Page 152 - Primer on Enhanced Oil Recovery

142 Primer on Enhanced Oil Recovery

Chemical flooding is accomplished by adding one or more chemical compounds
to the injected fluid. This leads either to: a decrease in the interfacial tension
between the reservoir oil and the injected fluid, or to an increase in the viscosity of
the injected fluid. High viscosity reduces the injected fluid mobility. This also leads
to increase in the reservoir sweep efficiency.
The main methods of chemical flooding technology are: polymer, micellar-
polymer and alkaline flooding. Polymer flooding is used to increase the viscosity of
the injected fluid and reduce its mobility. Micellar-polymer and alkaline flooding
are used to reduce the interfacial tension between the reservoir oil and the injected
fluid. This significantly improves the process of residual oil displacement. Polymer
flooding is usually used in the early stages of oil field development with sufficient
oil saturation in a porous medium. Micellar-polymer and alkaline waterflooding is
usually used at the final stage of field development. The process can be thought as
an effective wash of the residual oil

12.1 Polymer flooding

Continuous and effective oil production with high recovery rate is only possible if
necessary measures to influence the oil formation by the injection of large volumes
of water are implemented. The injection of large water volumes leads to the devel-
opment of the displacement front unsteadiness and a decrease in the sweep effi-
ciency. The solution to this problem is in creation non-Newtonian, heterogeneous
systems, that have adjustable rheological (e.g. flow) properties. All this can be
achieved by use of polymeric solutions.
One of the main problems in the flooding of oil reservoirs is the hydrodynamic
instability of the oil-water displacement front. The main factor affecting the hydro-
dynamic instability of the front is the ratio of the viscosities of oil and water in res-
ervoir conditions. Indeed, the critical wavelength of the perturbation λ (water-oil
front instability) is inversely proportional to the capillary number C a :

λB1=OCa (12.1)

v ðη 2 η Þ
2
1
Ca 5 (12.2)
σm
where: v speed of the front, η ; η are viscocities of oil and water, σ surface
1 2
tension and m porosity.
It can be seen from the presented expressions, ceteris paribus, that the higher the
difference in viscosity of oil and water, the higher the capillary number and the
lower the critical perturbation wavelengths. This leads to an early development of
the displacement front instability. It is seen as so named process of fingering, when
the displacement front is not a straight line anymore. The advance of water con-
tinues and leads to the relatively rapid breakthrough process. This means that