U.S. patent number 3,759,574 [Application Number 05/075,009] was granted by the patent office on 1973-09-18 for method of producing hydrocarbons from an oil shale formation.
This patent grant is currently assigned to Shell Oil Company. Invention is credited to Thomas N. Beard.
United States Patent |
3,759,574 |
Beard |
September 18, 1973 |
METHOD OF PRODUCING HYDROCARBONS FROM AN OIL SHALE FORMATION
Abstract
A method of producing hydrocarbons and optionally water-soluble
minerals from a subterranean oil shale formation containing zone(s)
of water-soluble minerals, by penetrating said formation with at
least one borehole and leaching or dissolving the water-soluble
minerals from the formation with a solvent fluid so as to form a
cavern(s) and/or interconnected cavities, followed by
fracturization and/or rubblization of the oil shale surrounding the
caverns or cavities, and thereafter injecting into fracturized
and/or rubblized zones, a pyrolyzing fluid to effect in-situ
hydrocarbon recovery therefrom.
Inventors: |
Beard; Thomas N. (Denver,
CO) |
Assignee: |
Shell Oil Company (New York,
NY)
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Family
ID: |
22122967 |
Appl.
No.: |
05/075,009 |
Filed: |
September 24, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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770964 |
Oct 28, 1968 |
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Current U.S.
Class: |
299/4; 423/206.2;
166/271 |
Current CPC
Class: |
E21B
43/281 (20130101); E21B 43/241 (20130101); E21B
43/2405 (20130101) |
Current International
Class: |
E21B
43/00 (20060101); E21B 43/24 (20060101); E21B
43/16 (20060101); E21B 43/28 (20060101); E21b
043/28 () |
Field of
Search: |
;166/271,272,259,261
;299/4,5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wolfe; Robert L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of copending application
Ser. No. 770,964, filed Oct. 28, 1968 and now abandoned. Copending
application Ser. No. 75,061 and Ser. No. 75,067 filed Sept. 24,
1970 also are continuations -in-part of the application and claim
subject matter similar to that claimed herein.
Claims
1. A method of producing hydrocarbons from a subterranean oil shale
formation containing zones of water-soluble minerals comprising the
steps of:
a. extending at least one well borehole into the water-soluble
mineral containing zone of the oil shale formation;
b. removing water-soluble minerals by leaching, dissolving or
solution mining with a non-acidic fluid, thereby creating porosity
in said zone of the formation;
c. effecting rubblization and fracturization of oil shale adjacent
leached zone (b);
d. injecting into said rubblized, fracturized oil shale a
pyrolyzing fluid; and
e. recovering hydrocarbons from said rubblized fracturized oil
shale.
2. The method of claim 1 wherein the leaching solution (b) is hot
water, and the pyrolyzing fluid is steam.
3. A method of producing oil from a subterranean oil shale
formation containing a zone of water-soluble minerals comprising
the steps of:
creating a cavity in the oil shale formation by circulating aqueous
a non-acidic solution-mining fluid into the water-soluble mineral
zone through a first well, and out of the water-soluble mineral
zone through a second well;
recovering the water-soluble mineral from aqueous fluid circulating
out of the second well;
fracturing and rubbling the oil shale formation surrounding the
cavity;
flowing a kerogen-pyrolyzing fluid into the fractured and rubblized
formation; and
recovering oil from the pyrolyzed treated fracturized and rubblized
formation.
4. A method for producing oil from a subterranean oil shale
formation having at least one zone which contains water soluble
minerals comprising the steps of:
extending at least one well borehole into said formation and into
said zone;
establishing fluid communication between said well borehole and
said zone at at least two spaced locations within said well;
circulating aqueous liquid from one of said spaced locations to
another in contact with said zone to dissolve water-soluble
minerals and leave a fluid-filled cavern within the oil shale
formation while
maintaining fluid pressures within said cavern below overburden
pressure within other regions in said oil shale formation;
generating fluid pressures within said oil shale formation
sufficient to create fractures and displace solid oil shale
material toward and into said cavern;
flowing a kerogen-pyrolyzing fluid from one of said locations to
another through the fractures and cavern within the oil shale
formation;
outflowing kerogen-pyrolyzing fluid from said well; and
recovering shale oil from outflowing portions of said
kerogen-pyrolyzing fluid.
