Note: Descriptions are shown in the official language in which they were submitted.
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PRODUCING WELL STIMULATION METHOD -_
COMBINATION OF THERMAL AND SOLVENT
BACKGROUND OF THE INVENTION
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1. Field of the Invention
This invention relates to an in-situ combustion
process for the cyclic thermal stimulation of heavy oil
around a producing well wherein oxygen or a fluid con-
taining a minimum of about 75~ by volume pure oxygen is
used as the oxidant so as to react with the oil to release
heat of combustion and to produce high concentrations of
carbon dioxide. The increased temperature, pressure, and
the dissolution of the C02 in the reservoir oil reduces
its viscosity and thereby increases oil production from
the well when it is returned to production.
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2 Desc_ ption of the Prior Art
Repetitive stimulation of oil producing wells
is a production practice of long standing. The phrase
"cyclic stimulation" is often used to reflect anticipated
production rate increases, the duration of which is rela-
tively short as compared to the total life of the well.
The cause of the production increase arises from either
(l) an increase in pressure driving reservoir fluids
toward the producing well, or (2) a decrease in resistance
; to flow of the fluids such as reduction in viscosity or
removal of impediments to flow in the reservoir rock
surrounding the well. The viscosity reduction may be
achieved through use of a low viscosity fluid solvent and
by increasing the temperature of the reservoir fluids and
rock in the proximity of the reservoir.
In wells producing heavy (viscous) oil, cyclic
thermal stimulation has become widespread in use. Two
somewhat different thermal stimulation techniques have
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been developed: (l) cyclic s~eam injection, and (~) cyclic
in-situ combustion. A typical cyclic steam stimulation
may include~ injection of steam into a producing
interval for a period that may extend up to several weeks,
depending on thickness of the reservoir, well spacing,
rate of steam injection, etc.; (2~ allowing a ~'soak"
period (which in some circumstances is not necessary); and
(3) returning the well to production. The heat introduced
into the reservoir rock continues to be effective for some
time in warming and reducing the viscosity of the oil,
thus increasing the production rate. The effects of the
stimulation will decline over a period of a few months
whereupon the treatment may be repeated.
Instead of using fuel-fed steam generators,
cyclic in-situ combustion may be used to heat the reser-
voir. ~ith this technique, air is injected into the
reservoir through the producing well, which, after igni-
tion, burns a small portion of the crude oil "in-situ",
generating heat which is conveyed outward from the well
into the surrounding reservoir by the flue gas formed and
by vaporized crude oil and water. Water may be injected
along with, intermittently, or following air injection to
form steam and hot water which will convey the released
heat of combustion farther into the reservoir. Although
2S this method of stimulation may utilize fuel of less value
than the steam process, wherein the steam is generated
prior to injection into the reservoir, use of the latter
process is generally favored. One major disadvantage of
the combustion method is the requirement of compressing to
injection pressure approximately four mols of nitrogen for
every single mol of oxygen in air to support the combus-
tion reaction. This increases cost and also dilutes the
carbon dioxide concentration in the flue gas1 greatly
diminishing its efficacy as a solvent gas for reducing
viscosity of the heavy oil.
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The method of this invention is a major improve-
ment in the combustion stimulation technique in that it
uses oxygen or a fluid containing a minimum of about 75
by volume pure oxygen as the oxidant injected into the
reservoir through the production well. The cycle of the
process would be similar to that used with air: i.e., (l)
inject the oxidant, which after ignition causes movement
of a burn front through the reservoir rock surrounding the
well; (2) allow a "soak~ period (which is optional); and
(3) return the well to production. The latter step
usually requires installation of a downhole pump to remove
produced liquids from the well.
The advantages resulting from the use of oxygen
or a fluid containing a minimum of about 75% by volume
pure oxygen includeo
l. Elimination of large amounts of "inert" gas,
i.e.l nitrogen, which is costly to compress for injec-
tion. Also the presence of the inert nitrogen gas as a
separate phase in the pores of the reservoir rock impedes
the flow of oil toward the well.
