Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF IMPROVED OIL RECOVERY BY SIM[JL~NEOUS
INJECTION OF W~ ;K WITH AN IN-SITU a)MBUSTION PROCESS
m is invention concerns a thermal oil recovery method
utilizing in-situ combustion which permits efficient recovery of
heavy oil from permeable, heavy oil-bearing reservoirs.
It has been proposed to recover oil, in the nature of heavy
viscous oils, from a subterranean reservoir by a method which is
commonly known as in-situ combustion. In this method, an
oxygen-containing gas is injected into the reservoir through an
injection well with ignition of oil within the adjacent reservoir
initiated by suitable means for establishing a combustion front.
m e reservoir is usually provided with one or more production wells
for the production of oil. As the flow of oxygen-containing gas to
the reservoir is continued, the combustion front is moved from the
injection well toward the production wells. m e heat generated by
burning reduces the viscosity of the oil which is displaçed before
the combustion front toward the production wells from which the oil
is recovered. m e combustion front in displacing the mobile oil
before it in the reservoir uses residual carbonaceous deposits as
fuel.
In this known process, the gaseous combustion products and
light hydrocarbons are considerably lighter than the oil and water
present in the reservoir and thus, because of gravity segregation,
tend to rise to the top of the reservoir when vertical communication
exists. Consequently, these products channel through the top of the
formation to the producing well thereby over-riding a major portion
of the reservoir and contacting only a small fraction of the
reservoir oil. This behavior results in inefficient oil recovery
and low vertical sweep efficiency.
Furthermore, in such in-situ combustion processes, large
quantities of heated rock are left behind in the reservoir. m is
heat is therefore lost, which greatly reduces the thermal efficiency
of the process.
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F--0938 -2-
According to the present invention, there is provided an
improved method for recovering oil, especially viscous or heavy oil,
from a permeable, heavy oil-bearing reservoir wherein in-situ
combustion is established in the lower portion of the reservoir,
water is injected into the upper portion of the formation at a
controlled rate and oil is recovered at a production well. An
oxidizing gas such as air or oxygen or mixtures thereof is injected
into the lower portion of the reservoir to form a combustion front
which advances through the reservoir toward a production well. Heat
generated by the in-situ combustion reaction heats the viscous oil
as it advances through the reservoir thereby reducing its
viscosity. The higher density water injected above the in-situ
combustion front tends to segregate to the bottom of the reservoir
because of gravitational forces, whereas the lower density products
of combustion tend to segregate to the top. In addition to
effective contact and heat exchange between the water and the
gaseous products of combustion, the water tends to fill gas-swept
channels thus impeding the flow of gases and diverting them to
previously unswept paths resulting in higher vertical sweep
efficiency. m e water passing through the combustion-heated
formation scavenges heat and becomes a hot water drive displacing
oil fr~m lower regions, not subjected to combustion, which further
improves recovery efficiency per BTU of heat injected. mis type of
vertical crossflow of fluids within a reservoir enables the
reservoir to be more efficiently heated over the areal extent of the
reservoir thereby greatly enhancing the recovery of oil. Mobility
control agents such as thickeners and water-soluble polymers may be
added to the injected water to improve its areal sweep efficiency.
The accompanying drawing is a cross-sectional view of an
injection well and a production well penetrating a subterranean,
permeable, heavy oil-bearing reservoir from which oil is to be
recovered by a method according to one example of the invention.
Referring to the drawing, a subterranean, permeable, heavy
oil-bearing reservoir 10, is overlain by overburden 12 and underlain
il50
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by basement formation 14. m e reservoir 10 is penetrated by an
injection well 16 and a production well 18.
m e injection well 16 has perforations 20 providing fluid
communication with the upper portion of reservoir 10 and
perforations 22 providing fluid communication with the lower portion
of the reservoir. A tubing string 24 extends from the earth's
surface to the lower portion of reservoir 10 forming an annular
space 26 between the tubing string and injection well casing 28.
A packer 30 seals the outer tubing surface from the inside
of casing 28. The top of injection well 16 is provided with means
31 for injecting fluid into the annular space 26 between tubing 24
and casing 28.
The production well 18 has perforations 32 providing fluid
communication with the lower portion of reservoir 10 for the
recovery of oil and gases through conduit 34 to the surface of the
earth.