5. A method for producing oil from a subterranean oil shale
formation having at least one zone which contains water soluble
minerals, comprising the steps of:
extending at least one well borehole into said formation and into
said zone;
establishing fluid communication between at least one well borehole
and said zone at at least two spaced locations within said
well;
circulating aqueous liquid from one of said spaced locations to
another in contact with said zone to dissolve water-soluble
minerals and leave a fluid-filled cavern within the oil shale
formation while
generating fluid pressure within said oil shale formation
sufficient to create fractures and displace solid oil shale
material toward and into said cavern;
flowing a kerogen-pyrolyzing fluid from one of said locations, to
another through the fractures and cavern within the oil shale
formation;
outflowing kerogen-pyrolyzing fluid from said well; and
recovering shale oil from outflowing portions of said
kerogen-pyrolyzing fluid.
6. The method of claim 5 including the step of establishing fluid
communication between said borehole locations through said zone
water-soluble mineral by hydraulically fracturing at least a
portion of said oil shale formation containing said zone.
7. A method of claim 5 including the step of establishing fluid
communication between said borehole locations through said zone
water-soluble mineral by explosively fracturing at least a portion
of said oil shale formation containing said zone.
8. The method of claim 5 including the step of establishing fluid
communication between said borehole locations through said zone
water-soluble mineral by electrically fracturing at least the
portion of said oil shale formation communicating with said well
boreholes.
9. The method of claim 5 wherein the step of circulating aqueous
fluid includes the step of imparting acidic properties to said
aqueous fluid and circulating said fluid liquid at pressures above
the overburden pressure.
10. The method of claim 5 wherein the step of circulating aqueous
liquid includes the step of imparting acidic properties to said
aqueous fluid and ciculating said aqueous fluid at pressures below
the overburden pressure.
11. The method of claim 5 wherein the step of generating fluid
pressures sufficient to create fractures is carried out by the step
of circulating fluid through said cavern at a temperature
sufficient to pyrolyze the kerogen within the oil shale adjacent to
the walls forming said cavern and to spall-off portions of said
walls into said cavern.
12. The method of claim 5 wherein the step of generating fluid
pressures sufficient to create fractures is carried out by the step
of pumping fluid explosives into said cavern; and
detonating said explosives so as to produce an initial pulse of
high pressure within the cavern followed by a pressure that becomes
lower than that within the adjacent oil shale formation thereby
displacing said solid material towards said cavern.
13. The method of claim 1 including the step of establishing fluid
communication between at least a pair of well boreholes within said
mineral containing zone, said communication being accomplished by
jetting aqueous liquid from each of said well boreholes to a point
intermediate said boreholes.
14. A method of producing oil from a subterranean oil shale
formation containing rich water-soluble mineral zones comprising
the steps of:
a. subjecting the formation to leaching of the water-soluble
minerals by injecting into the formation a non-acidic leaching
solution to leach out the minerals and thereby effecting a zone of
communicating cavities in the formation;
b. injecting a kerogen-pyrolyzing fluid into cavities zone (a) of
the formation so as to effect spalling and rubblization of the oil
shale;
c. continuing injection of the kerogen-pyrolyzing fluid to effect
oil extraction; and
d. recovering the oil.
15. The method of claim 14 wherein the solvent is an aqueous liquid
and the kerogen-pyrolyzing fluid is steam.
16. The method of claim 14 wherein the water-soluble mineral is
water-soluble carbonate, the water-soluble leaching solution is hot
water and the kerogen-pyrolyzing fluid is steam.
17. The method of claim 15 wherein the water-soluble mineral is
nahcolite.
18. The method of claim 5 wherein the water-soluble minerals are
recovered from the formation prior to injection of the
kerogen-pyrolyzing fluid.
19. The method of claim 3 wherein the dissolved water-soluble
mineral by-products are recovered prior to flowing
kerogen-pyrolyzing fluid into the formation.
20. The method of claim 5 wherein the aqueous liquid is hot water
and the kerogen-pyrolyzing fluid is steam.
21. The method of claim 20 wherein the water-soluble mineral is
water-soluble carbonate.
22. The method of claim 20 wherein the water-soluble mineral is
nahcolite.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the recovery of hydrocarbons and
optionally water-soluble minerals from underground oil shale
formations containing water-soluble mineral deposits. More
particularly, it relates to hydrocarbon recovery by in-situ thermal
fluid extraction of oil shale within a fracturized and/or rubblized
portion of a subterranean oil shale formation in and around a
cavern and/or interconnected cavities formed by leaching or
dissolving, e.g., solution mining of the water-soluble minerals
therefrom.
