Page 368 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

334 Reservoir Engineering

for mobility control). Polymers may be added to the alkaline mixture, and
polymer-thickened water can be used following the caustic slug.
Mechanisms. Alkaline flooding recovers crude oil by:

A reduction of interfacial tension resulting from the produced surfactants
Changing wettability from oil-wet to water-wet
Changing wettability from water-wet to oil-wet
Emulsification and entrainment of oil
Emulsification and entrapment of oil to aid in mobility control
Solubilization of rigid oil films at oil-water interfaces (Not all mechanisms
are operative in each reservoir.)

Technical Screening Guides
CTUb Od
Gravity 13" to 35" API
Viscosity e200 cp
Composition Some organic acids required
Reservoir
Oil saturation Above waterflood residual
Type of formation Sandstones preferred
Net thickness Not critical
Average permeability >20 md
Depth < about 9,000 ft (see temperature)
Temperature ~200°F preferred
Limitations.

Best results are obtained if the alkaline material reacts with the crude oiI;
the oil should have an acid number of more than 0.2 mg KOH/g of oil.
The interfacial tension between the alkaline solution and' the crude oil
should be less than 0.01 dyne/cm.
At high temperatures and in some chemid environments, excessive amounts
of alkaline chemicals may be consumed by reaction with clays, minerals,
or silica in the sandstone reservoir.
Carbonates are usually avoided because they often contain anhydrite or
gypsum, which interact adversely with the caustic chemical.
Problems.

Scaling and plugging in the producing wells.
High caustic consumption.

in-Situ Combustion [a861
Description. In-situ combustion or fireflooding involves starting a fire in the
reservoir and injecting air to sustain the burning of some of the crude oil. The
most common technique is forward combustion in which the reservoir is ignited
in an injection well, and air is injected to propagate the combustion front away
from the well. One of the variations of this technique is a combination of
forward combustion and waterflooding (COFCAW). A second technique is
reverse combustion in which a fire is started in a well that will eventually become