U.S. patent number 3,917,345 [Application Number 05/422,239] was granted by the patent office on 1975-11-04 for well stimulation for solution mining.
This patent grant is currently assigned to Kennecott Copper Corporation. Invention is credited to Donald H. Davidson, Ray V. Huff.
United States Patent |
3,917,345 |
Davidson , et al. |
November 4, 1975 |
Well stimulation for solution mining
Abstract
Methods for increasing the permeability of a subterranean
igneous rock formation penetrated by at least one well where a
hydraulic fluid is injected into the formation at a pressure
sufficient to cause diffusion into the natural fractures thereof
and thus increase the cross-sectional area thereof, maintaining the
pressure and injecting a second fluid which results, by the use
thereof, in substantially maintaining the increased cross-sectional
area after the pressure is reduced.
Inventors: |
Davidson; Donald H. (Bedford,
MA), Huff; Ray V. (Acton, MA) |
Assignee: |
Kennecott Copper Corporation
(New York, NY)
|
Family
ID: |
23673983 |
Appl.
No.: |
05/422,239 |
Filed: |
December 6, 1973 |
Current U.S.
Class: |
299/5; 166/280.2;
166/307 |
Current CPC
Class: |
E21B
43/283 (20130101); C09K 8/80 (20130101) |
Current International
Class: |
E21B
43/00 (20060101); E21B 43/28 (20060101); C09K
8/80 (20060101); C09K 8/60 (20060101); E21C
041/10 () |
Field of
Search: |
;299/2,4,5 ;423/38
;166/308,305,307,271,274,280 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
High-Concentration Hydrochloric Acid Aids Stimulation Results in
Carbonate Formations, Jour. Pet. Tech., Oct. 1966, p. 1294
chart..
|
Primary Examiner: Purser; Ernest R.
Assistant Examiner: Pate, III; William F.
Attorney, Agent or Firm: McCarter; Lowell H. Sniado; John
L.
Claims
What is claimed is:
1. In a hydrometallurgical operation for the in-situ underground
mining of a low grade copper-containing igneous rock formation by
the use of a copper leaching solution for extracting copper values
from said formation wherein at least one well extends from the
surface of the earth into the formation, the improvement which
comprises the steps of, prior to contacting said copper-containing
formation with said leaching solution, injecting, at a pressure
below the pressure producing new fractures in the formation, a
hydraulic fluid through said well into the formation at a pressure
sufficient whereby said fluid diffuses into the natural fractures
of the formation surrounding said well and causes said natural
fractures to increase in at least the cross-sectional area thereof
and while maintaining said pressure, injecting a second fluid into
said natural fractures through said well whereby the use of said
second fluid substantially results in maintaining the increased
cross-sectional area of said natural fractures after the pressure
is reduced.
2. The method as set forth in claim 1 wherein the hydraulic fluid
is water which is injected into said well of from 1,000 feet to
10,000 feet deep at a pressure of from about 1,000 psig to about
10,000 psig, wherein said pressure is less than about 1.0 psig per
foot of well depth, and below the pressure producing new fractures
in the formation.
3. The method as set forth in claim 1 wherein the formation
contains chalcopyrite and pyrite.
4. The method as set forth in claim 1 wherein said second fluid is
an acidic solution which is capable of reacting with said igneous
rock to increase at least the cross-sectional area of said natural
fractures.
5. The method as set forth in claim 4 wherein said acidic solution
comprises a mixture of HF and HCl.
6. The method as set forth in claim 4 wherein said acidic solution
comprises a mixture of H.sub.2 SO.sub.4 and HCl.
7. The process of claim 1 wherein said second fluid is selected
from the group consisting of propping agents, acidic solutions,
alkaline solutions, oxidizing agents and mixtures thereof.
8. The process of claim 1 wherein said second fluid is ammoniacal.
Description
This invention relates to the treatment of underground
depositbearing formations. More particularly, it relates to a
method for increasing the permeability of such underground
formations to enhance or stimulate the recovery of the desired
deposits therefrom.
The prior art considered in conjunction with the preparation of
this specification is as follows: U.S. Pat. Nos 2,944,803;
3,285,342; 3,387,888; 3,533,471; 3,561,532; 3,565,173; 3,587,744;
3,593,793; and 3,654,990. All of these publications are to be
considered as incorporated herein by reference.