2. Description of the Prior Art
Large deposits of oil in the form of oil shale are found in various
sections of the United States, particularly in Colorado and
surrounding states and Canada. Various method of recovery of oil
from these shale deposits have been proposed and the principal
difficulty with these methods is their high cost which renders the
recovered oil too expensive to compete with petroleum crudes
recovered by more conventional methods. Mining the oil shale and
removing the oil therefrom by above-ground retorting in furnaces
presents a disposal and pollution problem and also such processes
are also generally commercially uneconomical. In-situ retorting to
convert the oil shale to recover the oil contained therein is made
difficult because of the non-permeable nature of the oil shale. The
art discloses various means of improving oil recovery of oil from
oil shale such as described in U.S. Pat. Nos. 3,400,762 or
3,437,378, or 3,478,825 and particularly various means of
increasing permeability of oil shale formations as described in
U.S. Pat. Nos. 3,273,649 or 3,481,398 or 3,502,372, or copending
application Ser. No. 839,350, filed July 7, 1969. Although these
references are directed to an advancement of the art, the basic
technique for recovering oil from oil shale still requires
rubblization techniques such as by means of explosive devices,
e.g., nuclear energy which is expensive, difficult to control and
presents a radioactive contamination problem, all of which are very
undesirable.
OBJECTS OF THE INVENTION
It is an object of this invention to provide an improved method for
recovering hydrocarbons from a water-soluble mineral containing oil
shale formation by leaching or dissolving the water-soluble
minerals such as by solution mining so as to form a cavern and/or
interconnected cavities within the oil shale formation.
It is a further object of the invention to effect rubblization
and/or fracturization of the water-soluble mineral leached oil
shale formation surrounding the cavern and/or cavities so as to
form a permeable zone thereby enhancing in-situ thermal fluid
extraction (pyrolysis) of hydrocarbons therefrom.
Still another object of this invention is to effect in-situ
pyrolysis to produce hydrocarbons from oil shale subjected to
leaching, rubblization and/or fracturization as indicated in the
previous two paragraphs, and subsequently recovering the
hydrocarbons by suitable means.
Still another object of the present invention is to recover
water-soluble minerals from a rich water-soluble mineral containing
oil shale formation(s) that may be removed during the leaching
and/or solution mining, rubblization and/or fracturization, and/or
pyrolysis processes.
Still another object of the present invention is to sequentially
and/or simultaneously recover water-soluble minerals and
hydrocarbons from rich water-soluble mineral containing oil shale
formations that may be removed during the leaching and/or solution
mining, rubblization and/or fracturization and/or pyrolysis
processes.
Other objects of the invention will be apparent from the following
description.
SUMMARY OF THE INVENTION
The present invention is directed to recovery of hydrocarbons and
optionally water-soluble minerals from water-soluble mineral
containing oil shale formations by the following steps: (1)
subjecting a rich water-soluble mineral zone(s) of an oil shale
formation to a leaching, dissolving or solution mining process so
as to dissolve and preferably remove the water-soluble minerals,
thereby creating porosity to allow for thermal expansion of the oil
shale and establish communication through the treated zone(s), (2)
effecting in said leached zone(s) rubblization and/or
fracturization so as to form zone(s) of rubblized and/or fractured
oil shale with large surface area for more efficient heat treatment
by in-situ thermal fluid extraction (pyrolysis), and (3) injecting
into the rubblized and/or fracturized oil shale zone(s) a
pyrolyzing fluid to effect hydrocarbon recovery.
The water-soluble mineral(s) and hydrocarbons may be recovered
sequentially or simultaneously and if the latter, the two products
can be separated by suitable means such as settling or solvent
extraction above ground. The oil shale formation may contain more
than one zone of rich water-soluble minerals which zones may be
separated by impermeable oil shale layers of several feet to
several hundred feet and each of these water-soluble mineral layers
or zones can be leached or dissolved or solution mined in
accordance with the process of the present invention. Also, the
water-soluble mineral zones may contain the same or different
minerals such as carbonates, bicarbonates, halites or mixtures
thereof.
