Note: Descriptions are shown in the official language in which they were submitted.
I :~ 643~5
IN SITU COMBUSTION OF TAR SANDS WITH INJECTION
~[~e~
This invention relates to a method of recovery of heavy,
viscous, normally non-flowing hydrocarbons from subterranean
formations of tar sands. More particularly, this invention relates
to a thermal method of recovering valuable hydrocarbon products from
subterranean deposits of tar sands.
Increasing worldwide demand for petroleum products,
combined with continuously increasing prices for petroleum and
products derived therefrom, has prumpted a renewed interest in the
sourees of hydroc~rbons which are less accessible than crude oil of
the Middle East and other countries. One of the largest deposits of
such sources of hydrocarbons comprises tar sands and oil shale
deposits found in Northern Alberta, Canada, and in the Midwest
States of the United States. While the estimated deposits of
hydrocarbons contained in tar sands are enormous (e.g., the
estimated total of the deposits in Qlberta, Canada is 250 billion
barrels of synthetic crude equivalent), only a small proportion of
such deposits can be recovered by currently available mining
technologies (e.g., by strip mining). For example9 in 197~ it was
estimated that not more than about 10~ of the then estimated 250
billion barrels of synthetic crude equivalent o~ deposits in
Alberta, Canada was recoverable by the then available minin~
technologiesO tSee SYNTHETIC FUELS, March 1974, Pages 3-1 through
3-14). The remaining about 90% of the deposits must be recovered by
various in-situ techniques such as electrical resistance heating,
steam injection and in-situ ~orward and reverse combustion.
While operating details of all of such in-situ techniques
vary, a common objective thereof is to lower the viscosity of the
hydrocarbon deposits of tar sands and oil shale to the point where
such hydrocarbon deposits can be pumped to the surface of the
~ormation with equipment normally available at the formation site.
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Of the aforementioned in-situ recovery methods, in-situ
combustion (both ~orward and reverse) appears to be the most
promising method of economically recovering large amounts of
hydrocarbon deposits with currently available technolcgy. The
attractiveness of the in-situ combustion methods arises prlmarily
from the fact that it requires relatively little energy necessary
~or sustaining combustion o~ the hydrocarbon deposits. In
csntradistinction, other in-situ techniques, such as electrical
resistance heating and cteam inJection require considerable amounts
of energy, e.g., to heat the steam at the surface before it is
injected into the formation of tar sands.
Conventional in-situ combustion involves drilling o~ at
least two substantially vertical wells into the ~ormation, the wells
being separated by a horizontal distance within the formation. ûne
of the wells is designated an injection well, and the other a
production well. The recovery of hydrocarbons is accomplished by
raising the temperature around a bore hole to bitumen combustion
temperature with some type of a conventional down hole heater/burner
apparatus, and then supporting combustion by injecting an oxidizing
gas, e.g., oxygen or air into the formation. There are two basic
processes o~ in-situ combustion, viz., foIward and reverse
combustion. Forward combustion is initiated at the oxidant
inject~on well and the combustion ~ront propagates toward the
production well. Reverse combustion is initiated at the production
well and the combustion ~ront propagates toward the oxidant
injection well. Hydrocarbon vapors produced during the combustion
process are recovered at the surface of the formation and stored in
appropriate containers. The combustion is conducted at a
temperature not to exceed 1500 F. for about 12 months until the
viscosity of oil deposits is reduced to 700-8ûO cp, generally
oonsidered necessary for pumping the oil to the surface of the
~ormation. Further details o~ forward and reverse in-situ
:1., ~..
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combustinn techniques are set forth in SYNTHETIC FUELS, March 1974,
pages 3-4 through 3-14, and in THE TAR SANDS OF CANADA by F. W.
Camp, pages 27-34, Cameron Engineers, Inc., Denver, Colorado, 2nd
Edition (1974).
Howev~r, hereto~ore practiced in-situ combustion techniques
have resulted in relatively low recovery of bitumen ~rom
subterranean formations of tar sands. For example, the rates of
recovery have been reported to be less than about 50% of the total
deposits, e.g., SYNTHETIC FUELS, March 1974, pages 3-4 through 3-14.
