Page 74 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

66 Hybrid Enhanced Oil Recovery using Smart Waterflooding

viscosity: relative viscosity, specific viscosity, reduced Salinity, hardness, pH, temperature, oxidation
viscosity, inherent viscosity, and intrinsic viscosity. reduction, and ironic ions
The quantity of intrinsic viscosity is the fundamental Both HPAM and xanthan undergo the degradations by
indication of molecular weight and it is independent salinity, hardness, pH, oxidation reduction, and ironic
of polymer concentration. Eq. (4.3) defines the intrinsic ions, which are related to the chemical stability. They
viscosity as the specific viscosity at the infinite dilution are flexible polyelectrolyte. The salt can screen the
concentration. Many equations have been proposed to charged molecules of polymer, and the charged mole-
quantify the relationship between the intrinsic viscosity cules of polymer in a solution would be surrounded
and molecular weight of polymer. The equations by the opposite charged ions. Therefore, the polyelec-
explain that the viscosity is proportional to the molecu- trolyte polymer molecules would expand at the low
lar weight of polymer, because the polymeric solutions salt concentration because of more mutual repulsion
used in polymer EOR have relatively low concentration, of the charges along its chain and contract at the high
in which interactions between polymer molecules are salt concentration owing to less repulsion. The expan-
negligible. The relationships proposed by the studies sion in the molecular size of polymer increases the vis-
(Chauveteau, 1982; Huggins, 1942; Kraemer, 1938) cosity of polymeric solution. For the HPAM, the
are commonly used to describe in the light of the divalent cations, i.e., hardness, have more significant
intrinsic viscosity of polymer. Huggins (1942) devel- screening effect on the negatively charged molecules
oped the relationship between the specific viscosity of polymer compared with the monovalent cation. As
and polymer concentration at the low concentration HPAM undergoes more hydrolysis, the negatively
as in Eq. (4.4). Kraemer (1938) formulated the relation charged carboxyl group of HPAM interacts with the
between the inherent viscosity and intrinsic viscosity as divalent cations. It results in the decreasing viscosity
in Eq. (4.5). of polymeric solution, formation of gels, or precipita-
tion. The viscosity of xanthan is less sensitive to the
!
h sp salinity and hardness compared with HPAM. The pH
½hŠ¼ lim (4.3)
also influences the viscosity of HPAM. The polymer is
c poly /N c poly
neutral at low pH condition but completely charged
h sp 2
0 (4.4)
¼½hŠþ k ½hŠ c poly at high pH condition. Lowering pH neutralizes the
c poly
polyelectrolyte HPAM polymer and leads to the less
ln h r 00 2 expansion of polymer chain because of less electrostatic
h I ¼ ¼½hŠ k ½hŠ c poly (4.5)
effects. The effect of pH becomes more dominant in
c poly
low-salinity solutions. In terms of temperature, the
where c poly indicates the concentration of polymer; h is high temperature destabilizes both HPAM and xanthan
the non-Newtonian fluid’s viscosity: if the viscosity h is polymers and reduces the viscosities, i.e., thermal degra-
independent of the shear rate, then it is equal to the dation of polymer. In addition, the oxidative degrada-
Newtonian fluid’s viscosity m;[h] is the intrinsic viscos- tion of polyacrylamide polymer varies with the
0
ity; h sp is the specific viscosity; k is the Huggins constant; temperature. At the low temperature, the effect of dis-
h I is the inherent viscosity; h r is the relative viscosity; solved oxygen on viscosity of HPAM is not significant.
00
and k is the constant. However, even small amount of dissolved oxygen pro-
The intrinsic viscosity is also sensitive to the molec- vides the substantial reduction of the viscosity at the
ular expansion of the polymers as well as the molecular high temperature condition. Lastly, the high enough
weight. Most of the real polymers have nonlinear chains concentration of ferric ion (Fe ) cross-links the
3þ
of polymer. The real end-to-end size of polymer has a HPAM to form insoluble gel. The loss of viscosity is sig-
discrepancy to the apparent size of polymer in a solu- nificant because of insoluble gel formation.
tion. This difference differentiates the intrinsic viscosity
of real polymer from that of linear structure of polymer. Shear rate
The size of polymer chain is also affected by the solvent. In contrast to the Newtonian fluid of water, polymeric
Both HPAM and xanthan are not neutral. They are poly- solution suffers a non-Newtonian behavior by the shear
electrolytes with a number of negative charges, which stress. It is understood as the mechanical stability of
introduce the electrostatic repulsion between the polymer. Although the Newtonian fluid of water, theo-
different parts of the molecule. Therefore, the molecular retically, has a constant viscosity regardless of shear
expansion of polymer is influenced by the solutions. stress, the non-Newtonian fluid of polymeric solution