Page 354 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 321

cut. Thus, the ultimate oil recovery at a given economic limit may be 4%-10%
higher with a mobilitycontrolled flood than with plain water. Additionally, the
displacement is more efficient in that less injection water is required to produce
a given amount of oil.
The need to control or reduce the mobility of water led to the advent of
polymer flooding or polymer-augmented waterflooding. Polymer flooding is
viewed as an improved waterflooding technique since it does not ordinarily
recover residual oil that has been trapped in pore spaces and isolated by water.
However, polymer flooding can produce additional oil over that obtained from
waterflooding by improving the displacement efficiency and increasing the
volume of reservoir that is contacted. Dilute aqueous solutions of water-soluble
polymers have the ability to reduce the mobility of water in a reservoir thereby
improving the efficiency of the flood. Partially hydrolyzed polyacrylamides
(HPAM) and xanthan gum (XG) polymers both reduce the mobility of water by
increasing viscosity. In addition, HPAM can alter the flow path by reducing the
permeability of the formation to water. The reduction in permeability to water
that is achieved with HPAM solution can be fairly permanent while the per-
meability to oil can remain relatively unchanged. The resistance factor is a term
that is commonly used to indicate the resistance to flaw that is encountered by
a polymer solution as compared to the flow of plain water. For example, if a
resistance factor of 10 is observed, it is 10 times more difficult for the polymer
solution to flow through the system, or the mobility of water is reduced 10-fold.
Since water has a viscosity of about 1 cp, the polymer solution, in this case,
would flow through the porous system as though it had an apparent or effective
viscosity of 10 cp even though a viscosity measured in a viscometer could be
considerably lower.
The improvement in areal sweep efficiency resulting from polymer treatment
can be estimated from Figure 5-161. For example, if the mobility ratio for a
waterflood with a 5-spot pattern is 5, the areal sweep efficiency is 52%
at breakthrough. If the economic limit is a producing water41 ratio of 1OO:l
(f, G 100/101 = 0.99), the sweep efficiency at floodout is about 97%. If the
polymer solution results in the mobility ratio being lowered to 2, sweep effici-
encies are 60% at breakthrough and 100% at the same economic water-oil ratio.
A simplified approach to qualitatively observing the improvement with
polymers in a stratified system is illustrated in Figure 5-164. For example, if
the permeability variation is 0.7, the waterflood mobility ratio is 5, and the
initial water saturation is 0.3, the fractional recovery of oil-in-place can be
estimated. From the plot, R(l - 0.4 S,) = 0.29, and the fractional recovery, R,
is 0.29/[1 - (0.4)(0.3)] = 0.33. This R needs to be multiplied by the areal sweep
efficiency of 0.97 to yield a recovery of 32% of the oil-in-place. If polymers again
reduce the mobility ratio to 2 (and if no improvement in permeability variation
occurs), a fractional recovery of 0.375 is obtained. Since the areal sweep with
the polymer flood is loo%, a recovery of 37.5% of the oil-in-place is estimated.
Thus the improvement with polymers is estimated at 0.375-0.32 or 5.5% of
the oil-in-place. If the flow distribution with polymer solution lowered the
permeability variation (which is not likely), the incremental production could
be higher. These calculations are gross oversimplifications of actual conditions
and only serve as a tool to show that reducing mobility ratio with polymers can
improve the sweep efficiencies.
A properly sized polymer treatment may require the injection of 15%-25%
of a reservoir pore volume; polymer concentrations may normally range from
250 to 2,000 mg/L. For very large field projects, millions of pounds of polymer
may be injected over a 1-2 year period of time; the project then reverts to a