Screen factor degradation

The measured reduction in screen factor was 1%. Maerker s data also indicate that at 500 B/D, polymer screen-factor degradation should be at least 60% in the pilot. Pilot data indicate a 1 % reduction. On the basis of these comparisons, it is concluded that Nalco 3857 flowing in a high-permeability unconsolidated matrix may not experience the same level of degradation as the polyacrylamide used in Maerker s work. From Fig. 5, at a 600 ft/D velocity, a casing OD of 7 in., and a perforation length of 6 in., a maximum injection rate of 32 B-D/in. of perforation is predicted. This compares reasonably well with 16 B-D/in. that was achieved during pilot operations. 

Polyacrylamide El, with the lowest electrochemical degradation factor of 11.2 in Table 3, experiences the smallest reduction of resistance factor in the presence of univalent and divalent electrolytes, from 55.9 in river water to 49.5 in an 80/20 mixture of river and formation waters. These unusually large resistance factors probably resulted from the hydrodynamic resistance of the long linear polymer chain which is a unique characteristic of its gamma radiation manufacturing process. There appears to be some correspondence between the effect of electrolytes on viscosity and screen factor since polymers C and D1 with the lowest electrochemical degradation exhibit the greatest reduction in screen factor on... 

Figure 4.15. The Maerker correlation for screen factor loss caused by mechanical degradation in various consolidated cores (after Maerker, 1975).

Figure 4.16. Screen factor versus flux/D correlation for mechanical degradation in both consolidated cores and unconsolidated sand (from Seright, 1980 including data from Maerker, 1975, 1976).

Table 5.15 presents values of screen factors for some polyacrylamide solutions. The screen factor was initially used to correlate flow resistance in porous rocks. However, as discussed in the next section, the screen factor commonly is used to evaluate shear degradation of the polymer solution. The screen factor has been shown to be a direct measure of the viscoelastic characteristics of a polymer solution. 

Fig. 5,43—Laboratory degradation data percent losses of screen factor and viscosity as func< tions of flux through consolidated sandstone plugs for 600-ppm polyacrylamide solutions In 3.3% brine.

Seright also presented data for the flow of a polymer solution that was degraded before flow studies. Table S.17 shows results obtained when a polymer solution had been degraded by injecting the solution into a 150-md Berea core at a flux of 42.1 ft3/(ft2-D). The screen factor of the polymer solution decreased from 18.2 to 6.5, while the solution viscosity decreased from 2.56 to 2.10 cp. There were no shear-thinning effects on this polymer because the salinity was 3.3%. Table 5.17 summarizes the displacement data when this solution was used for fluxes ranging from 7.1 to 56.4 ft /fft -D). Measurements of the screen factor indicate that no further degradation occurred until the flux exceeded 42.1 ft3/(ft2-D). 

The effects of mechanical degradation for uniform materials can be correlated from the stretch rate of the fluid. Fig. 5.49 is the correlation between screen-factor loss and a group deter-... 

From Fig. 5.49, the screen-factor loss caused by mechanical degradation is about 67%. Thus, significant mechanical degradation of the polymer is expected at this injection rate. Because solution viscosity is not as sensitive to mechanical degradation as the screen factor, additional data are needed to evaluate the effects on solution viscosity. 

There is substantial loss in solution viscosity caused by mechanical degradation. Table 5.18 summarizes Carreau model parameters and screen factors for these solutions. The reduction in solution viscosity is a clear indication that the average molecular weight of the polymer was reduced by mechanical degradation. This reduction was confirmed by size-exclusion chromatography when a decrease in the molecular weight of about 68% was determined for the effluent from the flow experiment at a frontal-advance rate of 278 ft/D. 

Design procedures included laboratory tests on polymer rheology, relative permeability, shear degradation, screen factor, stability, and salinity effects. Computer simulations were performed to predict recoveries and to examine optimum polymer concentration. Field injectivity tests were conducted to examine injectivity behavior with time and to gain experience in surface handling of the polymer. Pressure-falloff tests were conducted in conjunction wiA the injectivity tests. Finally, the prqiamtion involved design of the polymer-injection plant and analysis of costs. 