2. The concentration (and partial pressure) of
the C02 formed in the combustion reaction is increased,
and correspondingly its solubility in the heavy oil is
increased. As a result, the viscosity of the heavy ~il
containing larger amounts of solvent gas is substantially
reduced, and oil production rate is increased accordingly.
3. The increased C02 content in the oil phase
increases the extent to which the "solution gas drive" can
contribute to the displacement of oil toward the produc-
tion well.
4. Ignition of the combustion reaction in-situ
is facilitated by the higher oxygen concentration of the
injected gas. "Auto ignition" will occur with a greater
number of crude oils, thus reducing the need to use down-
hole burners, electric heaters, or steam preheating to
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start the combustion reaction. (This does not preclude
the use of any of these methods where the crude oil
properties do not favor auto ignition.)
5. Water injection alonq with or intermittent
:5 to the injected oxidant may be used as in "wet combustion"
;using air and water. The advantages of increased heat
transport farther into the reservoir by the steam formed
in-situ from heat released by the combustion reaction also
apply with oxygen or enriched air combustion. The in-
creased solubility of C02 in the condensed water also
enhances its expulsion from the reservoir to the producing
well which also enhances the displacement of the heavy oil
toward the producing well.
`In U. S. Patent No. 3,174,543 to Sharp there is
described a method of recovering oil by producing carbon
dioxide in the reservoir region surrounding an injection
. well by in-situ combustion and then introducing water into
the reservoir to drive the carbon dioxide through the
reservoir to displace the reservoir oil toward a produc-
tion well. The present process is an in-situ combustion
stimulation process that takes place in the reservoir
immediately surrounding the bottom of a producing well
using oxygen or a fluid containing a minimum of about 75%
by volume pure oxygen as the oxidizing medium which
results in the formation o~ a combustion gas comprising a
high concentration of carbon dioxide. The carbon dioxide
readily dissolves in the oil and reduces its viscosity.
The heat generated in the reservoir by combustion also
reduces the viscosity of the oil phase thus improving its
flow thrnugh the formation when production is resumed. By
the process of this invention therefore, a more effective
recovery of the heavy crude oil is obtained.
Thermal oil stimulation processes using the so-
called "huff-n-puff" gas injection techniques are
- 35 disclosed in U. S. Patent No. 3,332,482 to Trantham,
3,369,604 to Black et al. and 3,465,822 to Klein.
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U. S. Patent to Trantham, 3,332,482, discloses a
process for the secondary recovery of viscous oil using an
in~situ combustion process at the bottom of a producing
well in which air is used as the oxidizing medium. In
this process, air is injected into the production well and
the oil surrounding the bottom of the well is ignited to
establish a combustion zone. Combustion is continued
until the reservoir is plugged by viscous oil which
results in a substantial increase in pressure. Combustion
is terminated and the well is opened for production so
that the compressed gases within the reservoir remote from
the production well and beyond the plugged area drives the
oil into the hot burned-out area between the plugged area
and the production well where it is heated, perhaps
upgraded somewhat, and finally recovered through the pro-
duction well. Inherent in this process is the production
of a gas, which is normally referred to as flue gas, which
gas is composed predominantly of nitrogen and lesser
amounts of carbon dioxide, carbon monoxide and other gases
derived from the crude oil. The carbon dioxide in the
flue gas is diluted by the nitrogen and other gases and is
much less soluble in the reservoir oil than a gas con-
sisting of substantially pure carbon dioxide or a gas
containing a higher concentration of carbon dioxide than
the flue gas produced by the use of air as the oxidizing
medium. The solubility in reservoir oil of carbon dioxide
formed with air combustion, at a given pressure, may be
five to ten times less than that formed from oxygen
combustion.
U. S. Patent No. 3,369,604 discloses a method
for stimulating producing wells using a combination of
steam stimulation and in-situ combustion wherein air, or a
mixture of air and oxygen is used as the oxidizing gas.
