Page 355 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

542 Reservoir Engineering

normal waterflood. The polymer flooding literature was reviewed in the late
1970s [387]. Recommendations on the design of polymer floods were recently
made available [388].

Variations In the Use of Polymers
In-Situ Polymerizatlon. A system is available in which acrylamide monomer is
injected and polymerized in the reservoir. Both injection wells and producing
wells have been treated.
Crosslinked or Gelled Polymers. Several methods are available for diverting
the flow of water in reservoirs with high permeability zones or fracture systems.
Some methods are only effective near the injection well while others claim the
treatment can be effective at some depth into the reservoir. Both producing wells
and injection wells can be treated.
One method is the aluminum citrate process which consists of the injection
of a slug of HPAM polymer solution, aluminum ion chelated with citrate ion,
and a second slug of polymer. Some of the polymer in the first slug adsorbs or
is retained on the surfaces of the reservoir. The aluminum ion attaches to the
adsorbed polymer and acts as a bridge to the second polymer layer. This
sequence is repeated until the desired layering is achieved. The transport of
aluminum ions through the reservoir may be a problem in certain cases, so the
effects of the treatment may be limited to near the wellbore.
Another method is based on the reduction of chromium ions to permit the
crosslinking of HPAM or XG polymer molecules. A polymer slug containing Cr*
is injected followed by a slug of polymer containing a reducing agent. When
the Cr* is reduced to Cr+3, a gel is formed with the polymer. The amount of
permeability reduction is controlled by the number of times each slug is injected,
the size of each slug, or the concentrations used. An alternate treatment involves
placing a plain water pad between the first and second polymer slugs.
In another variation of the above two methods for HPAM, a cationic polymer
is injected first. Since reservoir surfaces are often negatively charged, the cationic
polymer is highly adsorbed. When the foregoing sequential treatments are
injected, there is a strong attraction between the adsorbed cationic polymer and
the anionic polymers that follow.
Polymer concentrations used in these variations are normally low, on the order
of 250 mg/L. With low molecular weight polymers or if a very stiff gel is desired,
polymer concentrations of 1-1.5% are common. The type of polymers are similar
to those used in conventional polymer flooding, but the products used for
gelation command a higher price.
Methods developed recently, especially for fracture treatments, include CF
(acetate)-polyacrylamide, collordal silica, and resorcinol-formaldehyde.

Surfactant and Alkallne Flooding
Both alkaline flooding and surfactant flooding improve oil recovery by lowering
the interfacial tension between crude oil and the displacing water. With alkaline
flooding, the surfaaants are generated in situ when the alkaline materials react
with crude oil-this technique is normally only viable when the crude oil contains
sufficient amount of organic acids to produce natural surfactants. Other possible
mechanisms with the caustic mterials include emulsifbtion of the oil and alteration
in the prekential wettability of the reservoir rock.
With surfactant flooding, surfaceactive agents are mixed with other compounds
(such as alcohol and salt) in water and injected to mobilize the crude oil.