By water-soluble minerals present in the oil shale is meant to
include water-soluble silicates, halides, carbonates, and/or
bicarbonates salts, such as alkali metal chloride, carbonate,
bicarbonate and silicate, e.g., halite, trona, nahcolite and the
like.
The first or initial step should be so designed to create a cavern
or interconnecting cavities in the water-soluble mineral bed(s) or
zone(s) by dissolving, leaching or solution mining techniques
through at least one borehole penetrating said formation. Leaching
can be effected by cold or hot aqueous solutions either at
atmospheric or elevated pressures. When hot solutions are used such
as hot water or acidified hot water and/or steam, more rapid
dissolution is effected of certain water-soluble minerals such as
nahcolite, trona, halite to produce void spaces in the oil shale
formation thereby providing and enhancing well communication, space
for thermal expansion of the shale, and greater surface for contact
with subsequent pyrolyzing fluid. Water can be cold or hot or steam
or any other aqueous fluids can be used such as steam and/or water
containing acids, e.g., HCl, or HCl + HF, surfactants, sequestering
agents, etc. If the initial cavities are not in communication,
fracturing may be necessary.
If necessary, fracturing the formation either before or after
leaching by conventional means such as hydrofracturing, explosive
means, nuclear means, etc., may be desirable. The leaching
solutions can contain chemical agents to enhance dissolution of the
minerals. Under certain leaching conditions decomposition of
certain water-soluble minerals, e.g., bicarbonates, into
solublizing materials may take place of such minerals as dawsonite
and silicates which might be present in the formation, thereby
increasing the porosity of the formation. For example, when
nahcolite is dissolved with water, the pH of the dissolution fluid
is increased and thereby aids in the dissolution of silicates,
etc.
Leaching or solution mining of the water-soluble minerals such as
halite or nahcolite can be accomplished by a suitable solution
mining technique such as described in U.S. Pat. Nos. 2,618,475;
3,387,888; 3,393,013; 3,402,966; 3,236,564; 3,510,167 or Canadian
Pat. Nos. 832,828 or 832,276 or as described in copending
application Ser. No. 2,765 filed Jan. 17, 1970. Spalling and
rubbling can be accomplished by the method described in U.S. Pat.
No. 3,478,825 or by other means such as by hydraulic, explosive,
nuclear and/or electrical means. Preferably rubblization is
accomplished by hot fluid circulation through the cavern causing
the walls to spall and fracture. In-situ thermal recovery of oil
can be effected by a pyrolyzing fluid such as steam and/or hot
water or solvent extraction means.
The circulation of a pyrolyzing fluid not only effects oil recovery
but also effects thermal rubbling and/or fracturization. Also, if
the pyrolyzing fluid such as steam is used to extract and recover
oil, more minerals may be dissolved perpetuating the process.
By the term "pyrolyzing fluid" is meant a liquid or gas which by
means of thermal, chemical and/or solvent action, interacts with
the kerogen components of an oil shale to produce and entrain
hydrocarbon such as steam, Such a fluid can be hot fluids such as
hot water of steam, or mixtures of hot water and strea, hot
hydrocarbons and/or mixtures of such fluids with chemicals such as
acids, e.g., HCl and/or organic solvents, benzene, toluene, cumene,
phenol, etc. The kerogen pyrolyzing fluid can be heated by surface
or borehole-located heating devices. The kerogen-pyrolyzing fluid
can advantageously comprise or contain a solvent for the soluble
mineral, such as steam condensate or a hot aqueous solution of
organic and/or inorganic acid, having a temperature such as at
least one hundred degrees Fahrenheit, such as from about
450.degree. F to above about 1,500.degree. F and preferably from
about 550.degree. F to 1,000.degree. F. Where the
kerogen-pyrolyzing fluid contains or forms aqueous components, its
circulation through the treated oil shale formation can enlarge the
cavern, by solution mining the soluble minerals, while shale oil is
being produced. Also, simultaneously or sequentially pyrolyzing and
oil extracting fluids can be used such as steam followed by a
solvent such as phenol or benzene.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical sectional view, partly diagrammatic, of an
embodiment of the invention showing a formation penetration by more
than one well.
FIG. 2 is a sectional view of an embodiment of the invention, the
formation being penetrated by a single well.
FIG. 3 is a graphical illustration showing the solubility of sodium
chloride (NaCl) and sodium bicarbonate (NaHCO.sub.3) in water as a
function of temperature.