Presently many mineral mines, located throughout the Western United
States, are not in a position to be commercially developed because
of their low ore concentration and/or because they are located at
remote regions which makes them economically unfeasible for
conventional type exploitation. Even ore bodies possessing
relatively high ore concentration and located in accessible regions
may have its valuable deposit at a depth that conventional mining
may not be mechanically or economically feasible.
In recent years a process for tapping these underground ore bodies,
in-situ, has evolved which has resulted in recovering some mineral
values which were formerly considered inaccessible by conventional
mining techniques. The process consists mainly of drilling a well
into an underground ore deposit and then introducing a leaching
solution to contact the ore therein. The leaching solution
dissolves the mineral within the ore deposit and thereafter the
pregnant solution is recovered and processed by conventional
extraction means to recover the particular mineral values
therefrom.
In conjunction with this in-situ solution mining, it is known in
the art that various ores, for example, chalcopyrite and most other
primary sulfide minerals, are not effectively dissolved by the
leaching solution or the rate of withdrawal solution from the ore
body is slow because of the low permeability of the formation and
thus such process is impractical.
It is also known in the art that the recovery of minerals and
fluids from underground formations of relatively low permeability
can be enhanced by fracturing the formation rock to create areas of
high permeability. One commonly employed technique for fracturing
such formations is hydrofracturing. In this technique, a fracturing
fluid is injected into the formation through a wellbore at a
pressure above the formation break-down pressure. The fracture
initiates at the wellbore and hopefully propagates outward into the
formation in a radial manner. While this technique is generally
useful, one serious problem is encountered. If, after the fracture
has reached the desired limits, the pressure is substantially
reduced, the fracture settles back into place preventing the
desired circulation of leaching solution and/or solvent. If, on the
other hand sufficient pressure is maintained to keep the fracture
open until sufficient material has been dissolved to provide a
definite flow channel, the fracture continues to extend beyond the
desired limits. Thus, the fracture may extend outside the treatment
zone to a porous formation which creates a problem of leakage from
the solution mining operation.
The aforementioned disadvantages inherent in the prior art
processes are now overcome by practicing the processes of the
present invention.
Accordingly, it is an object of the present invention to provide an
improved method for stimulating the recovery of materials from
underground deposits.
It is another object of the invention to provide an improved
process for increasing the permeability of igneous rock
formations.
It is another object of the invention to provide for increasing the
cross-sectional area of natural fractures in the formation around a
well-bore.
It is a further object of the invention to provide a process for
maintaining the increased cross-sectional area of natural fractures
once said fractures are initially opened.
It is a further object of the invention to provide a process for
increasing the amount of copper metal recovered from solution
mining of igneous rock formations containing chalcopyrite.
These and other objects of the present invention will be readily
apparent in conjunction with the description of the present
invention hereinafter set forth, including the appended claims.
The objects of the present invention are preferably accomplished by
a process in which prior to any hydrometallurgical operation being
conducted on the underground deposits, there is injected into the
formation, via the well, a hydraulic fluid at a pressure sufficient
to cause diffusion of said fluid into the natural fractures of the
formation but below pressure at which new fractures would be formed
in the formation and which consequently causes the fracture to
increase in at least the cross-sectional area thereof. While
maintaining said pressure, a second fluid is injected into said
natural fractures via said well. The second fluid is of such
character that it causes the increased cross-sectional area of the
natural fractures to be substantially maintained after the pressure
is reduced. In this manner it has unexpectedly been found that the
permeability of the formation can be substantially increased
without incurring any of the prior art disadvantages including high
pressures need for initiating new fractures which present
engineering and safety problems. Thus, the natural fractures of the
formation are treated to produce the desired end result and there
is no need for conducting a "fracturing" operation, i.e. to produce
new fractures in the formation, which requires substantial time,
money and effort. For example, the use of explosives implanted in
crevices, cracks, or fissures is common in mining and quarrying
operations. Such explosives have included both solid and
liquid-type explosives. The detonation of an explosive device or
materials in a wellbore to achieve explosive fracturing of the
surrounding formation, however, suffers from the same disadvantage
noted above with respect to hydrofracturing operation, in addition
to the difficulty of propagating the fracture at increasing
distances from the injection wellbore. Explosive fracturing by the
detonation of an explosive device in a wellbore also requires a
subsequent clean up operation before recovery of operations can be
begun at that wellsite, increasing both the time and expense
involved in such a treating action. Explosive fracturing also
presents numerous safety problems; it has been experienced in the
past that several people have been killed in conjunction with the
utilization of explosives for carrying out the desired end result;
i.e., fracturing underground formations.