U. S. Patent No, 3,465,822 to Klein, discloses a
thermal oil stimulation process in which in-situ combus-
tion is initiated around a well by air injection followed
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by injection of water and injection of inert gas, sequen-
tially, and thereafter opening up the well to flow of
fluids, including oil.
Also, in a Society of Petroleum Engineer of AIME
article, SPE 9228, presented on September 23-26, 1979, in
Las Vegas, Nevada, entitled "A Parametric Study of the
` C2 ~UF-n-PUF Process" there is disclosed the results of
Y Mathematical Model studies of the use of carbon dioxide as
a solvent gas in cyclic well stimulation. The carbon
10 dioxide is not prepared in the well by in-situ combustion
as in the present process and offers no advantages asso-
ciated with the heat generated by oxygen combustion of the
reservoir oil.
None of the prior art discloses the improved
15 method of recovering oil around a well using in-situ
combustion stimulation wherein the oxidizing medium is
oxygen or a fluid containing a minimum of about 75% by
volume pure oxygen so as to produce increased concen-
trations (and partial pressures) of carbon dioxide in the
20 combustion gases. The carbon dioxide dissolves in the
reservoir oil reducing its viscosity, thereby ~acilitating
its flow to the production. The viscosity of the reser-
voir oil is further reduced by the heat generated in the
reservair by combustion.
25 SUMMARY OF THE INVENTION
This invention is directed toward a method for
: the cyclic thermal stimulation of heavy oil producing
wells by in-situ combustion around the producing well
, using oxygen or a fluid containing a minimum of ~bout 75%
by volume pure oxygen as the oxidizing medium which
results in improved recovery of the oil from the reser-
voir. The use of such an oxidizing medium produces a
combustion gas comprising high concentrations o~ carbon
dioxide and water in the form of steam. The steam aids in
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carrying heat farther into the reservoir; the carbon
dioxide is an efFective solvent in that it dissolves in
the heavy oil that occurs at even greater distances in the
reservoir beyond the combustion and steam heated zone
thereby reducing its viscosity. During combustion, the
heat generated is absorbed by the reservoir which extends
radially from the production well which results in further
reduction of the viscosity of the heated heavy oil as it
flows subsequently toward the producing well. Combustion
10 may be continued until the combustion zone travels a
radial distance in the range of about 5 to 50 feet and
then production operations are resumedO In addition, when
combustion has been carried out in the stated portion of
the reservoir, the production well may be shut in for a
15 predetermined interval of time to enhance the solvent
effect of the carbon dioxide and the thermal effect of
combustion. The length of this soak period will depend
upon the field characteristics of the producing well.
Water may also be mixed with the oxidant to enhance the
20 transport of heat farther into the reservoir thereby
increasing the effectiveness of the thermal effects.
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DESCRIPTION OF THE PREFERRED EMBODIM~NT
In accordance with the present invention, oil is
25 recovered from a reservoir by cyclic thermal stimulation
of one or more producing wells using an in-situ combustion
process wherein oxygen is used as the oxidizing fluid
instead of air. Although the preferred oxidizing fluid is
pure oxygen, some sacrifice in the actual performance of
30 the process may be needed to make it more practical and
economically feasible and therefore the oxidizing fluid
may contain a minimum of about 75% by volume pure oxygen.
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The oxygen, upon reacting with the hydrocarbons in the
reservoir, yields principally gaseous carbon dioxide and
water as follows:
(CHn)m + (m + 4 ) 2~ m C0 ~ mn H 0
The carbon dioxide acts as a solvent since it will dis-
solve in the reservoir oils and therefore appreciably
lower the viscosity of the oil even in the absence o~ the
thermal effects. The amount of dissolution will depend on
the local reservoir pressure and temperature, but will be
substantially greater than that experienced iF air is used
because of the higher concentration of carbon dioxide.