FIG. 4 is a schematic illustration of a method for providing
communication between a pair of well boreholes in accordance with
the techniques of this invention.
FIG. 5 is a schematic illustration partially in vertical section
illustrating the mechanism of single-well salt leaching.
FIG. 6 is a graphical representation of maximum rate of nahcolite
leaching as a function of leaching fluid temperature.
FIG. 7 is a graphical representation of minimum time required to
leach a nahcolite cavity of 100-foot radius as a function of
leaching fluid temperature.
FIG. 8 is a graphical representation showing estimated maximum time
to leach a nahcolite cavity of 100-foot radius as a function of
leaching fluid injection rate and temperature.
FIGS. 9-12 show graphical representations of various process
parameters as a function of time in an example application of the
process of this invention where the rubbling rate is 0.02 feet per
day.
FIGS. 13-16 show graphical representations of various process
parameters as a function of time in an example application of the
process of this invention where the rubbling rate is 0.1 feet per
day.
FIGS. 17-20 show graphical representations of various process
parameters as a function of time for an example application of the
process of this invention where the rubbling rate is 0.5 feet per
day.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 of the drawing, a plurality of well boreholes
are shown penetrating into a subterranean oil shale formation 9
which contain rich zones of water-soluble minerals 10, 10a and 10b.
An injection well borehole 11 is shown extending into oil shale
formation 9 and rich soluble mineral zone(s) 10 or multizones such
as 10a and 10b that are located within the oil shale formation 9
and are also encountered by well borehole 12. Well boreholes 11 and
12 are illustrated as having casings 13 and 14, respectively,
cemented in place in their respective boreholes by suitable
sealants 15 through 16, respectively. Although only a single
injection well borehole 11 and a single production well borehole 12
have been illustrated, obviously various combinations of one or
more injection and production wells may be provided by one skilled
in the art.
In carrying out the method of this invention, the location of zones
rich in substantially water-soluble minerals is determined in a
conventional manner.
Fluid communication between well boreholes 11 and 12 (FIG. 1) and
the zones rich in water-soluble minerals therebetween may be
established by solution mining a cavern or cavities 23, through the
soluble mineral zones. Communication can be enhanced by means of
conventional hydraulic, electric, and/or explosive fracturing
techniques, all well known in the art. Where, for example,
subterranean stresses in and around soluble mineral zones 10, 10a
and 10b are conducive to the formation of horizontal fractures, the
fluid communication between well boreholes 11 and 12 and the
soluble mineral can be established by a conventional hydraulic
fracturing technique. Referring to FIG. 1, after fluid
communication has been established between a pair of wells, aqueous
leaching or solution mining liquid is injected through tubing 17
down well borehole 11, out through perforations 18 opposite any or
all of the soluble beds through the bed 10, 10a and/or 10b up
borehole 12 through tubing via perforation 19 creating a leached
cavern 23. The aqueous liquid may comprise water and/or steam or
aqueous solutions of acid or acid-forming materials and is
circulated at pressures either above or below the over-burden
pressure. The circulating aqueous liquid dissolves the
water-soluble minerals and mineral by-products thereof are
recovered from the fluid flowing out of well borehole 12, for
example, by conventional evaporation and/or precipitation
procedures.
Fluid communication can also be established in one borehole between
at least two spaced portions of the well borehole and the
water-soluble minerals (as for example, in FIG. 2 communication is
through the tubing string the ends of which are open to the
water-soluble minerals and some distance apart.) Thus a single well
may be utilized by a dual zone completion arrangement as shown in
FIG. 2 such that fluids can be injected at one point of the well
and produced from another point of the same well. In FIG. 2, the
wellbore is 26, the casing is 27, the sealant is 28, within the
casing are the injection tubing string 29 and production tubing
string 30, the borehole 26 penetrates oil shale formation 9 with
mineral zone(s) 10 or or multizones 10a and 10b.
Fracturing pressures are generated within the oil shale formation 9
while lower pressures are maintained within the cavern 23 which is
formed within oil shale formation 9 by the removal of the
water-soluble minerals. These pressures are preferably generated by
merely circulating hot fluid through cavern 23. As the walls of the
cavern(s) 23 (23a FIG. 2) are heated kerogen is pyrolyzed within
the cavern walls and the pressures of the pyrolysis products
increase until portions of the walls are spalled into the cavern 23
creating a rubblized zone 24 (24a FIG. 2) and surrounding fracture
area 25 (25a FIG. 2).
