Note: Descriptions are shown in the official language in which they were submitted.
10369Z8
This invention pertains to an oil recovery method,
and more specifically to a method for recovering oil or
petroleum from a subterranean, viscous petroleum-containing,
unconsolidated formation such as a tar sand deposit.
~lhere dr-e Known to exist tnroughout the world many
subterranean petroleum containing formations from which the
petroleum cannot be recovered by conventional means because
of the relatively high viscosity thereof. The best known of
such viscous petroleum containing formations are the so-called
tar sands or bituminous sand deposits. The largest and most
famous such deposit is in the Athabasca area in the north-
eastern part of the province of Alberta, Canada, which is
known to contain over 700 billion barrels of petroleum. Other
extensive deposits are known to exist in the western part of
the United States, and Venezuela, and lesser deposits in
Europe and Asia.
Tar sands are frequently defined as sand saturated
- with a highly viscous crude petroleum material not recoverable
in its natural state through a well by ordinary production
methods. The hydrocarbons contained in tar sand deposits are
usually highly bituminous in character. The tar sand deposits
are generally arranged as follows. Fine quartz sand is
coated with a layer of water and the bituminous material
; occupies most of the void space around the wetted sand grains.
; 25 The balance of the void volume may be filled with connate
water, and occasionally a small volume of gas which is usually
air or methane. The sand grains are packed to a void volume
of about 35%, which corresponds to about 83% by weight sand.
The balance of the material is bitumen and water. The sum of
bitumen and water will almost always equal about 17~ by weight,
with the bitumen portion varying from around 2% to around 16%.
.b~`'
, ~
--1--
, !
lU3692~
It is an unusual characteristic of tar sand deposits
that the sand grains are not in any sense consolidated, that
is to say the sand is essentially suspended in the solid or
nearly solid hydrocarbon material. The API gravity of the
bitumen usually ranges from about 6 to about 8, and the
specific gravity at 60F is from about 1.006 to about 1.021.
Approximately 50% of the bitumen is distillable without crack-
ing, and the sulfur content may be as high as between 4 and
5% by weight. The bitumen is also very viscous, and so even
if it is recoverable by an in situ separation technique, some
on-site refining of the produced petroleum must be undertaken
in order to convert it to a pumpable fluid.
Bitumen may be recovered from tar sand deposits by
mining or by in situ processes. Most of the recovery to date
has been by means of mining, although this is limited to
instances where the ratio of the overburden thickness to tar
sand deposit thickness is economically suitable, generally
defined as one or less. In situ processes have been proposed
which may be categorized as thermal, such as fire flooding or
steam injection, and steam plus emulsification drive pro-
cesses. Generation of the heat necessary to mobilize the
bitumen by means of a subterranean atomic explosion has been
seriously considered.
The known processes most closely resembling that of
the present invention are disclosed in U.S. Patents 2,412,765;
3,608,638 and 3,838,738. The '765 patent discloses a process
in which low boiling solvents such as propane and butane are
employed to recover oil from partially depleted formations.
In this process the oil recovery is made as complete as
possible and the Eormation is a consolidated or self-supporting
-2-
~o36~;~z~
formation. In the process of the '638 patent a hydrocarbon
solvent such as benzene, platformate or kerosene, at a
temperature in the range of 300-700~F, is injected into the
top of tar sands at an injection well and forced through the
fGl-llatioll .o an a~Jaccn~ prGduction well. Injection of the
solvent and production of oil are continued to maintain a
gaseous phase across the top of the formation. The tar sand
oil is made more mobile as a result of heatiny and dissolution
of the solvent into the oil, whereby the oil drains into the
production well and is lifted to the surface. A requirement
of solvents to be used in the latter process is that the sol-
vent is miscible in the tar sand oil without precipitating
cor.stituents in the oil. Preferred solvents are those which
have good solvent properties such as aromatic hydrocarbons or
mixtures of hydrocarbons containing substantial amounts of
aromatic hydrocarbons. Highly paraffinic hydrocarbons such as
liquefied petroleum gases are said not to be suitable because ~ -
of their inability to dissolve the asphaltic constituents of
; the tar sand oil. The '738 patent is directed to a process
in which petroleum is recovered from tar sand deposits by
first creating a fluid communication path low in the formation.
