Page 28 - Fundamentals of Enhanced Oil and Gas Recovery

16 Amirhossein Mohammadi Alamooti and Farzan Karimi Malekabadi

1.10.5 Gravity Drainage Drive
Gravity drive mechanism is active in both gas cap and water drives, where the differ-
ences in densities act as source of energy. The energy is provided by upward liberated
gas movement to the primary or secondary gas cap, and also downward movement of
water to the aquifer. The main gravity drainage drive mechanisms are found in satu-
rated reservoirs, where the upward movements of gas bubbles push down the oil
toward the well. In order to have optimized recovery of the gravity drainage drive,
wells should be completed near the lowest possible part of oil layer. Numerous factors
affecting the efficiency of this mechanism include vertical permeability, dip of the res-
ervoir, relative permeabilities, and production rates.
In many reservoirs where both water and primary gas cap are available, these drive
mechanisms exist simultaneously. In other words, the dominant drive mechanism is a
combination of drive mechanisms.

1.11 MECHANISMS OF OIL TRAPPING AND MOBILIZATION

1.11.1 EOR: What, Why, and How?
Oil recovery through the reservoir life is classified in three main steps: primary, sec-
ondary, and tertiary, called EOR. During the reservoir recovery period, primary oil
production is the early stage in which oil comes up to the surface by natural energy
or some artificial lift tools, including gas lift or pumps. Consequently, the secondary
recovery is followed to maintain the reservoir pressure by water or gas injection to
aquifer or gas cap, respectively. In many reservoirs the secondary recovery is launched
immediately after primary production. The vast and techniques implemented to
extract residual oil are categorized in tertiary recovery.
The reservoirs that cannot produce oil by natural depletion are candidates for the
tertiary recovery implementation. Numerous effective parameters in the oil reservoir
shift the production plan to the tertiary recovery. Wettability adversity, reservoir depth,
and hydrocarbon viscosity are some parameters making the EOR usage inevitable.
Also some part of the residual oil immobilized in primary or secondary production
can be displaced by some advanced EOR methods.
Depending on reservoir characteristics, various EOR methods can be carried out.
The EOR processes are categorized as the thermal method, which incorporates heat
transfer to bring up the viscous crude oil; gas injection, which uses nitrogen and car-
bon dioxide in both miscible and immiscible approaches; and chemical techniques,
which are used not only to improve the Waterflood sweep efficiency, but also to
reduce the oil surface tension.