Accordingly, it is a primary object of this invention to
provide an improvement in the prior art known in-situ combustion
processes.
It is an additional object of this invention to provide a
process for in-situ combustion of tar sands which results in
improved rates of recovery of bitumen.
These and other objects have been attained by introducing
into the formation, prior to the commencement of a conventional
in-situ combustion process, a relatively light hydrocarbon gas. The
gas is introduced into the tar sand formation through wells drilled
to sufficient depths to reach the bottom or near the bottom of the
~ormation. The relatively light hydrocarbon gas may optionally
contain a proportion of condensable hydrocarbons which aid in the
combustion of tar sands.
The relati~ely iight hydrocarbon gas introduced into the
formation is any readily available gas that is substantially
noncondensable at the temperature and pressure of the formation,
i. e., any hydrocarbon containing gas derived from a liquid whose
boiling point (or condensation point of the gas) is less than
230K (degrees Kelvin) (-43C) under ambient pressure of about 1
atmosphere. Preferably, the condensation point of such gas is 100
to ~30K (-173 to ~43C) and most pre~erably 110 to 184K
~-163C to -89C) under ambient pressure of about 1 atmosphere.
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Suitable gas is natural gas, and low boiling alkenes and alkanes of
Cl to C3, e.g., methane, e~hane, ethene, propane, propene,
preferably methane, ethane and natural gas and most preferably
natural gas.
It is to be understood that ambient pressure of about 1
atmosphere does not necessarily designate pressure of exactly one
(1) atmosphere, insofar as the ambient pressure may vary depending
on the altitude of the tar sands formation. Thus, the term "ambient
pressure" as used herein encompasses pressures of 0.95 atmospheres
to 1.05 atmospheres.
The gas introduced into the formation can either be a
substantially pure homogeneous gas having the aforementioned
properties9 or it can be a mixture of any of the gases suitable for
use with the process of the present invention. Ik will be apparent
to those skilled in the art, that if a mixture of gases is used, the
relative proportion of each individual gas must be such that the
condensation point of the mixture must not be lower than the
condensation point of a pure gas as speci~ied abovz. It will also
be apparent to those skilled in the art that the gas may contain a
small proportion, e.g., 1% to 1~% of higher hydrocarbons, e~g., of
up to C7.
The gas is introduced into the formation of tar sands
either by means of bore holes drilled speci~ically for that purpose
or through injection or production wells. The relatively light
hydrocarbon gas is introduced under a pressure of 35 atm to 100 atm,
preferably 60 atm to 80 atm, and most preferably 65 atm to 70 atm,
and at a temperature of -40C to 100C, preferably 0C to
60C, and most preferably 25C to 35C. The well through
which the gas is introduced is drilled to reach the bottom of the
formation of tar sands or near the bottom thereof. In any event7
the point of entry of the gas into the formation may not be more
than 0% to 50% of the height of the formation, preferably 1% to 40~,
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and most pre~erably ~% to 25% of the height of the fonmation,
measured from the bottom thereo~. In this connection, the height of
the formation of tar sands is de~ined as the total thickness of the
formation, measured from a beginning point below the surface of the
earth where the amount of tar sands in the formation is at least
80~, preferably 100%, to the point above said beginning point of the
formation wherein the relative amount of tar sands in the formation
is at least 95~, preferably 100%.
The rate of introduction of the gas into the formation will
vary, depending on the type of the gas used in a particular
embodiment. Generally speaking, the rate of introduction of the gas
and the time required for the introduction thereof into the
formation will be such that the injection will continue until the
formation contains at least lO0 to 500 cu ft/bbl of oil present in
the sand, preferably 250 to 350 cu ft/bbl o~ oil present in the
sand. Most pre~erably, the formation will be relatively substan-
tially saturated with the gas injected therein. ~n this connection,
a point o~ relative saturation of a formation with the gas is
defined as the point at which the formation cannot absorb
appreciable additional quantities of gas beyond those which have
already been absorbed.