The hydraulic fluid is any liquid which is capable of being pumped
at below fracturing pressures. It is to be understood that the term
"hydraulic fluid" is not intended to mean that it creates new
fractures, since the inventive concept herein is the use of such
fluid (and a second fluid hereinafter defined) to dilate natural
fractures without creating any substantial new fractures. However
fluids known in the art as "fracturing fluids" or "hydraulic
fracturing fluids" may be used without departing from the scope of
the invention, it being understood that the process claimed herein
operates below the fracturing pressure of the ore body. Although a
solvent for the metal values in the formation can be used,
preferably the initial opening will be made with a non-solvent.
Thus, suitable fluids, hydraulic or fracturing, include alcohols
and glycols, water and brine or any other organic or inorganic
liquid which meets the requirements of being pumpable at the
necessary pressure. In most cases, the preferred fluid will be
water or water saturated with the material of the formation being
treated. It is within the skill of the art to add thickening
agents, fluid loss additives, corrosion inhibitors, bactericides
and the like to the fluid when desired.
The second fluid which is used, in the present invention processes,
is either an aqueous slurry containing a propping agent or a
solution, acidic or alkaline, hereinafter referred to as reactive
solution, which is capable of reacting with the rock formation via
corrosion, dissolution and/or oxidation to increase at least the
cross-sectional area of the natural fractures, and in some cases
the length thereof by relatively short distances.
Suitable propping agents include sand, glass beads, resin beads
such as polyethylene or other polymeric beads, metallic pellets
such as aluminum pellets, walnut hulls or shells, ceramics, and the
like.
The propping agent can be added initially with the hydraulic or
fracturing fluid, but will preferably not be added until the
opening is increased.
It is desirable to control the particle size of the propping agent
since the purpose thereof is to provide effective propping and/or
to prevent sealing of the natural fractures. For example, a U.S.
Sieve Size of between about 12 and 40 mesh would be quite suitable.
This is not a critical feature and thus the mesh size should be
determined based upon the material used, solution flow rates
desired, economics and the like. Likewise, the quantity of the
propping agent in the aqueous slurry is ascertainable in a similar
fashion with the only restraint being that the slurry must be
pumpable. Sand, for example, can be used in a weight range of about
1 to 5 percent based on the total weight of said slurry.
The reactive solution, i.e. one of the above described "second
fluids", can be any acidic solution which has a pH of less than
6.5, preferably less than 2.0 and more preferably in the range of
from about 0.5 to about 1.5 and which is reactive with the rock
formation. The acids which can be used include, without limitation,
HF, HCl, H.sub.2 SO.sub.4, and mixtures thereof. "Mud acid", i.e.
an aqueous solution containing 3% by weight HF and 13% by weight
HCl, is a preferred "second fluid". The acidic solution may also
comprise or contain an oxidizing agent such as water soluble ferric
salts like ferric sulfate. For example, see U.S. Pat. No.
3,574,599. Another preferred second fluid is a mixture of H.sub.2
SO.sub.4 and Fe.sub.2 (SO.sub.4).sub.3, a one molar solution and
one-half molar solution respectively.
The reactive solution may also be alkaline such as an ammoniacal
solution. For example, see U.S. Pat. No. 3,278,232.
The pressure used to pump the hydraulic or fracturing fluid and/or
the second fluid into the natural fractures is any pressure which
does not cause any substantial new fracture formation and is
usually less than about 1.0 pounds per square inch gauge (psig) per
foot of well depth. Thus, the pressures can easily be within a
range of about 1,000 psig to about 10,000 psig with wells of 1,000
to 10,000 feet deep. It is to be understood then that the pressure
is a critical feature of the present invention in order to obtain
the desired end results.
In the practice of this invention for the stimulation of
underground formations to increase their fluid productivity and/or
permeability, a hydraulic or fracturing fluid is placed in a well
penetrating the formation to be treated, optionally through the
piping, adjacent and in contact with the face of the formation to
be treated. If desired or required, packing is employed to isolate
and confine the fracturing fluids to a portion of the well exposing
the formation to be treated. Pressure is then applied via the
hydraulic fluid so as to build up the pressure on the formation
exposed to the hydraulic fluid to a value great enough to cause
diffusion into the natural fractures.
