3
and withstanding harsh environment in reservoirs, [14, 15] including high molecular
weight PAM, hydrophobically modified PAM, [16-18] comb-shape PAM, [19, 20] ionic
modified PAM[21] and PAMs with combined modification. [22-25]
High molecular weight PAM is one of the earliest polymers employed in polymer
flooding. High Mw PAM, through
its hydrodynamic volume, increases the viscosity of
the fluid so that a large, retained viscosity can be achieved. [16] Meanwhile, temperature
and salt resistance can be enhanced to some extent. [26] This kind of PAM has been
applied in oilfields more than any other kind of PAMs. [7] However, when subjected to
elevated shearing, mechanical
degradation could happen, leading to an irreversible loss of
viscosity. [27]
Alternatively, such viscosity control can be obtained by taking advantage of
hydrohobically associating polymer with lower molecular weight, of which the main
polymer backbone is water soluble, whereas it contains a small amount of
hydrophobic
monomers, such as fatty esters or other saturated carbon units. [18, 28] The main
hydrophilic domain renders solubility of the PAMs chain in aqueous phase. For the
lipophilic groups distributed along the polymer chain, the amount of them is small, yet,
they prefer to associate therefore to form physical linkages due to the unfavorable
aqueous solvent conditions. These physical linkages exist both within molecules and
between molecules hence rendering a kind of dynamic colloid network structure, which
could be broken when
susceptible to high shearing, but re-form in a reduced shearing,
thus this viscosity loss owing to shearing is reversible.
Based on the development of hydrophobically modified PAMs, both hydrophobic
and hydrophilic functional groups were introduced in PAMs polymer chain, resulting in
4
the preparation of comb-shape PAMs. [29, 30] It was reported that comb-shape PAMs
has superior salt-resistance through building a hindering structure and generating a
random coil configuration. Compared to the traditional polymer chain
curling or collapse
when subjected to brine, comb-shape PAMs do not fully collapse under a high salinity
environment. An extensive investigation for field application has been done in China.
[23, 31]
A number of ionic co-monomers were introduced into PAMs and generated
another type of PAMs, polyelectrolyte, to increase the salt tolerance of polymer solution.
[32, 33] 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 repulsion so that solution viscosity sensitivity to brine is reduced.
Under higher salt concentrations, amphoteric polymers have superior salinity-tolerance
and temperature resistance compared to polyelectrolytes with pendant cationic or anionic
groups.
Besides the mentioned type PAMs, combined modification
methods have been
employed in producing PAMs to achieve a desired performance, including preparing
PAMs with employing at least two modifications, [34] such as incorporating both
hydrophobic and ionic functional element in PAMs, as well as making hybrid PAMs by
introducing inorganic functional groups. [35, 36] Figure 1.3. shows
the timeline of the
development of polyacrylamides for EOR.
Gel treatment is another cost-effective and effective method to enhance oil
recovery. Polymer gel treatments of the injection wells are being developed to
preferentially limit flow through conduit zones. [37-39] Polymer gels can be cost-
5
effective methods to improve sweep efficiency and reduce excess waste water production
during oil recovery. When gels carried by fluid are injected into reservoir, they are prone
to preferentially enter channels or fractures with low pressure, and gels may then
aggregate or plug a region when the flow path becomes too narrow.
Once conduit
channels are plugged, flow is shifted to other places of the reservoir, so more of the
remaining oil can be displaced. [40]
Figure 1.3.
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