According to a preferred mode of operation, the lower
portion of the oil-bearing reservoir 10 is ignited in the vicinity
of perforations 22. After ignition, an oxygen-containing gas such
as oxygen or air or mixtures thereof is injected through tubing
string 24 into the lower portion of the reservoir through
perforations 22. A combustion front is formed which progressively
advances from the injection well to the production well. After the
combustion front has advanced a sufficient distance, water is
injected through injection means 31 into the space 26 formed between
casing 28 and tubing 24 and outwardly through the perforations 20
into the upper portion of the reservoir 10.
m e water, since it has a relatively high density, tends to
segregate to the bottom of the formation because of gravitational
forces, whereas the relatively low density combustion gases tend to
segregate to the top. m e water also tends to fill gas-swept
channels thus impeding the flow of gas and diverting it to
previously unswept paths resulting in higher vertical sweep
efficiency. The water passing through the combustion-heated
F-0938 ~4~
reservoir scavenges heat and becomes a hot water drive displacing
oil from lower regions of the reservoir, not subjected to
combustion, which further improves recovery efficiency.
m e combustion front and water advance through the
reservoir 10 contacting the oil and reducing its viscosity and
displacing the oil towards production well 18. Admixtures of oil
and gases enter production well 18 through the lower perforations
32, pass up through conduit 34 and are recovered at the surface of
the earth.
Once the combustion front is sufficiently near to the
production well which can be ascertained by a rise in the
tem~erature at the production well, further injection of the
oxidizing gas is discontinued. Water injection may be continued
until water breaks through at the production well 18.
Water may be injected with the oxygen-containing
combustion supporting gas after the initiation of in-situ combustion
so as to absorb heat from the combustion 20ne, which is a technique
known in the art as wet combustion. The amount of water injected,
in relation to the oxygen-containing combustion supporting gas, will
vary depending upon the a unt necessary to keep the reservoir below
excessive temperature levels. It must not be so great, of course,
as to extinguish combustion as would be evidenced by the composition
of the gases produced from the reservoir.
The oxygen-containing gas used to support combustion can be
air, substantially pure oxygen, or oxygen-enriched air.
The amount of water injected into the upper portion of the
reservoir is much larger than that used in conventional wet
combustion processes in order to scavenge sufficient heat from the
burned out portion of the reservoir and also to restrict gas flow in
the burned zone. However, the injection rate must be controlled or
intermittently discontinued to prevent the water from overriding the
combustion front or extinguishing it. m e st appropriate amount
of water injected into the upper portion of the reservoir may vary
over a wide range depending on many factors such as reservoir, oil
F-0938 ~5~
zone thickness, well spacing, reservoir permeability, and other
variables of operation such as the rate of air injection to support
the combustion process. In general, however, where air is injected
into the low portion of the reservoir, the water injected into the
upper portion will be in the range 0.1 to 2.0, more preferably 0.3
to 1.0, barrels of water per 1000 cubic feet of air injected.
In another embodiment of the invention, separate injection
wells may be used for the injection of the oxidizing gas such as air
or oxygen and water into the selected portions of the reservoir.
For example, two or more closely spaced injection wells may be used
with air or oxygen injected near the bottom of the reservoir through
one well and water injected near the top of all, or selected,
separate wells as dictated by preferred engineering practices.
In another embodiment of the process of this invention, the
areal sweep efficiency of the water injected into the upper portion
of the reservoir may be improved by the addition thereto of mobility
control agents such as thickeners or water-soluble polymers. m ese
agents increase the viscosity of the water and hence decrease its
bility with a resulting increase in displacement efficiency.
Examples of thickeners and water-soluble polymers useful in
connection with this process are disclosed in U. S. Patent Nos.
3,500,918 to Holm and 3,710,861 to Steeg,
m e present invention may be carried out utilizing any
suitable injection and production system. m e injection and
production systems may comprise one or more wells extending from the
surface of the earth into the oi~-bearing formation. Such injection
and production wells may be located and spaced from one another in
any desired pattern. For example, a line drive pattern may be
utilized in which a plurality of injection wells are arranged in a
more or less straight line toward a plurality of production wells in
a more or less straight line parallel to a line intersecting the
plurality of injection wells. In addition, a circular drive pattern
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may be used in which the injection system comprises a central
injection well and the production system comprises a plurality of
production wells about the injection well in a ring pattern such as
a 5-spot or 7-spot well pattern.
