are larger than water molecules and some are larger than pores in the porous medium. Because of this, the polymers do not flow throughout the pore space in contact with the brine. The fraction of the pore space that is not in contact with the polymer solution is designated PV (Pore Volume) inaccessible and has been observed in all types of porous media for both polyacrylamide as biopolymer and is considered a general characteristic of the flow of polymers.
3. Reduction in permeability. The polymers reduce the apparent permeability of the rock. Reducing the permeability depends on the type of polymer, the amount of polymer held, the pore size distribution, and average size of the polymer relative to the pores of the rock. The reduction in permeability is determined experimentally by first displacement of polymer solution through the porous media and after displacement of the polymer with brine and brine permeability is measured after all the polymer has been displaced. In practice it is convenient to describe the reduction of permeability in terms of the permeability of the brine, this is done by defining resistance factor.
The characteristics of polymers injectivity in enhanced oil recovery processes
The mobility conditions maintenance is essential in this process. Unfortunately, increasing the injection viscosifiers can be reduced injectivity, slow decay of the liquid, and retarding oil production injection patterns. It can estimate the injectivity miss associated with the injection of polymer solutions if fractures are not open and estimate the degree of fracture extension if it is open. It is therefore necessary to examine the 3 main properties of polymers that affect injectivity:
1) Waste in the polymer.
2) Rheology of polymer in the porous medium.
3) mechanical degradation of the polymer.
Application window for the implementation of polymers as improved method of recovery
Since the polymer injection is not always sustainable for all the oilfield, should bear in mind certain criteria, which give an indication of whether this process is possible at all.
Characteristics of oilfield
The mineralogy is important with regard to the compatibility of the polymer solutions, for example, in shale formations polymer mixture with water to be evaluated because the water absorption phenomena of some clays and dissolution of carbonate formations carbonates with high temperatures that can change the properties of the polymer solution.
The oilfield depth is critical only when this has to do with reservoir temperature, injection pressure and fracture pressure.
Temperatures below 200 ° F ensure a stable polymer solution, however these values may vary depending on the type of polymer and product; aspects that are modified by temperature have to do with the tendency to flocculate, acceleration of decomposition reactions and adsorption of the rock.
The reservoir pressure is not critical if this allows the injection pressure is less than the fracture pressure and not so high that requires expensive pumping equipment.
The porosity of the reservoir should be of medium to high (higher than 18%) to ensure a good storability.
The absolute permeability is considered good from 50 to 250 md. Moderate permeability values (between 15 and 50md) cause higher injection pressures. Permeability values considered very good (between 250 and 1000MD) and excellent (greater than 1000MD) ensure greater recoveries with conventional water injection and cause the polymer injection is costly and difficult to justify. The concept of permeability variation connected with reservoir heterogeneity is better than the permeability only to determine areas of applicability polymer injection. The heterogeneous reservoirs are good candidates for this process for two reasons: 1st, the polymer solutions reduce the permeability of the rock. 2nd, the solution has a tendency to shift to areas of the site have not been swept and areas where water injection results in an unsatisfactory sweep.
Characteristics of fluids
The oil viscosity that directly controls the mobility ratio of oil and water should not exceed 150 to 200 cp. 100cp lower viscosities are preferred, however, very low viscosities allow only small improvements. In high viscosity crudes mobility variations can be substantial polymer. Thermal recovery methods are competitive when oil viscosities are high.
The proportion of water-oil at the beginning of the project should be low, even zero. This will mean greater movable oil saturation. The application of fluid injection from the beginning of a secondary recovery waterflood instead allow greater changes.
Improved polymers properties
The effect of polymer injection efficiency is improved areal and vertical sweep, reducing the radius of the water-oil mobility and divert fluid injected into the larger reservoir sectors. The process does not improve the efficiency of movement, as it has no effect on existing capillary forces and interfacial tensions.