This is followed by injecting a heated fluid, aqueous or non-
-aqueous, into the fluid communication path. This in turn is
followed by injecting a volatile solvent such as carbon
- 25 disulfide, benzene or toluene into the preheated flow path
and continuing to inject the heating fluid. The more volatile
solvent is vaporized and moves upward into the formation
where it dissolves petroleum, loses heat and condenses there-
after, flowing down, carrying dissolved bitumen with it into
the preheated flow path. The low boiling point solvent
(~
~0369ZE~
effectively cycles or refluxes within the formation and is
not produced to the surface of the earth. Bitumen is trans-
ferred from the volatile solvent to the heatiny fluid, con-
tinually passing through the communication path, and bitumen
S and heating fluid are recovered together as a mixture or
solution.
U.S. Patent 2,862,558 is less closely related to
the present invention but is of interest in that viscous
petroleum is recovered by this process from consolidated
sandstone formations, such as the sarton tar sands, by the
injection of steam and solvent vapor - kerosene vapor, for
example - with what is described as improved results over the
abovementioned '765 patent.
It has not been recognized in preceding patents or
elsewhere in the prior art that complete removal of the
hydrocarbons or bitumen from a tar sand is undesirable. When
the tar content is completely removed, the remaining sand
particles are not connected to each other and effectively
become free-flowing. This tends to result in formation sub-
sidence and in blockage of the pump intake and/or production
line in the well by sand bridging.
Despite the many proposed methods for recovering
bitumen from tar sand deposits, there has still been no
successful exploitation of such deposits by in situ processing
on a commercial scale up to the present time. Accordingly,
in view of the lack of commercial success of any of the methods
proposed to date and especially in view of the enormous re-
serves present in this form which are needed to help satisfy
present energy needs, there is a substarltial need for a
satisfactory method for recovery of bitumen from tar sand
, deposits.
--4--
103692B
A preferred embodiment of the invention is
illustrated by Figure l, which is a diagrammatic vertical
sectional view through a sinyle well extending through an
overburden and an unconsolidated tar sand and bottoming in
S an underlying impermeable formation. The well is adapted
for introduction of hot solvent vapors and for removal of
the bitumen solution formed. Surface means for separating
the solvent from the produced bitumen and returning it as
hot vapors to the tar sand are included.
The invention is an in situ solvent fractionation
process for recovering bitumens from unconsolidated tar sands,
said process consisting essentially of,
(l) introducing hot solvent vapors in a formation
consisting essentially of a bitumen-containing tar disposed
among discrete sand particles, said vapors being so composed
and at such a temperature as to condense, upon contact with
unfractionated tar in said formation, to form a liquid and
to selectively dissolve bitumens out of said tar as a more
soluble fraction,
(2) allowing the resultant solution of bitumens
to drain below the level at which said hot vapors contact
unfractionated tar and collecting it, thereby leaving in
place a less soluble bituminous fraction of said tar as an
undissolved residue capable of supporting itself and the
sand particles among which it is disposed;
(3) removing the collected solution from said
formation; and
(4) recovering the dissolved bitumen content of
said solution.
~s'
~,,
10369Z~ I
Preferably, the process is regulated so that said
undissolved residue constitutes at least 5 wt. % o~ the un-
fractionated tar.
In a particularly preferred mode of operation the
vapors are introauced io, and the solution removed from,
said formation, both through the same bore; optionally, said
bore passes through the formation and is bottomed in an under-
lying strata. In this embodiment, the underlying strata is
impermeable or the portion of the bore (wall and bottom)
therein and the zone of contact between the layers is render-
ed impermeable by appropriate procedures of known types; for
example, by horizontal fracturing, injection of a curable
resin solution and curing.
In a continuous version of the preceding mode the
solvent content of the solution is recovered, reheated,
; revaporized and reintroduced as hot vapors to the formation.
In a more economic and highly preferred mode of
operation the hot vapors comprise a hydrocarbon obtained by
- distilling, cracking or otherwise treating bitumens recoverea
from said formation.
~he temperature of the hot solvent vapors upon
introduction to the formation is preferably within the range
of from about 90-150C, most preferably within the range of
95-120C.
The process of the present invention is character-
ized by the f,act that the temperature of the solvent vapors
introduced to the formation is the most important parameter
of the process. The type or types of hydrocarbons present
in the solvent is a less important parameter. However, in
some instances more effective recoveries are experienced
" .
6-
,..................................................................... .
"
1036,9Z8
with solvents which are aromatic or which at least include some aromatics.
Preferably the solvent vapors are a mixture of vapors of an aromatic solvent
which is benzene, toluene, xylene or mixtures thereof and of an aliphatic
solvent which is heptane, octane, nonane, decane or mixtures thereof. In
the preferred system the mole ratio of aliphatics to aromatics in said
vapors is within the range of from about 60/40 to about 40/60. Solvents
boiling within the range of about 90-150C at atmospheric pressure are
preferred. Lower boiling solvents may be employed at elevated pressures.