A 500-ppm solution of polyacrylamide is to be injected into a reservoir through a slimhole completion. This completion is 2 /g-in. tubing cemented to the surface in a 5 8-in. hole. The productive formation was completed open hole. The polymer injection rate is expected to be 1 B/D-ft, The reservoir is a consolidated sandstone with a porosity of 0.20 and a permeability to brine at Sor of 10 md. Using Maerker s correlation, estimate the loss in screen factor and solution viscosity resulting from shear degradation this polymer. 

Dilute stream shear testing was performed on the EP injection skid, where Injectivity would not be Impaired. The sheared polymer solution was sampled at the wellhead, and numerous tests were performed to detect the changes in polymer solution quality. Viscosity, screen factor, concentration of polymer, core plug injectivity and filtration performance were measured to evaluate the shear degraded polymer solution. 

To estimate the maximum rate polymer can be injected, the maximum polymer stretch rate was calculated to occur in the reservoir matrix just outside the perforations. The difficulty in estimating the true velocity (and hence shear degradation) of the polymer bank in the reservoir is the uncertainty in diameter of the perforations and the extent they protrude into the sand matrix. Calculations based on Maerker s work indicate that the sandpack data of Fig. 5 should undergo a 78% reduction in screen factor at 352 ft/D (true velocity). 

Fig. 9—Viscosity and screen factor of backproduced samples show no polymer degradation.

Table 1 summarizes the results of the field sample analyses standardized for polymer concentration. The quality of the field injection sampled collected both at the well and bottom compared closely to that of a laboratory weighed and dried polymer standard prepared in the field using normal laboratory procedures. This 300 mg/l polymer solution had a measured screen factor of 12. 9 and a viscosity, measured at 115 seconds" shear rate, of 3. 93 cp. Laboratory derived correlations showed that this standard corresponded to a mobility reduction effect, expressed in terms of a resistance factor, of 32. The close comparison between this unsheared standard and field injection samples implied that neither the surface nor bottom hole sampling induced any shear degradation. 

Interpretation of the core flooding data to obtain a correlation relating resistance factors to the field measured screen factors was complicated by the presence of additional calcium ion and by chemical degradation. 

It was assumed that chemical degradation would have the same effect on resistance factor as that measured for screen factor. Using this assumption, a correlation between field screen factor and resistance factor was derived from the laboratory core flooding experiments. 

Fig. 5 - Chemical degradation of field polymer samples effect on screen factor.

In order to receive maximum economic benefit from injected polymer, one would select the highest molecular weight polymer available, subject to the limitation that it must be able to pass through the pore throats of the reservoir rock and, not cause excessive injectivity losses. A number of polymer samples were obtained from suppliers and tested for viscosity, screen factor, and shear degradation oharaotaristics in Sleepy Hollow fresh water (Table 2). Because this water has such low salinity, polyacrylamide polymers provided the best mobility reduction characteristics at low concentration, and shear degradation was shown to be at an acceptable level. 

Swab samples showed approximately 16% loss in screen factor from the wellhead and 15% loss in viscosity (Table 4). Polymer and tracer concentration measurements and, gel permeation chromatography analysis showed that the loss in properties was due to degradation of the polymer and not to dilution. 

In general, stabilizers function by reaction with proplnt decompn products. A number of methods have been described and a preliminary evaluation of these methods was conducted by several labs under a cooperative program. Based on results from these screening tests, the PicArsn spec tropho tome trie methods for available stabilizer and primary degradation products were selected for further study. The initial phase of this program was an attempt to standardize the necessary spectrophotometric factors. Significant differences with regard to the factors obtained... 

Several attempts have been reported by recovery teams to recover LNAPL by the use of vacuum wells set above the fluid surface. While these efforts have attained marginal success, several factors have been seen to interfere. Vacuum wells set in the vadose zone tend to encourage airflow from the surface downward through the soil, as well as to extract vapors. The result is often the enhancement of biological degradation near the wells, which tends to cause the well screens to become plugged with biomass. Routine maintenance of the wells is required to keep them functioning properly. 

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