The results of this laboratory study on the retention, hydrodynamic retention and rheology of AN 113 VLM and VHM polymers are presented in the following section with the objective to see the role of molecular weight in polymer performance of polymer-enhanced waterflooding through porous media.
3.1. Retention Measurement
Figure 2 shows the normalized concentration of the injected and effluent polymer solution plotted versus the pore volume injected. These breakout curves are used to estimate polymer retention by finding the area between the two polymer slug injection curves. OriginPro® software (by OriginLab, Northampton, MA, USA) was used to estimate the area between the curves, which was then used to calculate retention in microgram of polymer per gram of rock.
Although the second polymer slug curves for both polymers are quite similar, the first slug curves show a significant deviation, which points out that the molecular weight does play a significant role in adsorption behavior.
Figure 3 shows the polymer retention for the two polymers as a bar chart. The retention was significantly higher (55 µg/g) for the higher molecular weight polymer (AN113 VHM) as compared to the lower molecular weight polymer (AN113 VLM).
The exact reason for the difference in retention capacity between the high and low molecular weight polymers is unknown. It could be due to mechanical entrapment or because of the molecular weight difference itself since the polymer retention is reported as the mass of the polymer.
3.2. Effect of Rate on Polymer Retention
Polymer retention experiments were performed at four different flow rates using AN 113 VLM (low MW) and AN 113 VHM (high MW) polymers. The effluent absorbance was detected at 300–320 nm wavelength and converted to polymer concentration using the laboratory determined calibration curves. Polymer retention was determined from the area between the breakout curves of first and second polymer slugs.
Figure 4 shows the hydrodynamic retention of the two polymers at different flow rates (1, 3, 5, and 8 cm
3/min) corresponding to frontal velocities of 22, 66, 110, and 176 ft/day.
Table 4 lists the hydrodynamic retention results of both polymers.
Polymer retention increased with growing interstitial velocity for both polymers due to hydrodynamic retention (HDR), as expected. However, the HDR and its sensitivity to injection rate were both significantly greater for lower M
W polymer (AN 113 VLM) when compared to the higher M
W polymer (AN 113 VHM), as shown in the bar chart
Figure 5. The adsorption is generally considered to be the dominant retention mechanism [
20,
30] and the results of high M
W polymer are consistent with this consensus. In case of a low M
W polymer, however, the HDR was much more significant than the adsorption. This observation is important for selecting the M
W of polymer in reservoirs, where the velocity is the highest near the wellbore.
The exact reason for the significant increase in HDR of low MW as compared to the one with high MW polymer is not known. However, this observation might be explained by the fact that the physical volume of the solvated polymer molecule is higher for higher molecular weight polymers; thus, the number of molecules retained in a given stagnant zone will be less for higher molecular weight polymer. Another possible reason could be the osmotic pressure, which is the thermodynamic driving force for mixing. For large molecules, this force is relatively small as compared to the viscous forces which are proportional to the polymer injection rate.
The increase in hydrodynamic retention for lower M
W polymer is suggesting that more polymer molecules will reside in a stagnant zone if their size is small. However, the results are different than the once which were observed by Chen et al. [
27]. The difference can be explained on the basis of the order of magnitude difference in permeability of the cores in the two studies. Chen’s study used a low permeability core (167 mD), which encounters a greater retention contribution from mechanical entrapment. This study, on the other hand, used a high permeability core (2000 mD) in which the chances of mechanical entrapment are greatly reduced.
To see if there is a correlation between polymer retention and interstitial velocity, the data in
Table 4 was plotted in
Figure 6. A good correlation between polymer retention increase factor (RIF) and interstitial velocity was observed. For both low and high molecular weights, the hydrodynamic retention increased almost linearly with increasing interstitial velocity within the range of velocities tested and are likely to be encountered at field conditions. The velocity effect was more pronounced in AN 113 VLM (low molecular weight) polymer than AN 113 VHM for the reasons discussed earlier.
3.3. Reversibility of Hydrodynamic Retention
It is generally understood that HDR is a reversible phenomenon [
28]. However, the role of MW on the reversibility of HDR in sulfonated polyacrylamide polymers has not been previously published. Two polymer core floods were performed using AN 113 VLM and AN 113 VHM to investigate this role.
Before the start of the HDR tests, it was presumed that the cores had already completed adsorption as a result of previous tests. The cores were flushed out with 80 PV of brine injection. Then the first polymer slug was injected at 176.44 ft/day; 4 cm3 of effluent samples were collected frequently. The reversible polymers were flushed out with 80 PV of brine injection. The second polymer slug was then injected at the same rate of 176.44 ft/day and effluent samples were collected.
The breakout curves are shown in
Figure 7. In addition to the two 176.44 ft/day polymer injection breakout curves, the “Second 22 ft/day” breakout curve is also included. This curve was determined in an earlier experiment during the adsorption-free slug injection run; it is used in
Figure 7 as the reference slug for determining the adsorption during each of the two 176.44 ft/day polymer slug injection.
Since neither of the breakout curves at a high interstitial velocity of 176.44 ft/day approached (matched) the adsorption-free “Second 22 ft/day” reference breakout curve, it is evident that extra HDR was encountered in both. Moreover, the close match between the first and the second 176 ft/day breakout curve indicates that the HDR did not change during the subsequent identical polymer injection. This is only possible if the HDR during the first 176 ft/day polymer injection was completely reversed by the 80 PV of washout brine injection before the second 176 ft/day polymer injection. Since the reversibility of HDR was observed in both low and high MW polymers, it could be surmised that the MW did not have any effect on HDR.
3.4. Permeability Reduction
Differential pressures were noted during pre and post polymer brine injection at 22 ft/day and the Residual Resistance Factor (RRF) was calculated as follows:
where ΔP
pre is the differential pressure (0.018 psi for AN 113 VLM and 0.02 psi for AN 113 VHM) during brine injection before polymer introduction and ΔP
post is the differential pressure during brine injection after the core had been flooded with polymer at various rates.
The RRF is an effective indicator of permeability reduction caused by the polymer retention and also indicates its reversibility.
Table 5 shows the values of RFF along with the differential pressures during brine injection after each polymer slug injection.
The data in
Table 5 (also presented in
Figure 8 as a bar graph) shows that the adsorption is not completely reversible since even after injecting significantly large volumes (80 PV) of brine, the ΔP during brine injection at the identical rate did not revert to the pre-polymer condition. This resulted in an RRF higher than unity (1.05, 1.80) for polymers of both molecular weights. This shows that adsorption causes irreversible permeability reduction.
The fact that the pressure drop during brine injection, intended to flush out the polymer, always reached the same level (0.019 or 0.032 psi) indicates that the brine was able to completely flush out all hydrodynamically retained polymer even though the polymer slugs prior to the brine flush were injected at variable rates (22, 66, 110, 176 ft/D) for various tests.
It is noteworthy to mention that the RRF value determined for AN 113 VLM (low MW) was lower than RRF value for AN 113 VHM (high MW), indicating a higher permeability reduction for polymers with higher molecular weights. However, the hydrodynamic retention in both polymers did not alter permeability permanently and was completely reversible regardless of the MW.