However, this is limited by the pressure exerted by the over-burden. For
example, in order to obtain a boiling point over lOO~C with a solvent such as
propane, pressures equivalent to more than 1000 feet of impermeable over-
burden would be needed. Most of the tar sands of the world are not that deep.
Also, it is preferable to operate near atmospheric pressure in order to mini-
mize loss of the solvent vapors.
Since the bitumen is a mixture of hydrocarbon types (aliphatic,
naphthenic, aromatic), the preferred solvent is a mixed solvent of the type
obtained by distilling, cracking or otherwise treating the material to be
dissolved. As a general rule, bitumens having a higher content of asphaltenes
-~ are more readily dissolved by aromatic solvents.
- 20 It is particularly to be noted that the present process is not
directed to complete removal of the tar from the tar sand being processed.
An important purpose of the present process is to essentially avoid the slump- -
ing or collapse of the formation which would result if all of the tar were
removed.
Referring now to Figure 1, a tar sand formation 1 is located between
an over-burden 2 and an impermeable underlying formation 3, a well-bore
indicated generally by the numeral 4 passes through the over-burden and tar
- sand, penetrates into the underlying layer and is bottomed therein. The well
casing 5 preferably extends just deeply enough to contain and support the
packer 7, located at the interface between the tar sand and over-burden. The
- 7 ^
~036 ~
portion ~ of bore 4 within the tar sand layer is unlined to permit facile
flow of hot vapors and leachate (the solution formed by the leaching pro-
cess). The portion 8 of bore 4 in the underlying layer 3 constitutes a
liquid sump or reservoir into which the leachate drains. Pump 9 pumps the
solution to the surface through insulated pipc 10 (which passes through
packer 7). The produced bitumen solution is pumped into a still 11 heated
by means 12. Solvent is distilled off and returned to the tar sand forma-
tion through an insulated line 13. The distillation bottoms, which are a
concentrated solution or solvent-free, liquid bitumen, pass through pipe 14
and drains into a sump 15. Pipes 10 and 13 pass through a conventional
closure member 17 at the top of the casing. As the volume of the vapor-
filled portion of the tar sand formation increases, additional solvent is
added as necessary from tank 18 through pump 19 and line 20. The amount of
additional solvent required will be such as to provide a vapor volume equal
to the liquid volume of the bitumens being recovered. The furthest point of
approach of the vapors into the tar sand formation is indicated by a wavy
line 16.
The hot solvent vapors are introduced to the tar sand formation at
a rate such as to exert a small, preselected back pressure, of a few pounds
per square inch gauge preferably 0.5 to 5 pounds per square inch. Thus, a
sufficient superheat to maintain a small temperature gradient between the
point of injection of the solvent vapors into the formation and the interface
between the leached and unleached portions thereof is provided. In this
manner, any heat losses, such as may result from the presence of foreign
bodies - shale breaks, for
- 8 -
r
1036,92~
example - are accommodated, the t:emperature of the leached
portion of the formation is maintained above the condensation
point and further leaching of the residual tar is avoided.
When all the bitumen is extracted from a true tar
sand, the residual sand has no inherent strength and crumbles.
; Some of the so-called "tar sand" formations are actually
sandstone formations similar to an oil-bearing sand for-
mation. This rock type is not as readily extracted by the
vapor solvent method. Tests of this method on plugs cored
from asphalt rocks gave a maximum recovery of only 30-50
with any of the usual solvents.
For efficient bitumen recovery by the present
method, a plurality of closely-spaced wells is drilled into
the tar sand formation. The spacing selected will depend
somewhat on the depth and thickness of the formation but on
. ~he average will be within the range of about 25-200 feet.
These wells may function both to supply hot vapors to the
formation and to recover the solution formed therefrom, or,
alternatively, some wells may be used for vapor input until
breakthrough to an adjacent well has occurred. Thereafter,
the adjacent well may be used for solution recovery.
When the portion of the tar sand formation which
is penetrated by the wells has been leached until further
bitumen recovery would be uneconomic or would cause collapse
of the sand, introduction of hot vapors is discontinued and
the solvent is recovered from the formation by flooding with
water. This cools the formation and condenses any vapors,
the vapors amounting to about one pound of solvent per
barrel of porous base, depending upon the molecular weight
3~ of the solvent.
s
'~' _g_
~ .