In an alternative embodiment, the light hydrocarbon gas may
contain a small praportion of hydrocarbons which condense at the
temperature and pressure conditions of the tar sand formation. The
condensed hydrocarbons are dissolved in the tar sands, making the
latter easier to burn during the subsequent in-situ combustion. The
condensable hydrocarbons used for such purpose must have a condensa-
tion point of at most 100C under ambient pressure conditions of
about one atmosphere. Suitable condensable hydrocarbons for such
purpose are: all hydrocaroons of C4 to C7~ such as alkanes,
alkenes and aromatics, e.ga, n-butane, isobutane, n pentane,
isopentane, hexane, all of its isomers and heptane and all of its
isomers9 benzene, and toluene, preferably normal pentane and
isopentane, hexane, heptane and all of the isomers thereof.
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The amount o~ condensable hydrocarbons present in the light
hydrocarbon gas injected into the formation is 1% to 10%, preferably
2% to 8%, and most pre~erably 3% to 5% by volume. The condensable
hydrocarbons dissolve relatively easily in the ~ormation o~ tar
sands, thereby aiding in the combustion of tar sands when in-situ
combustion is initiated. Thus, the viscosity of the condensable
hydrocarbons must be 0.01 centipoise (cp) to 0 5 centipoise at 40nc,
preferably 0.05 centipoise to 0.3 centipoise~ most pre~erably 0.10
centipoise to 0.15 centipoise. The density of the sondensable
hydrocarbons must be 0.~ to 0.75 g/cm3, preferably 0062 to 0.67
g/cm3, most preferably 0.65 g~cm~.
The relatively easily condensable hydrocarbons present in
the light gaseous hydrocarbcns stream can either comprise a single
homogsneous hydrocarbon substance encompassed by any one of the
generic groups enumerated above, or they can be a mixture of any o~
such substances, so long as the relative proportions of the
individual components of such mixtures are such that the
condensation point, the viscosity, the density and other properties
of the mixture fall within the range o~ the respective properties of
the relatively easily condensable hydrocarbons specified above.
A~ter the injeotion o~ the light gaseous hydrocarbons
stream, either with or without condensable hydrocarbons, is
completed, the in-situ combustion proceeds in the usual manner,
i~e.7 the temperature of the tar sands ~ormation is brought to or
near the combustion temperature and oxygen or air is injected into
the ~ormation in a conventional manner as described in S.M. Faroug
Ali, ~ , THE
JOURNAL OF PETROLEUM TECHNOLOGY, pp. 477-486, (April, 1972).
In any event, once the combustion o~ tar sands has begun,
the stream of light hydrocarbons previously introduced into the
formation aids in the combustion9 thereby markedly accelerating the
entire combus~ion process and increasing the yield of gases and
liquid hydrocarbons obtained there~rom.
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The ~ollowing examples illustrate specific non-limiting
embodiments o~ the invention. All temperatures are in degrees C, all
pressures in atmospheres, and all percent proportions in percent by
volume, unless otherwise indicated.
Example 1
A sample of tar sand containing 1~% by weight o~
petroliferous material is subjected to a laboratory simulated in-situ
combustion test. Air is then injected and the tar sand is heated so
as to initiate combustion. Difficulty is experienced in the ignition
and in sustaining the combustion of the tar sand~ The dif~iculty on
initiating combustion is due, it is beliaved, to the lack of
volatiles in the oil deposited on the sand.
In contrast, the experiment of Exampl~ 1 is repeated, but
this time, methane is injected under pressure until the amount
absorbed is equivalent to 300 cu ft/bbl of oil contained in the
sand. After this injection is completed, the above described
procedure, air injection ~ollowed by heating, is carried out. The
tar sand is ignited, the ~lame sustained and the cil is collected.
Example ~ is repeated with a natural gas containing 5~ C2
and heavier hydrocarbons, of which the condensables comprise 2-3%.
Similarly, the tar sand is ignited, the flame sustained and oil is
collected.
