Page 372 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

838 Reservoir Engineering

problem with polymer flooding is the decrease in injectivity which must accompany
any increase in injection fluid viscosity. If the decreased injectivity is prolonged,
oil production rates and project costs can be adversely affected. Injection rates
for polymer solutions may be only 40%-60% of those for water alone, and the
reduced injectivity may add several million dollars to the total project costs.
Other problems common to the commercial polymers are cited earlier.
Moderately low gravity oils (13'45" API) are normally the target for alkaline
flooding (see section on "Alkaline Flooding"). These oils are heavy enough to
contain the organic acids, but light enough to permit some degree of mobility
control. The upper viscosity limit (~200 cp) is slightly higher than for polymer
flooding. Some mobile oil saturation is desired, the higher the better. The
minimum average permeability is about the same as for surfactant/polymer (>20
md). Sandstone reservoirs are preferred since carbonate formations often contain
anhydrite or gypsum which react and consume the alkaline chemicals. The
alkaline materials also are consumed by clays, minerals, or silica; this con-
sumption is high at elevated temperatures so the maximum desired temperature
is 200°F. Caustic consumption in field projects has been higher than indicated
by laboratory tests. Another potential problem in field applications is scale
formation which can result in plugging in the producing wells.
Crlteria for Thermal Methods
For screening purposes, steamflooding and fireflooding are often considered
together. In general, combustion should be the choice when heat losses from
steamflooding would be too great. In other words, combustion can be carried
out in deeper reservoirs and thinner, tighter sand sections where heat losses
for steamflooding are excessive. Screening guides for in-situ combustion are
given earlier in Section "In-Situ Combustion." The ability to inject at high
pressures is usually important so 500 ft has been retained as the minimum depth,
but a few projects have been done at depths of less than 500 ft. Since the fuel
and air consumption decrease with higher gravity oils, there is a tendency to
try combustion in lighter oils if the fire can be maintained, but no projects have
been done in reservoirs with oil gravities greater than 32" API [403].
In summary, if all screening criteria are favorable, fireflooding appears to
be an attractive method for reservoirs that cannot be produced by methods used
for the lighter oils. However, the process is very complicated and beset with
many practical problems such as corrosion, erosion and poorer mobility ratios
than steamf looding. Therefore, when the economics are comparable, steam
injection is preferred to a combustion drive [378].
Screening criteria for steamflooding are listed earlier in section "Steam-
flooding". Although steamflooding is commonly used with oils ranging in gravity
from 10"-25" API, some gravities have been lower, and there is recent interest
in steamflooding light oil reservoirs. Oils with viscosities of less than 20 cp are
usually not candidates for steamflooding because waterflooding is less expensive;
the normal range is 100-5,000 cp. A high saturation of oil-in-place is required
because of the intensive use of energy in the generation of steam. In order to
minimize the amount of rock heated and maximize the amount of oil heated,
formations with high porosity are desired; this means that sandstones or
unconsolidated sands are the primary target, although a steam drive pilot has
been conducted in a highly fractured carbonate reservoir in France. The product
of oil saturation times porosity should be greater than about 0.08 [400]. The
fraction of heat lost to the cap and base rocks varies inversely with reservoir
thickness. Therefore, the greater the thickness of the reservoir, the greater