Page 155 - Primer on Enhanced Oil Recovery

Chemical EOR 145

The polymer solution flow in a porous medium is significantly different from
the flow defined in the laboratory rheometer. Moreover, diluted polymer solutions
which are characterized by shear liquefaction in measurements on the rheometer,
can in a porous medium exhibit dilatant rheology, i.e. shear thickening. Such behav-
ior is explained by a retention of polymer molecules by the pore space. At the same
time, the polymer macromolecules unfold which increases shear rate and escalate
resistance to flow. Retention of the polymer can occur by adsorption or even
mechanical processes. In layered-inhomogeneous and micro-inhomogeneous porous
media the rheology of the diluted polymer solution is S-shaped (see Fig. 12.3).
At the same time, the dilatant nature of the flow is replaced by pseudoplastic.
The change in flow rheology occurs at sufficiently high shear rates and not is
observed in the actual oil containing rock. In the formation high-permeable zones
higher flow rate is observed compared to the flow rate in low-permeable zones,
which leads to an increase in viscosity (decrease in solution mobility). This leads to
the flattening of the filtration profile and an increase in the sweep efficiency.
In the course of polymer flow in a porous medium, connate water is also dis-
placed along with the oil. As a result, the polymer solution interacts directly with
the reservoir rock, leading to the polymer adsorption from the solution. At the same
time, the concentration of the polymer in the solution decreases; a layer of water
with a low polymer concentration is formed.
3
According to various researcher data, the polymer adsorption can reach 150 g/m ,
which is significantly less than the adsorption of surfactants. Usually one of the
main requirements for polymers is their minimal adsorption on the surface of a
porous medium. However, this is a simplified view of the efficiency of oil displace-
ment with a polymer solution. Adsorption plays an important role in the mechanism
of polymer flooding, because it reduces the mobility of the polymer solution (by
reducing the permeability) and increase the coverage of the reservoir sweep. At the
same time, adsorption should not exceed the limits of the experimentally deter-
mined optimal range, since with high adsorption, the front of the polymer move-
ment lags far behind the oil displacement front (the displacement front is formed in
this case by water with very low polymer content). This reduces the efficiency of
the process due to the fact that oil is displaced mainly by inactive water.

Figure 12.3 Polymer solution rheology for heterogeneous porous media.