30
polymers are widely employed in EOR as viscosity control agents for enhanced oil
recovery, drilling fluid additives, and also for modifying flow and
stability properties of
aqueous solutions and gels.[139, 140] They also can be mixed in surfactant solution
working as interface stabilize taking advantage of electrostatic forces in other
flooding.[141] In preparing hydrophobically modified PAM, the main method is to
introduce hydrophobic associative groups onto the PAM chain. Compared to hydrophobe,
ionic co-monomers are water soluble and regarded as “environmental-friendly” and
“designable” material solutions, which has drawn recent attention.[33, 142-146] These
polyelectrolytes are synthesized by free radical polymerization of acrylamide and their
derivatives
by the copolymerization method, which includes solution, precipitation, and
emulsion techniques.[17, 147, 148]
Fernandez[143] presented cationic, water-soluble polymers prepared by
copolymerizing AM, N-vinyl prrolidone (NVP), and methacrylamide propyl
trimethylammonium chloride (MP) via free radical polymerization in aqueous solution.
Characterization indicated AM and NVP tended to form alternating copolymers during
the polymerization. The introduction of NVP effectively limited the hydrolysis of the
primary amide groups in the polymer chain and allowed
formation of inter-chain
hydrogen bonding so that thermal stability was improved.
Zou, et al.[144, 149] synthesized a series of water-soluble anionic acrylamide
polymers with cyclodextrin side chains, which endow a polymer molecular recognition
function. The cup-type structure appears to hold surfactant resulting in a 20% reduction
of the surfactant loss caused by stratum absorption in the process of polymer/surfactant
flooding. First, the allyl-β-cyclodextrin (allyl-β-CD) was synthesized from β-CD and
31
allyl bromide using sodium hydroxide as an alkaline catalyst. A series of copolymers
were then prepared from allyl bromide,
sodium acrylate, and acrylamide at various mole
ratios under redox initiated free radical polymerization in the presence of achelating
agent.
Through the introduction of both cationic and anionic groups, amphoteric
polymers are synthesized. In freshwater, amphoteric polymers are coiled due to the
intermolecular attractions by the cationic and anionic groups; however,
in brine the
polymer will become extended due to charge dilution of the ionic groups in the molecular
chain by the salt ions in the brine so that solution viscosity sensitivity to brine is reduced.
Under higher salt concentrations, amphoteric polymers of larger molecular weight have
superior salinity-tolerance and temperature resistance compared to polyelectrolytes with
only pendant cationic or anionic groups. When the numbers of cationic and anionic
groups are equal, i.e., the average net charge of the macromolecular chain equals zero,
the polymer chain extension remains approximately constant so that
the solution viscosity
changes only slightly in reservoirs with different mineral salt concentration. Good
salinity resistance could therefore be achieved.
Rodrí
guez, et al.[145] synthesized groups of anionic and zwitterionic modified
PAMs using a gel polymerization process and studied the rheology of the products as
well as mixtures. One modified PAM was a copolymer (PAM-S) of AM with the sodium
salt of 2-acrylamide-2-methyl-1-propane sulfonic acid and the other consisted of the
zwitterionic copolymer (PAM-Z) of AM with a sulfobetaine methacrylate monomer. The
obtained bulk gel was cut into small pieces, dried, and fine powders were achieved.
Solutions were prepared by mixing the
polymer powders, brine and the surfactant
32
cetyltrimethylammoniump-toluene sulfonate (CTAT). The mixed polymer and surfactant
formed worm like, elongated micelles and a viscosification synergy behavior was
explored. In the wormlike micellar system, the surfactant associations worked
similar to
polyelectrolyte associations to form shear-reduced cooperative structure sand shear-
thickening behavior together with the two modified PAMs so that overall a shear
thickening behavior was observed.[150] Furthermore, greater increases in micellar
length promoted the solution viscosity due to formation of an entangled micellar network.
Gui, et al.[151] synthesized a zwitterionic PAM/VPPS copolymer containing sulfobetaine
groups by copolymerizing AM and 4-vinylpyridine propylsulfobetaine (4-VPPS). They
first prepared 4-VPPS monomer through the ring opening reaction of 1,3-styrene with 4-
vinylpyridine (4-VP).[152] The 4-VPPS product was then copolymerized with AM,
utilizing redox initiation in a NaCl solution.
After precipitation in acetone, washing by
water, and drying, a fine powder was obtained.
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