Page 293 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

260 Reservoir Engineering

into the reservoir via one wellbore and production of oil and/or gas from
another wellbore. Conventional means of secondary recovery include the immis-
cible processes of waterflooding and gas injection. Currently in the United
States, waterflooding is the dominant secondary recovery method in that about
half of the oil production is recovered from waterflood projects. After secondary
recovery, a substantial amount of oil may remain, and attempts to recover oil
beyond primary and secondary recovery are referred to as tertiary recovery. Any
method that recovers oil more effectively than plain waterflooding or gas
injection is defined as enhanced recovery. The more sophisticated enhanced
methods may be initiated as a tertiary process if they follow waterflooding or
gas injection, or they may be a secondary process if they follow primary recovery
directly. Many of the enhanced recovery projects are implemented after water-
flooding. (Enhanced recovery methods are discussed later.)

Pressure Malntenance
Pressure maintenance is a secondary recovery process that is implemented
early during the primary producing phase before reservoir energy has been
depleted. Pressure maintenance projects, which can be accomplished by the
injection of either gas or water, will almost always recover more oil reserves
than are recoverable by primary producing mechanisms. For example, the return
of gas to the formation early in the primary producing history of a field will
permit higher rates of oil production.

Gas InJection

Historically, both natural gas and air have been used in gas injection projects,
and in some cases nitrogen and flue gases have been injected. Many of the early
gas injection projects used air to immiscibly displace crude oil from reservoirs.
The injection of hydrocarbon gas may result in either a miscible or immiscible
process depending on the composition of the injected gas and crude oil
displaced, reservoir pressure, and reservoir temperature. Hydrocarbon miscible
injection is considered as an enhanced recovery process and is discussed later.
Although the ultimate oil recovered from immiscible. gas injection projects
will normally be lower than for waterflooding, gas injection may be the only
alternative for secondary recovery under certain circumstances. If permeability
is very low, the rate of water injection may be so low that gas injection is
preferred. In reservoirs with swelling clays, gas injection may be preferable. In
steeply-dipping reservoirs, gas that is injected updip can very efficiently displace
crude oil by a gravity drainage mechanism; this technique is very effective in
low-permeability formations such as fractured shales. In thick formations with
little dip, injected gas (because of its lower density) will tend to override and
result in vertical segregation if the vertical permeability is more than about 200
md [254]. In thin formations especially if primary oil production has been by
solution-gas drive, gas may be injected into a number of wells in the reservoir
on a well pattern basis; this dispersed gas injection operation attempts to bank
the oil in a frontal displacement mechanism. In addition to the external gas
injection into reservoirs with dip as just described (which may be into a primary
or secondary gas cap), a variation called attic oil recovery involves injection of
gas into a lower structural position. If there is sufficient vertical permeability,
the injected gas will migrate upward to create a secondary gas cap that can
displace the oil downward where it is recovered in wells that are already drilled.