Alternatively, fracturization and/or rubblization can be
accomplished by conventional means such as hydraulic, explosive
means and the like. To provide additional void space, if necessary,
further leaching can be conducted.
Finally, a kerogen-pyrolyzing fluid such as steam is circulated
from well borehole 11 (FIG. 1) through the rubblized zone 24 and
fractured zone 25 of oil shale formation 9 and out of well borehole
12. Hydrocarbon materials are then recovered from the heated fluid
circulating out of well borehole 12 by means well known in the art.
Removal of hydrocarbons from the oil shale provides additional void
space enlarging the original rubblized zone, perpetrating the
process. Similar techniques can be applied to single wells as shown
in FIG. 2.
Conventional equipment and techniques, such as heating means,
pumping means, separators and heat exchangers may be used for
pressurizing, heating, injecting, producing and separating
components of the heated fluid circulating through the oil shale
formation 9. The production of the fluid may be aided by downhole
pumping means, not shown, or restricted to the extent necessary to
maintain the selected pressure within the oil shale formation
9.
The fluid circulated through rubblized zone 24 and fractured zone
25 (FIG. 1) to recover oil shale from oil shale formation 9 may
comprise any heated gas, liquid or steam. Oil shale reactive
properties may also be imparted to the circulating fluid as
discussed hereinabove.
Where the oil formation contains a zone rich in substantially
water-soluble minerals in which zone the soluble minerals occur in
the form of adjacent but discrete nodules or lenses 31, or the
like, the present process is applied as described above. In this
situation, the caverns comprise a network of relatively small
cavities that are interconnected by fractures.
EXAMPLES
Leaching Phase
A. In a continuous oil shale formation containing a nahcolite bed,
a pair of wells are completed into a nahocolite layer at 2,100 feet
with a downhole well separation of 70 feet. Solution mining of the
nahcolite (NaHCO.sub.3) by injection of hot water therein provides
both communication between the wells and the void space necessary
to effect fragmentation and subsequent in-situ thermal treatment of
the formation to recover oil.
In such a situation a bulk density (p) was found to be a 2.2 gm/cc
and the permeability (K) was found to be 0.065 millidarcy for the
nahcolite layer at about 2,055 feet. Experimentally, samples of
this nahcolite were found to be completely dissolved in hot water,
leaving 6 percent by weight insolubles.
Minimum volumes of water required to establish a channel 1 foot
wide, three feet high and 70 feet long (between two wells about 50
feet apart, for example) which contains 13.4 tons of nahcolite may
be determined from the solubility of sodium carbonate and
bicarbonate in water.
As can be seen in FIG. 3 the solubility of pure NaHCO.sub.3 in
water at formation temperature (90.degree. F) is about 30 lbs/bbl.
Thus, a minimum of 700 bbls of water is required to establish
communication between wells. On the other hand, a cylindrical
cavity of the same height but 50 feet in radius contains 1,620 tons
of nahcolite, and requires at least 10.sup.5 bbls of water at
formation temperature.
Water requirements may be reduced by a factor of five if the water
is heated to 400.degree. F (.DELTA.T = 310.degree. F). Heating the
water also has the added advantage that it results in a higher
dissolution rate. Thus heating the water results in a shorter
operating life, and requires the handling of relative small volumes
of water. On the other hand, it requires the use of heaters with
their attendant requirements of water quality and fuel supply.
Also, the water disposal lines may become plugged with precipitate
as the temperature of the line drops at the surface.
If the water is injected at formation temperature, a slight
reduction in temperature takes place. The heat of solution of
sodium bicarbonate is 4 kcal/mole, which results in as much as a
10.degree. F drop in the solution temperature. Because the solution
is not saturated, the observed temperature drops are in fact much
smaller and thus may be discounted.
The addition of acids, such as 15 percent HCl to mining solutions
is beneficial since it generally may be expected to result in a
reduction in operating time, because of the high rate of reaction
between the acid (HCl) and nahcolite. For example, injection of an
acid solution into the wellbore will speed up the rate at which the
cavity is made.
Communication may be established between the two wells by means of
mechanical nozzles having controllable orientation through which
the solvent is introduced. As illustrated schematically in FIG. 4,
where the uncertainty in orientation of the nozzles is .+-.
