Page 357 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

344 Reservoir Engineering

the increasing interest in CO, and nitrogen or flue gas methods, they are
separated from the hydrocarbon miscible techniques.
Hydrocarbon miscible flooding can be subdivided further into three distinct
methods, and field trials or extensive operations have been conducted in all of
them. For LPG slug or solvent flooding, enriched (condensing) gas drive and
high pressure (vaporizing) gas drive, a range of pressures (and therefore, depths)
are needed to achieve miscibility in the systems.
Unless the reservoir characteristics were favorable, early breakthrough and
bypassing of large quantities of oil have plagued many of the field projects. In
addition, the hydrocarbons needed for the processes are valuable, and there is
increasing reluctance to inject them back into the ground when there is some
question about the percentage that will be recovered the second time around.
Therefore, in the U.S. in recent years the emphasis has been shifting to less
valuable nonhydrocarbon gases such as CO,, nitrogen, and flue gases. Although
nitrogen and flue gases do not recover oil as well as the hydrocarbon gases (or
liquids), the overall economics may be somewhat more favorable.
Nitrogen and Flue Gas Flooding

As previously mentioned, nitrogen and flue gas (about 87% N2 and 12% CO,)
are sometimes used in place of hydrocarbon gases because of economics.
Nitrogen also competes with CO, in some situations for the same reason. The
economic appeal of nitrogen stems not only from its lower cost on a standard
Mcf basis, but also because its compressibility is much lower. Thus, for a given
quantity at standard conditions, nitrogen will occupy much more space at
reservoir pressures than CO, or even methane at the same conditions. However,
both nitrogen or flue gas are inferior to hydrocarbon gases (and much inferior
to CO,) from an oil recovery point of view. Nitrogen has a lower viscosity and
poor solubility in oil and requires a much higher pressure to generate or develop
miscibility. The increase in the required pressure is significant compared to
methane and very large (4-5 times) when compared to COY Therefore, nitrogen
will not reduce the displacement efficiency too much when used as a chase gas
for methane, but it can cause a significant drop in the effectiveness of a CO,
flood if the reservoir pressures are geared to the miscibility requirements for
CO, displacements. Indeed, even methane counts as a desirable “light end” or
“intermediate” in nitrogen flooding, but methane is quite deleterious to the
achievement of miscibility in CO, flooding at modest pressures.
Carbon Dioxlde Flooding

CO, is effective for recovery of oil for a number of reasons. In general, carbon
dioxide is very soluble in crude oils at reservoir pressures; therefore, it swells
the net volume of oil and reduces its viscosity even before miscibility is achieved
by the vaporizing gas drive mechanism. As miscibility is approached, both the
oil phase and the CO, phase (which contains many of the oil’s intermediate
components) can flow together because of the low interfacial tension and the
relative increase in the total vohmes of the combined CO, and oil phases
compared to the water phase. However, the generation of miscibility between
the oil and CO, is still the rhost important mechanism, and it will occur in GO2-
crude oil systems as long as the pressure is high enough. This so-called “mini-
mum miscibility pressure” or MMP has been the target of several laboratory
investigations and is no longer a mystery. The 1976 NPC report [380] showed
that there is a rough correlation between the API gravity and the required MMP,