Polyacrylamide and its derivatives for oil recovery

3.2
 
Organically Cross-linked Polyacrylamide Gels 
The most popular HPAM/organic cross-linker system is based on the 
condensation reaction between phenol and formaldehyde.[200, 257] Chang, et al.[199] 
synthesized gels based on a cross-linking reaction between polyacrylamide, and phenolic 
compounds, e.g., phenol resorcinol, and formaldehyde. This system was patented by 
Pfizer with the trade name Floperm 325
TM
and the polymer concentration was typically in 
the range of 10,000 to 20,000 ppm.[204] Flowperm 325
TM 
gelation occurred at pH greater 
than 9 and the gel temperature sensitivity limit was about 149°C (300°F) or more when 
using a combination of other polymers, such as phenol/formaldehyde and 
phenol/formaldehyde/chromium(III) crosslinking system.[258] Seright, et al.[259] and 
Zhuang, et al.[260] both confirmed that the gelation time of this system was very 
sensitive to salinity and that gelation did not occur when pH was below 7.5. A 
sulfomethylated resorcinol-formaldehyde (SMRF) system was then developed to improve 
the salinity tolerance and pH sensitivity.[261, 262]
Albonico,et al.[203] presented a phenol-formaldehyde cross-linked polymer gel 
system. The investigators chose low molecular weight HPAM, co-polymeric PAM-
AMPS and terpolymeric acrylamide/N-vinyl pyrrolidone/acrylamide-3-propane sulfonic 
acid (PAM-VP-AMPS) as test samples using phenol-formaldehyde as the cross-linker. 

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46 
Their results showed that the phenol and formaldehyde gelation system can withstand 
temperature as high as 120°C. In slim-tube experiments (Slim tube tests are performed to 
observe actual solvent-oil interaction in a physical simulation of reservoir pore space.), 
these gelants possessed excellent injectivity under 140°C. Slim tube tests are performed 
to observe actual solvent-oil interaction in a physical simulation of reservoir pore 
space.[263] Hutchins et al.[258] also showed the phenol-formaldehyde cross-linked 
gelling system could remain stable in oxygen-free seawater at 150°C (300°F) for 12 
months and 177°C (350°F) for five months. With a goal of longer delay time for gelation 
to prevent gelation before the reactants have reached the elevated temperature target 
zones, Huchins et al.[258] reported an organic crosslinking system comprising HPAM 
and a crosslinking combination of hydroquinone (HQ) and hexamethylenetetramine 
(HMTA). The gel was stable for 12 months at 149°C (300°F) and 5 months at 177 °C 
(350°F).
Dovan, et al.[264] described another crosslinking system that can be applied at 
higher temperatures featuring new cross-linkers, HMTA, terephthaldehyde, terephthalic 
acid, and glutaric acid as primary cross-linkers and HQ, dihydroxynaphthalene, and gallic 
acid as secondary cross-linkers. The primary cross-linkers were chemical compounds that 
can form unstable gel networks with HAPAM polymer (Hydrophobically Associative 
Polyacrylamide), while the secondary cross-linkers could stabilize the gels formed by the 
primary cross-linkers at high temperature. This system has a gelation time of several days 
at 168°C, but has not yet been tested in a field application. Due to environmental 
concerns, the use of HQ and formaldehyde are discouraged.

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47 
Polyethyleneimine (PEI) was also an organic cross-linker utilized to make 
thermally stable PAM copolymer gels.[245, 265] Morgan, et al.[266] reported a gel 
system based on acrylamide/t-butyl acrylate copolymer (PAtBA) cross-linked with PEI. 
The PEI crosslinking system was shown capable of propagating eight times farther than 
chromium-based systems under equivalent conditions because the crosslinking reaction is 
dependent on a nucleophillic attack by amine nitrogen from PEI at the carbonyl carbons 
of the t-butyl acrylate. Unlike many metal-based ionomer cross-linkers, PEI is not 
susceptible to hydrolysis.[245] Al-Muntasheri, et al.[267, 268] performed a series 
experiments to test PEI/PAM-AtBA gels under high temperature and pressure. The 
reaction mechanism is showed in Figure 11. They first reported the possibility of 
crosslinking PAM with PEI at temperatures up to 140°C (285°F) and pressures up to 30 
bar (435psi). In bulk, the gels were stable up to130°C (194°F) for at least 8 weeks. At the 
same temperature but 69 bar (1000psi) pressure, the PEI gel was stable for 3 weeks. The 
PEI/PAM-AtBA copolymer crosslinking system has also been proven successful in 
controlling water production over a temperature range from 27 to 177°C (80°F to 
350°F).[265] 
NRR'
NR
NRR'
n
C
H
2
C
NH
2
O
PAM chain
PAM chain
NR
NR
O
O
n
NR
NR
O
O
n
+
+
2R'NH
2