10.degree., the nozzles may be directed from both wells A and B,
with the orientation of the nozzles ranging from 0.degree. to
15.degree. from their centerlines. This procedure insures eventual
communication between the wells and reduces the time to obtain
communication.
The degree of saturation of the effluent liquid is closely related
to the mean residence time of the fluid in the subsurface, the
circulation pattern of the fluid, and the rate at which the
nahcolite goes in solution. The solution efficiency may be
increased by increasing the residence time, that is, by increasing
the operating time. Where sufficient water capacity is available
and the operating time is to be kept low, it would appear that low
solution efficiencies may be tolerated, especially if it is not
intended to heat the water. On the other hand, the mining effect
may be greatly enhanced if fragments resulting from jetting are
removed as solids.
After solution mining to form the cavern, the formation is
fractured in the vicinity of the cavern and oil is recovered
therefrom by means of in-situ oil recovery means as is well known
in the art.
B. Results for a single well leaching to a 100-foot radius was
determined experimentally for a nahcolite layer oil shale. The
leaching rate results show that leaching rates are a function of
temperature.
FIG. 5 shows the mechanism of single well salt leaching. Fresh
water enters at the top of the formation and flows along the top of
the cavity. Once it reaches the salt layer it dissolves the salt,
becoming denser. The denser fluid then flows to the bottom of the
cavity along the edge of the salt. There are two important
parameters which control the rate of frontal advance of the cavity,
natural convection and diffusion in the vertical direction. The
slowing of the frontal advance is caused by diffusion in the
vertical direction from the salt solution to the incoming fresh
water. As the concentration of salt in the water reaching the
leading edge of the cavity increases, the rate of frontal advance
slows proportionally.
An experiment was scaled for 2,000 bpd at room temperature in a
6-foot layer of NaCl. This corresponds to scaling nahcolite
leaching in the same size layer at 8,300 bpd and 300.degree. F. It
was found that the rate of frontal advance was constant out to the
scaled test radius of 100 feet. The concentration of salt in the
produced solution increased from 12 percent of saturation to 85
percent of saturation during the course of the experiment.
Using the results of the experiment, estimates were made of the
maximum leaching rate of the subject nahcolite layer as a function
of the temperature of the fluid at the leading edge of the cavity.
Since a perfectly circular pattern was not obtained in the
experiment, the minimum leaching rate was used in the estimates. It
was also assumed that there were 20 percent insolubles in the
nahcolite and that their only effect was in reducing the available
surface area for leaching. FIG. 6 shows the rate of leaching as a
function of the temperature of the fluid at the leading edge. FIG.
7 shows the minimum time required to leach a 100-foot radius as a
function of temperature. It appears that a flow rate of 2,000 bpd
should be practical for a 6-foot layer.
The test showed that the production well was producing saturated
solution when the frontal advance rate decreased and the maximum
time required to leach a 100-foot radius can be calculated from a
material balance and the solubility of nahcolite in water. FIG. 7
shows this minimum leaching time as a function of leaching time and
flow rate. In making the calculations for FIG. 7, the constraint
that the rate of advance could not exceed the maximum values given
in FIG. 6 was used.
FIG. 8 shows the effect of temperature on water injection rates
leaching a cavity with a radius of 100 feet.
It should be noted that the temperature at the leading edge of the
advancing front will not be the same as the injected temperature
due to heat losses to the shale. The temperature drop will be
roughly proportional to the temperature difference between the
injected fluid and the initial shale temperature and will increase
as the front advances.
Rubblization Phase
Following the leaching phase rubbling using hot water and steam on
the oil shale was performed. This consisted of cementing a large
rectangular block of oil shale into a stainless steel container
such that the lower 3-1/2 inches of the block was unconfined and
was contacted with hot water or steam. A spring-loaded plate
positioned below the block allowed for the detection of any falls
occurring during the experiment. Thermocouples placed in the steam
chamber and into the shale block monitored the temperature at these
points. Pressures surrounding the shale were maintained at 900 to
1,000 psi with nitrogen gas.
Three tests (A.sub.1, B.sub.1 and C.sub.1) were run under
essentially the same conditions. The first, A.sub.1, utilized a
lean shale block (8 gal/ton); the lower face of the block was
contacted with 500.degree. F steam for a 6-day period. At the
conclusion of the test, the shale container was opened and the
block examined, and it was only evidented that the steam induced
considerable cracking and rubbling. No oil was recovered during or
after the experiment.