1036928
It should be noted that limited coproduction of
sana and a corresponding degree of formation slumping in
the vicinity of the well bore(s) can be anticipated in the
practice of the present invention (which is not predicated
on the complete avoidance of these effects). However, the
temperature and solvent composition required to substantially
avoid such effects can be determined by laboratory tests
(such as in the following examples) carried out on samples
of the tar sand formation to be leached. Allowance must of
course be made for the fact that the proportion of sand
in a small sample lost (coproduced) will be substantially
larger than in leaching of the tar sand in place.
In some instances, it may be adviseable to extend
the casing (element 5 in the drawing) down to the bottom of
the bore (element 4), as a foraminous section adapted to
act as a sand filter, thereby counteracting any tendency for
subsidence to occur around the bore.
Example 1 - Solvent Effectiveness Tests
An open-top container is set up with boiling
solvent in the bottom, a cooling coil around the top for a
condenser, and an opening so that a sample in a wire screen
basket can be lowered into the heated vapors. Any solvent
condensing on the sample runs down through the screen and is
caught in an aluminum dish suspended below the basket but
above the liquid. Samples are prepared from irregular small
pieces of the tar sand by placing them in a mold and com-
pressing them to 6000 psi to form a cylinder about l-inch
long and l-inch in diameter. The tarred sample in the wire
basket is lowered into the solvent vapors until no more
condensation occurs, usually less than one-half hour. Then
--10--
1036~
the sample is remo-~ed, cooled and weighed. Any sand removed
with the solvent is separated and weighed and the residual
sand (the leached sample) is weighed. The sum of these
weights is subtracted from the original sample weight to
find the weight o~ the tar extracted from the sample. The
leached sand sample from the basket is then broken down and
thoroughly washed with Chlorothene~ (l,l,l-trichloroethane)
to remove any residual tar. The final sand weight is used
to calculate the original total tar content and then th~
percent recovered by the solvent vapor being tested.
Cores cut from undisturbed tar sands are also ex-
tracted by this method and the results are comparable,
within experimental error, to those obtained with the pressure
molded samples.
Solvents of different types and having different
boiling points were tested with five different tar sands
and one asphalt rock.
The vapor compositions and temperatures employed
and the results obtained are given in the following table.
(Samples suspended in steam vapor at 100C for
an hour gave no recovery. The sample did not even slump.
Heating the sample for two hours at 200C also gave no
recovery.)
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--14--
103692~
The preceding tests, surprisingly, indicated that
the boiling point of the solvent is more critical for ex-
traction than the type or types of hydrocarbon in the solvent,
Solvents boiling above 90C recovered more than 90~ of the
bi~umen from ~tha~asca tar sand and solvents boiling at or
above 115C recovered 90% or more bitumen from the two
California tar sands tested. On the other hand, solvents
boiling below 80C recovered less than 50~ of the bitumen in
all cases and below 50C there was almost no recovery,
Samples from the Santa Cruz area in California did
show some solvent type discrimination, Pure naphthenic sol-
vents were 10-20% less effective than aromatic or mixed
solvents of the same boiling points. This presumably is
attributable to the higher asphaltene content of the Santa
Cruz tar sands, In contrast, bitumen from Asphalt Ridge, Utah
did not extract as well with aromatics as with aliphatics at
corresponding temperatures.
Example 2
A laboratory experiment was devised to approximate
field conditions and estimate production rates~ A core four
inches in diameter and four inches high with a 5/8" diameter
center hole was formed in the shape of a doughnut by compres-
sing Athabasca tar sand fragments into an appropriate mold at
2500 psi, This sample was extracted by injecting a 50/50
mixture of toluene and heptane vapors into the top of the
center opening and removing condensed solvent and extracted
tar a,t the bottom through a liquid trap,
The rate of bitumen production was measured and
extrapolated to estimate the rate of production that could be
expected from an actual producing well. Extrapolation was
10369,i~
based on the ratio of surface area exposed in a well 12" in
diameter through 100 feet of tar sand formation, compared to
the surface exposed in the hole of the laboratory sample.
This ratio is about 5420:1. After the flows stabilized, the
recovery rate was ~.3 to 0.5 ml/min. which translates to
about 15-25 barrels per day for a 100 foot pay zone.
Another 4" doughnut-shaped core was divided into
three layers by impermeable membranes placed horizontally and
spaced vertically at equal intervals within the core to
simulate shale breaks in a formation. When this core was
extracted by the above method, the recovery rate was about
0.2 ml/min. or equivalent to 10 B/D, a 30% reduction in
output.
It is to be understood that the practice of the
present invention is not limited to the examples given herein
and is limited only as defined by the following claims.
-16-
