Page 359 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

346 Reservoir Engineering

Thermal Recovery
In-Sltu Combustion

The theory and practice of in-situ combustion or fireflooding is covered
comprehensively in the recent SPE monograph on thermal recovery by Prats
[378]. In addition, the continuing evolution of screening criteria for fireflooding
[398,399] and steamflooding [400] have been reviewed and evaluated by Chu.
A recent appraisal of in-situ combustion was provided by White [401] and the
status of oxygen fireflooding was provided by Garon [402].
Part of the appeal of fireflooding comes from the fact that it uses the world’s
cheapest and most plentiful fluids for injection: air and water. However, sig-
nificant amounts of fuel must be burned, both above the ground to compress
the air, and below ground in the combustion process. Fortunately, the worst part
of the crude oil is burned; the lighter ends are carried forward in advance of
the burning zone to upgrade the crude oil.

Steam Flooding
Of all of the enhanced oil recovery processes currently available, only the
steam drive (steamflooding) process is routinely used on a commercial basis.
In the United States, a majority of the field testing with this process has
occurred in California, where many of the shallow, high-oil-saturation reservoirs
are good candidates for thermal recovery. These reservoirs contain high-viscosity
crude oils that are difficult to mobilize by methods other than thermal recovery.
In the steam drive process, steam is continuously introduced into injection
wells to reduce the viscosity of heavy oil and provide a driving force to move
the more mobile oil towards the producing wells. In typical steam drive projects,
the injected fluid at the surface may contain about 80% steam and 20% water
(80% quality) [380]. When steam is injected into the reservoir, heat is transferred
to the oil-bearing formation, the reservoir fluids, and some of the adjacent cap
and base rock. As a result, some of the steam condenses to yield a mixture of
steam and hot water flowing through the reservoir.
The steam drive may work by driving the water and oil to form an oil bank
ahead of the steamed zone. Ideally this oil bank remains in front, increasing in
size until it is produced by the wells offsetting the injector. However, in many
cases, the steam flows over the oil and transfers heat to the oil by conduction.
Oil at the interface is lowered in viscosity and dragged along with the steam to
the producing wells. Recoverability is increased because the steam (heat) lowers
the oil viscosity and improves oil mobility. As the more mobile oil is displaced
the steam zone expands vertically, and the steam-oil interface is maintained.
This process is energy-intensive since it requires the use of a significant fraction
(25%-40%) of the energy in the produced petroleum for the generation of steam.
In steamflooding, the rate of steam injection is initially high to minimize heat
losses to the cap and base rock. Because of reservoir heterogeneities and gravity
segregation of the condensed water from the steam vapor, a highly permeable
and relatively oil-free channel often develops between injector and producer.
Many times this channel occurs near the top of the oil-bearing rock, and much
of the injected heat is conducted to the caprock as heat loss rather than being
conducted to oil-bearing sand where the heat is needed. In addition, the steam
cannot displace oil efficiently since little oil is left in the channel. Consequently,
neither the gas drive from the steam vapor nor the convective heat transfer