The second test, B.sub.1, was essentially a repeat of A.sub.1,
using a richer shale (27 gal/ton) and a different heating medium,
hot water instead of steam. The water temperature was held constant
for a 1- or 2-day period and then raised in 50.degree. F
increments. The water temperature was raised and held constant at
300.degree. F for 16 hours. Several large cracks (1/4 inch to 1/2
inch wide) were developed even at these mild temperatures. After a
day's delay, the test was restarted and a major fall occurred
(water temperature = 350.degree. F). Smaller falls of 5 to 10
pounds occurred at 25 hours. The test was terminated after 312
hours; the maximum temperature, 520.degree. F, maintained for the
last 51 hours. No oil was detected in the effluent water stream,
but the outlet lines were found to be coated with a tarry residue
readily soluble in benzene.
The results of B.sub.1 indicated that rubbling took place even at
mild temperatures (350.degree. F).
Test C.sub.1 was run under conditions similar to B.sub.1 and the
specific conditions are shown in Table 1.
Table 1
---------------------------------------------------------------------------
C.sub.1 -- TEST CONDITIONS
Water Temp. Time at Temp. Shale Temp. Pressure (.degree.F) (hours)
(.degree.F) (psi)
__________________________________________________________________________
300 48 185 975 350 48 210 1000 400 48 245 1000 450 48 275 1000 x
__________________________________________________________________________
.sup.x temperature was then reduced in 50.degree. increments. Total
test time = 312 hours (13 days)
Upon examining the shale at the termination of the test, it was
apparent that a large slab (54 lb) had almost entirely separated
from the main body of the shale block. The major fractures formed
did not continue through the entire slab. There was good
correlation between the position of the vertical cracks
(perpendicular to the bedding plane) and the positions of
distortions occurring in the bedding. Fractures also occurred at
sites where there was marked variation in the mineral content.
These variations were in the form of streaks of crystalline
material.
Heating the shale four days at 520.degree. F resulted in greatly
increased fracturing over that resulting from heating to
450.degree. F. After heating at 450.degree. F, many cracks had
formed, but none completely cleaved the slab. After heating to
520.degree. F, a number of these cracks had been considerably
widened and had propogated through the entire extent of the slab.
The strain, measured for the slab, had increased to 0.057 and
average slab thickness increased from 4 to 4-1/2 inches. No oil was
produced with the effluent water.
Peculiar to test C.sub.1 was the correlation between the positions
of bedding plane distortions and the occurrence of vertical cracks
upon heating. The previous sample B.sub.1 was very evenly bedded
and did not show this behavior.
In summary, the amount of fragmenting and fracturing of oil shale
increased with increasing richness of the oil shale sample. There
was a significant increase in fracturing at T - 520.degree. F over
that produced below 450.degree. F in unconfined shale samples. Good
correlation exists between the positions of vertical (perpendicular
to the bedding) cracks and the positions of distortions in the
bedding plane.
Recovery Phase
Calculations were made to estimate the performance of a shale oil
recovery project in accordance with the method of this invention
wherein steam is used as the pyrolyzing fluid to effect hydrocarbon
recovery as well as recovery of other products as shown in FIGS.
9-20.
The basic data used for the calculations were:
a. steam injection at 625.degree. F, 95 percent quality,
b. 10 tons of steam condensed coming down injection pipe,
c. initial temperature of shale is 100.degree. F,
d. shale contains 39 percent by weight nahcolite,
e. shale richness was 24.2 gal/ton (gross),
f. interval considered was 100 feet high,
g. initial cavity diameter was 22 inches,
h. 20 percent by weight of the produced hydrocarbon flowed in the
gas phase, and
j. production well temperature was 450.degree. to 500.degree.
F.
FIGS. 9-20 show the produced oil and hydrocarbon gas, the injected
steam and the produced water; the produced NaHCO.sub.3 ; and the
oil-steam ratio. FIGS. 9-12 are for a rubbling rate of 0.02 ft/day,
FIGS. 13-16 are for a rubbling rate of 0.1 ft/day, and FIGS. 17- 20
are for a rubbling rate of 0.5 ft/day.
It is understood that various changes in the detailed described to
explain the invention can be made by persons skilled in the art
within the scope of the invention as expressed in the appended
claims. I claim as my invention:
* * * * *