Page 166 - gas transport in porous media

Chapter 8: Gas Injection and Fingering in Porous Media
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capillary tube (slim tube) at a series of successively higher pressures. If we plot
the amount of displaced oil versus pressure, the resulting curve usually has a break
at about 95% recovery, the pressure at which is taken as the minimum miscibility
pressure. However, we must keep in mind that many crudes contain asphaltenes that
precipitate and do not dissolve, even after a series of theoretical multiple contacts
between the crude and propagation of the mixture of the injection fluid and the crude’s
non-asphaltenic components.
In what follows we describe some of the issues that are most important to gas
injection processes.

8.10.1 Reservoir Characterization and Management
In virtually all gas injection projects, the most critical decisions are made only
after lengthy computer simulations that attempt to optimize the amounts of the fluid
injected, the injection and production rates, and other operation variables. Thus, uti-
lizing a realistic model of the reservoir is very important and, in fact, the accuracy
of any predictive simulation technique is limited directly by the accuracy with which
the reservoir can be described. Despite the extensive sets of experimental data that
are used as the input parameters, the sophistication of the simulators, the size of the
computers on which the simulations are carried out, and the large number of cases
that are simulated, the simulator’s predictions may still be subject to very large uncer-
tainties. Therefore, a much less expensive pilot flood is usually carried out first, and
the reservoir’s model is tuned by changes in the values of the input parameters in
order to make the simulator’s output fit the pilot data.
For the field-scale projects that have been carried out, calculated optimal CO 2
injection volumes have ranged from 20 to 50% of the hydrocarbon pore volume. The
predicted CO 2 utilization factors range from 5 to 15 Mcf CO 2 /bbl of recovered oil.
The projected ultimate oil recoveries range from 5 to 30% of the original oil-in-place.
These numbers represent only the consensus of the current expectations. Significant
revisions of many of these estimates may be required after a large amount of actual
full-scale production data become available.

8.10.2 Mobility Control
As discussed above, the control of unfavorable mobility ratios is recognized as a major
technologicalproblemofgas-floodEOR,assuchmobilityratiosproducethefingering
phenomena which are a principal reason for the failure of many of the liquefied
petroleum gas floods of the 1950s and early 1960s (Craig, 1970). These problems
occur because the injection gases have very small viscosities at the temperatures and
pressures at which they are used, hence producing an unfavorable mobility ratio. The
problems associated with the mobility are greatly aggravated, and their theoretical
and experimental study greatly complicated by the facts that, (1) the reservoir rock
is highly heterogeneous at every length scale, from sub-millimeter to kilometer, and
(2) the number of fluid phases is often three rather than two.