Page 377 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 343

oil saturation) or during a polymer flood oil recovery test. If pqlymer retention
tests are conducted with only water initially present in the core, a higher level
of retention will result from the increased surface area available to the polymer
solution in the absence of oil. Effluent samples from the core are collected
during both the polymer injection and a subsequent water flush. These samples
are analyzed for polymer content. From a material balance, the amount of
polymer retained in the core is calculated. Results are usually expressed in lbs
per acre-ft. Excessive retention will increase the amount of polymer that must
be added to achieve the desired mobility control. The level of polymer retained
in a reservoir depends on a number of variables: permeability of the rock,
surface area, nature of the reservoir rock (sandstone, carbonate, minerals, or
clays), nature of the solvent for the polymer (salinity and hardness), molecular
weight of the polymer, ionic charge on the polymer, and the volume of porosity
that is not accessible to the flow of polymer solution. Polymer retention levels
often range from less than 100 lb/acre-ft to several hundred Ib/acre-ft.
Surfactant and Alkali Testing

Laboratory tests consist of measuring the interfacial tension (IFT) between
the crude oil and the injected solution (alkaline or surfactant additive). This is
usually done with a spinning drop interfacial tensiometer. With surfactants, the
requirement for measuring tensions can be minimized by performing vial tests
to determine solubilization parameters that can be correlated with IFT. Other
tests include determining relative permeabilities, wettability, and total fluid
mobilities. Once the optimum conditions are found, results of oil recovery tests
with the chemical flood additives are conducted, usually at waterflood residual
oil saturation.

CO, Flooding
For the gas injection projects, the trend in this country is toward the use of
carbon dioxide although the full impact of CO, flooding will be felt in several
years since construction of CO, pipelines into the west Texas area was completed
in the 1980s. Carbon dioxide flooding is not a truly miscible process; that is, it
does not dissolve in all proportions with crude oil. However, CO, can extract
light to intermediate components out of the crude oil. This C0,-rich mixture
can develop miscibility and effectively displace additional crude oil. The main
limitation involved is the very low viscosity of CO, that results in fingering of
CO, through the more viscous crude oil. This causes premature breakthrough
of the CO, and reduces the amount of oil recovered per unit volume of CO,.
A prediction of the minimum pressure required to achieve miscibility can be
made if the reservoir depth and basic properties of the crude oil are known.
Laboratory tests often consist of some means of determining the minimum
miscibility pressure, often by observing the oil displacement efficiency by CO,
in a small-diameter tube (slim tube) packed with sand or glass beads. Carbon
number distribution of the crude will be of value in determining if sufficient
amounts of the C, to C,, components are present.
Thermal Recovery

Viscosities of very viscous crude oils can be reduced by the use of thermal
recovery methods. Firef looding or in-situ combustion involves starting a fire in
the reservoir and injecting air to sustain the burning of some of the crude oil.