The time required to pump the hydraulic fluid will depend upon
several variables including, without limitation, initial
permeability, size of formation being treated, type fluid used and
the like. When the pump pressure indicates a pressure just below
fracturing pressure of the formation for the depth of the well,
then the second fluid is injected for a sufficient period of time
for the propping agents to be positioned or for the reacting
solution to react with the sides of the fractures. Again, these
times will vary depending upon several variables and can range, for
example, between about 30 minutes to about 10 hours.
While the above process has been described with reference to the
use thereof as a preliminary treatment of a formation prior to
solution mining, it is within the scope of the present invention
that an operating well could be "shut down" and treated in this
fashion in order to increase the permeability thereof.
The processes of the present invention are uniquely effective in
conjunction with the in-situ mining of underground igneous rock
formations which contain copper metal values in the form of
chalcopyrite and pyrite ores. It has been found that the
stimulation of a low permeability deposit is an important factor
for an economically viable in-situ mining operation. In the
particular case relating to the underground (or solution) mining of
chalcopyrite and pyrite ores, this stimulation permits the leaching
solvent to contact more of the copper minerals, thus increasing
both the leach efficiency and copper loadings. Both of these
parameters are critical for economically mining deep-lying low
grade copper ores by in-situ mining techniques.
Subsequent to the above described stimulation process, the copper
leaching solution is injected in order to subsequently recover the
copper values. The copper leaching procedures can be carried out in
any manner known to those skilled in the art of in-situ mining such
as those procedures described in U.S. Pat. Nos. 3,278,232;
3,574,599; 3,640,579; and 3,708,206, all of which publications are
incorporated herein by reference.
It is to be understood that the stimulation process can be used at
any time where one so desires. Preferably the hydraulic fluid is
used as a pretreatment of the formation or deposit. However, it is
also within the scope of the invention that said fluid can be
employed where deposits have already been subjected to
hydrometallurgical operations.
It is a preferred embodiment of the present invention to utilize
the process for the solution mining of copper from subterranean
formations in a particular pattern design of injection and
production wells. It is preferred that the injection and production
wells either be drilled in concentric patterns about each other
with a single production well contained within the center of the
pattern, for example a five-spot, or that the injection and
production wells be drilled in offsetting line patterns so as to
form a line drive mechanism within the copper formation. Generally,
the distance between the injection and production wells will be
from 20 to 1,000 feet, with particular depth, thickness,
permeability, porosity, water saturation of the formation, and
economic value of the copper mineral contained therein being the
engineering constraints upon which the design of the solution
mining patterns are based. Therefore, through patterned well
completion in the copper formation, the process may be used
sequentially across the copper deposit through a series of line
drive wells or concentric pattern wells so that the entire copper
deposit may be leached.
EXAMPLE I
An ore body 100 acres in area and averaging 500 feet in thickness
lies at an average depth of 4,000 feet below the surface of the
earth in Arizona. Samples of the ore shows that it is composed
primarily of granitic igneous rock and that it contains
chalcopyrite as the principal copper mineral. The ore samples also
show that it contains approximately 1.4 weight percent chalcopyrite
and that the total copper content of the ore averages 0.5 percent.
The volume of ore in the deposit is, therefore, 10.sup.4 acre-feet
or 4.356 .times. 10.sup.8 cubic feet. The specific gravity of the
granitic ore is 2.6. Therefore, the total weight of the ore in the
deposit is 3.54 .times. 10.sup.7 tons, and the copper content of
the ore body is 3.54 .times. 10.sup.8 pounds.
Approximately 5 wells (each 4500 feet deep) are drilled into the
ore body in an array such as to provide a five-spot pattern, and
the wells are completed (sealed and cased to 4,000 feet) such that
fluids may be either injected or produced from individual wells. By
measurements on core samples and by injection and production tests
on individual wells, it is determined that the void volume within
the randomly-oriented fracture system is equivalent to 2 percent of
the bulk ore volume, that the fracture spacing averages 6 inches,
and that the permeability of the ore body to liquid averages about
25 millidarcys.
