Page 186 - Primer on Enhanced Oil Recovery

176 Primer on Enhanced Oil Recovery

of the polymer solutions close or into the challenging zones allows reservoir flexi-
ble management. The injection volumes then are not so big, and this allows rela-
tively inexpensive additional oil recovery despite high advanced polymer prices.

14.4 Disperse systems

Creating some structures within or adding some particles to water modifies water
flow (rheology) properties. The water flow properties start to deviate from
Newtonian behavior. It is not exactly just viscosity increase. The system properties
start to depend on shear rate, applied force and system history. For instance, viscos-
ity can be decreasing over flow time (so named thixotropy), or the viscosity can
increase over the flow time (rheopexy). The system behavior becomes very com-
plex and can, in principle, be tuned to suit the purpose.
Disperse system behavior is defined by the dispersed particle interactions. If par-
ticles attract to each other than they will form some bigger conglomerates. At high
enough concentrations this will lead to soft solid-like behavior. Repulsion forces
between particles makes each particle to occupy certain, bigger than the particle,
volume.
Many terms are used to describe disperse systems but they all have an unifying
name colloidal system. Thermodynamics and in many cases chemical reactions
at the reservoir temperature define oil displacing properties of the systems with dis-
persed particles and structures. Earlier mentioned emulsions and foams also broadly
fall into this category. Very important point to mention is that oil extraction is a
dynamical process of matter movement through porous media. In general, porous
media acts as a filter. Filter can either do nothing to the passing material (no inter-
action apart from hydrodynamics and capillary forces) or it can start accumulation
part of the passing media. To add to the complexity the passing media (even oil
from one part of reservoir passing through another part) can modify the filter
(porous rock).
Properties of disperse systems are also very much defined by the concentration
of dispersed media. Viscosity is be affected by the dispersed media concentration
(volume fraction of dispersed particles) very dramatically at some concentrations.
At low concentration (volume fraction of dispersed particles) the particles move
freely by Brownian motion. The solution is then in a liquid state. The solution
would have viscosity may be few times higher as compared to pure liquid. At
around 50% concentrations particles can start forming crystalline domains. Liquid
and crystalline phases co-exist. The viscosity rises up may be ten times. At around
58 vol fraction % the solution becomes a glassy solid with the viscosity rising by
another hundred times or so. All mentioned properties only attainable in a static or
at a very low shear rate. As shear rate increases the viscosity is reduced very signif-
icantly. As particle sizes get smaller the viscosity reduction on shear rate depen-
dence is reduced. Particle shape and size distribution have complicated influence on
viscosity.