The water formed will be initially in the form of steam
which will aid in conveying the heat of combustion farther
into the reservoir, enhancing the effect of the heat
released~
By using oxygen or a fluid containing a minimum
of about 75% by volume pure oxygen as the oxidizing
medium, the large amount of nitrogen introduced into the
well when air is used would be eliminated, along with the
deleterious effect of gas phase nitrogen on the perme
ability of the liquid oil phase. Water injection along
with the oxidizing medium after combustion is initiated
may be used to moderate the high temperature generated and
~; to obtain greater distances of penetration into the reser-
voir for more effective heat distribution. lt would not
necessarily add gaseous products to be subsequently
produced.
For the purpose of simplicity in describing the
invention, reference sometimes will be made herein to only
one production well in my in-situ combination stimulation
process. However, it will be recognized that in practical
application of the invention, a plurality of such wells
may be used and in most cases will be utilized.
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In carrying out this invention, an oxidizing gas
comprising oxygen or a fluid containing a minimum of about
. 75~0 by volume pure oxygen is injected into a producing
well and combustion is initiated in any suitable conven-
tional mannex such as by locating an electrical or gas-
fired heater within the well so as to initiate a combus-
tion zone around the well and generate combustion gases
consisting principally of carbon dioxide and water in the
form of steam. Continued injection of oxygen moves the
resulting combustion zone outward into the reservoir and
the carbon dioxide in the combustion gases dissolves in
the reservoir oil reducing its viscosity. The heat
generated by combustion also lowers the viscosity of the
reservoir oil surrounding the production well and the
lS steam aids in conveying the heat of combustion farther
into the reservoir.
; Combustion is continued through the reservoir
around the production well until the combustion zone
advances a radial distance of about 5 to 50 feet from the
production well. Combustion is then terminated and the
production well is returned to a producing operation.
An alternative method of carrying out the inven-
tion is to shut in the production well after the combus-
tion zone has moved a radial distance of about 5 to 50
feet from the production well to allow a soak period in
- which heat generated in the reservoir will distribute
itself and also allow the carbon dioxide to more effec-
tively dissolve in the heavy oil at greater distances from
the well thereby lowering its viscosity. For cptimum
results, the length of the soak period will vary depending
upon the characteristics of the producing well such as
depth, rate of production, frequency of stimulation
periods and size of stimulate treatment. After the soak
period is terminated, the well is then returned to
producing operation.
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Another embodiment of this process is to inject
water continuously or periodically with the oxidizing
fluid in the production well after combustion is initiated
which serves to obtain greater distance of penetration of
combustion heat into the reservoir for more effective heat
distriDution. The water serves to recuperate the heat
storsd in the burned-out reservoir, which would otherwise
be wasted. This heat is then used to evaporate water.
The steam thus formed condenses downstream of the combus-
tion zone, where it contributes to further heating of thereservoir. This technique is known as wet combustion. As
another variation, water may be injected after oxygen
injection has been terminated whereupon the water would be
converted into steam upon passing through the higt- tempe-
rature zone created by the combustion and subsequentlywould extend the distance into the reservoir that would be
benefited by the heat of combustion.
The substantial concentration of carbon dioxide
produced in the reservoir in-situ acts as a local pres-
surizing agent, a solvent in the oil phase lowering theviscosity of the oil, and together with the thermal
~- effects of oombustion stimulates the reservoir and
significantly increases the production rate of the oil.
The oxygen used may be obtained from any type o~
separation plant capable of providing the desired purity.
A highly expedient approach is to inject oxygen into the
production well that may be supplied from cryogenic units
from which the oxygen i.n liquid phase is pumped at any
desired pressure level and thereafter passes through a
heat exchanger to vaporize the high pressure liquid
oxygen. This eliminates the need for compressor and
; attendant equipment. The cryogenic units may be portable
and operated at the well site. Equally effective is use
of oxygen available in the gaseous phase which may be
compressed with gas compression equipment to the pressure
level desired for injection into a well.
