Page 367 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 399

waterflooding consists of adding water soluble polymers to the water before it
is injected into the reservoir. Low concentrations (often 250-2,000 mg/L) of
certain synthetic or biopolymers are used; properly sized treatments may require
1596-2596 reservoir PV.

Mechanisms. Polymers improve recovery by:

Increasing the viscosity of water
Decreasing the mobility of water
Contacting a larger volume of the reservoir

Technical Smming Guides
cmtde oil
Gravity >25" API
Viscosity 450 cp (preferably < 100)
Composition Not critical
Reservoir
Oil saturation >lo% PV mobile oil
Type of formation Sandstones preferred but can be used in
carbonates
Net thickness Not critical
Average permeability >10 md (as low as 3 md in some cases)
Depth c about 9,000 ft (see temperature)
Temperature ~200°F to minimize degradation
Limitations.

If oil viscosities are high, a higher polymer concentration is needed to
achieve the desired mobility control.
Results are normally better if the polymer flood is started before the water-
oil ratio becomes excessively high.
Clays increase polymer adsorption.
Some heterogeneities are acceptable, but for conventional polymer flooding,
reservoirs with extensive fractures should be avoided. If fractures are
present, the crosslinked or gelled polymer techniques may be applicable.
Problems.
Lower injectivity than with water can adversely affect oil production rate
in the early stages of the polymer flood.
Acrylamidetype polymers lose viscosity due to shear degradation or increases
in salinity and divalent ions.
Xanthan gum polymers cost more, are subject to microbial degradation,
and have a greater potential for wellbore plugging.
Alkaline Flooding [386]

Description. Alkaliie or caustic flooding involves the injection of chemicals such
as sodium hydroxide, sodium silicate or sodium carbonate. These chemicals react
with organic petroleum acids in certain crudes to create surfactants in situ. They
also react with reservoir rocks to change wettability. The concentration of the
alkaline agent is normally 0.2 to 5% slug size is often 10% to 50% FV, although
one successful flood only used 9% PV, (but this project also included polymers