Page 363 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 329

Nevertheless, some EOR processes can be rejected quickly because of unfavor-
able reservoir or oil properties, so the use of preferred criteria can be helpful
in selecting methods that may be commercially attractive. If the criteria are too
restrictive, some feasible method may be rejected from consideration. Therefore,
the guidelines that are adopted should be sufficiently broad to encompass
essentially all of the potential methods for a candidate reservoir.
For convenience, brief descriptions of the eight most common enhanced
recovery methods are provided in the following sections. These descriptions list
the salient features of each method along with the important screening guides.
A few general comments are offered here on the relative importance of some
individual screening guides to the overall success of the various methods. In
addition, we will make some observations on the method itself and its relation-
ship to other enhanced recovery choices that may be available.

Hydrocarbon Misclble Flooding [386]
Description. Hydrocarbon miscible flooding consists of injecting light hydro-
carbons through the reservoir to form a miscible flood. Three different methods
are used. One method uses about 5% PV slug of liquidifed petroleum gas (LPG)
such as propane, followed by natural gas or gas and water. A second method,
called enriched (condensing) gas drive, consists of injecting a 10%-20% PV slug
of natural gas that is enriched with ethane through hexane (C, to CJ, followed
by lean gas (dry, mostly methane) and possibly water. The enriching components
are transferred from the gas to the oil. The third method, called high pressure
(vaporizing) gas drive, consists of injecting lean gas at high pressure to vaporize
C,-C, components from the crude oil being displaced.

Mechanisms. Hydrocarbon miscible flooding recovers crude oil by:
Generating miscibility (in the condensing and vaporizing gas drive)
Increasing the oil volume (swelling)
Decreasing the viscosity of the oil

Technical Screening Guihs
Crude oil
Gravity >35" API
Viscosity e10 cp
Composition High percentage of light hydrocarbons (C, -C,)
Reservoir
Oil saturation >30% PV
Type of formation Sandstone or carbonate with a minimum of
fractures and high permeability streaks
Net thickness Relatively thin anless formation is steeply
dipping
Average permeability Not critical if uniform
Depth >2,000 ft (LPG) to >5000 ft (high pressure gas)
Temperature Not critical
Llmltations.
The minimum depth is set by the pressure needed to maintain the generated
miscibility. The required pressure ranges from about 1,200 psi for the LPG
process to 3,000-5,000 psi for the high pressure gas drive, depending on
the oil.