Petrographic examination of core samples taken from the ore body
shows that about 2 percent of the rock surface area exposed by the
fractures is covered by the chalcopyrite mineral and that the rock
matrix bounded by the fracture system is substantially cubical in
configuration.
Thus, the surface-to-volume ratio of the ore blocks bounded by the
fractures is approximately equal to that for cubically shaped
blocks and the surface area to volume ratio for the ore blocks is
equal to 6/L, where L is the length of the side of a cube. In this
case L=0.5 feet, and the surface area to volume ratio is equal to
12 square feet/cubic foot.
The total surface area of ore exposed by the fracture network is
equal to 12/4.356 .times. 10.sup.8 or 5.227 .times. 10.sup.9 square
feet. The surface area of the chalcopyrite mineral exposed by the
fracture system is equal to 2 percent of the total surface area, or
1.045 .times. 10.sup.8 square feet.
Laboratory tests within the ore samples showed that ferric sulfate
solutions will dissolve copper from the chalcopyrite of the ore
body at a rate equal to 0.0002 pound of copper per square foot of
chalcopyrite surface area per day. The initial maximum rate of
copper production attainable from the ore body by in-situ leaching
with ferric sulfate would be 0.002 .times. 1.045 .times. 10.sup.8 =
209,000 pounds of copper per day. The laboratory tests also showed
that, by allowing a 0.4 molar solution of ferric sulfate to react
completely with the chalcopyrite and other minerals in the ore, a
pregnant leaching solution containing 3.0 pounds of copper per
barrel (42 gallons) could be obtained. Therefore, in order to
supply 0.4 molar ferric sulfate solution to the ore body at the
optimum rate; i.e., at the rate sufficient to produce the maximum
amount of copper and at the same time allow total reaction of the
ferric iron, the 0.4 molar ferric sulfate solution must be injected
initially at a rate equal to 69,700 barrels/day. The required
average residence time for the solution within the ore body is
fixed by the injection rate and the void volume of the ore body;
##EQU1## The injection and withdrawal rates of the wells is thus
regulated to permit the ferric sulfate solution to remain in the
ore body for approximately 22 days.
Utilizing the above set of conditions, the wells are operated for a
sufficient period of time to reach equilibrium and the copper
produced averages about 187,000 pounds per day.
The injection of the leaching solution is then terminated and a
hydraulic fluid (water) is pumped into the natural fractures for a
period until the pressure reaches about 3,150 psi. While
maintaining this pressure, an aqueous slurry containing two (2)
percent by weight sand is injected into each well for approximately
4 hours at the same pressure 3,150 psi. After this 4-hour period,
the wells remain inoperative for 30 minutes and then the leaching
treatment is initiated under the same conditions specified
heretofore. After equilibrium has been established, it is
determined that copper is now being produced at an average rate of
235,000 pounds per day. Thus, the use of the present invention
stimulation process has resulted in a 25% increase in production
directly as a result of the enlargement of old fractures without
attempting to create new or artificial fractures within the ore
body.
EXAMPLE II
Example I above is repeated in toto with the exception that "mud
acid" is used in place of the aqueous slurry of sand in order to
enlarge the natural fractures. Substantially the same magnitude of
increase in copper production is obtained as that obtained using
said sand slurry.
While Examples I and II have been described as applicable to the
copper sulfide ores, it should be understood that the process is
also applicable to ores bearing native copper and also to ores of
copper oxides and silicates where the copper is present in the
cuprous valence state. When the copper is present in its elemental
or lower valence state, it is susceptible to oxidation by ferric
iron to form solutions of cupric sulfate.
It should also be understood that while it is preferred to conduct
the process in an ore body between an input and withdrawal well, a
single well process is also included within the scope of the
invention. In a single well process, the leaching solution will be
injected through a well, permitted to remain in contact with the
ore body for a period of time, and then withdrawn through the same
well. The pregnant leaching solution is then passed to a copper
recovery stage, a regeneration stage and ultimately reinjected.
While the processes have been described as particularly effective
in the in-situ mining of copper-bearing deposits, it is also within
the scope of the present invention to treat other types of
mineral-bearing deposits which contain, for example, nickel,
silver, gold, molybdenum, uranium and the like.
The present invention has been described herein with reference to
particular embodiments thereof. It will be appreciated by those
skilled in the art, however, that various changes and modifications
can be made therein without departing from the scope of the
invention as presented.
* * * * *