Page 81 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

reservoir. Similar to vaporization process, in condensation gas drive process a relatively
small amount of vapor will be created. As the volume of dropouts is relatively small
and could be followed by subsequent vaporization or condensation processes, it is of
no significant concern in design process.

3.2.1.2.2 Vaporization/Condensation Gas Drive
Recent investigations on the process of MCM have suggested that an alternative mech-
anism, one that involves both vaporization and condensation, is often responsible for an
efficient process of MCM. This is what has been shown by Zick [30] through a series of
experiments on MCM. As explained in this work, reservoir oil was loaded into a PVT
cell and a specific amount of gas was then injected to the cell. After equilibrium was
reached, a small amount of liquid and gas phases were sampled and analyzed. Second
contact was then proceeded with a fresh gas. This process continued for seven contacts.
Zick [30] noticed that this process was not only a CGD, because if it was, then the
density of the liquid phase would decrease monotonically and that of the gas phase
would increase monotonically. Zick [30] deduced that such a maximum minimum-
like behavior represents a combination of vaporization and condensation process,
which is called vaporization/condensation gas drive.
Based on these observations, Zick [30] proposed a mechanism for the vaporiza-
tion/condensation process as follows:
• First, it was assumed that oil/gas system is composed of four major groups of com-
ponents as follows:
• Lean components (C 1 ,N 2 , and CO 2 )
• Light intermediate components, which are named as enriching components
(C 1 C 4 )
• Middle intermediate components (ranging from C 4 through C 10 on the low-
molecular-weight side up to C 30 on the high side that are generally not in the
injected gas but in the reservoir oil and may be vaporized from the oil to the
gas phase)
• High molecular weight components that cannot be vaporized from the oil with
significant amount.
• As enriched gas, which contains components of groups a and b contacts reservoir
oil, light intermediates condense and transfer from gas phase to the oil. Eventually,
oil gets lighter. The gas moves faster ahead, and fresh gas contacts the oil again.
Thus oil density will further decrease. If this process continued until the oil
became miscible with the injected gas, it would be the CGD process. But there is
a counter effect explained in subsequent items.
• The middle intermediates are not originally in the gas phase; thus they are stripped
from the oil into the gas. As lighter components are being stripped from oil, it
tends to be enriched in very heavy fractions and thus becomes less similar to the