Method for in situ minefields

A circulation tubing string is disposed about the lixiviant return tubing string in the production hole of an in situ minefield. The circulation string extends from the surface to a point near a packer which defines the top of the leaching interval. A fluid coupler provides a fluid flow path between the annular cross-section regions between the production hole casing and the circulation string, and between the circulation string and the lixiviant return string. A flow of cooling fluid is maintained from the surface through the circuital path defined by the annular cross-section regions and the fluid coupler, particularly including the exterior surface of the production tubing string. The fluid is maintained at a temperature and flow rate so that the pregnant leach liquor flows in the production tubing string between the leaching interval and the surface is characterized by a predetermined temperature drop.

BACKGROUND OF THE INVENTION 
The present invention relates to in situ mining of metal values, and more 
particularly, to downhole heat exchangers for cooling the pregnant 
leaching liquor in the production holes of an in situ minefield. 
The subject matter of the present invention is related to the subject 
matter of the Patent Application Ser. No. 724,548, filed on Sept. 20, 
1976, and entitled "In Situ Mining Method and Apparatus". That application 
is incorporated by reference in the present application. 
As noted in the incorporated reference, much contemporary effort is 
directed to the development of processes in hardware permitting the 
efficient and economic extraction of metal values from the low grade 
porphyry ores residing in relatively large, deep-lying deposits, whereby 
the metal value extraction may be accomplished with minimal environmental 
impact. 
Generally, in situ mining processes require at least two bore holes drilled 
to the lowermost level of the desired leaching interval in the ore 
deposit. A packer and lixiviant injector is then affixed to the interior 
of a first, or injection, hole at the top of the desired leaching 
interval. Leach liquor is pumped down the injection hole and into the 
leaching interval to establish a relatively high pressure reservoir of 
leach liquor in the portion of the injection hole in the leaching 
interval. A relatively low pressure is established in one or more nearby 
production holes at portions of those holes lying within the leaching 
interval. Lixiviant from the injection hole passes through fissures in the 
ore along a pressure gradient between the injection hole and the 
production holes. As the lixiviant passes through the ore, metal values 
are leached. The pregnant leach liquor is pumped to the surface by way of 
the production holes and processed to recover the leached metal values. 
The effective heat transfer coefficient in production holes is severely 
limited by the surrounding rock formation and the cement used to case the 
hole (for example, for a 91/2 inch diameter production hole, the effective 
heat transfer coefficient for the rock formation may be of the order of 
1.5 to 0.7 Btu/hr.degree.Fft.sup.2, with the cement casing and other 
factors reducing this to provide an overall heat transfer coefficient in a 
range of 0.6 to 0.4 Btu/hr.degree.Fft.sup.2). At these levels, and with 
the typical production hole flow rates on the order of 120 gpm, the heat 
exchange between the pregnant leach liquor and the rock formation, as the 
liquor travels to the surface, is very small and the leach liquor 
effectively arrives at the surface substantially at the average geothermal 
temperature of the leaching interval. 
As exemplified by the incorporated reference, in-situ mining techniques 
require considerable chemical processing at surface plants to extract the 
leached metal values from the pregnant liquor returned by way of the 
production holes. For example, in the ammoniated lixiviant process 
disclosed by the incorporated reference, the surface processing requires 
an ion exchange plant, operating at a near-atmospheric pressure 
environment for the leach liquor. However, for deep well insitu minefields 
(on the order of 3,000 feet), the ambient geothermal temperature in the 
leaching interval is typically on the order of 100.degree. C. Accordingly, 
in deep well in-situ minefields, the pregnant leach liquor removed from 
the production holes is also at a temperature of the order of 100.degree. 
C. At these temperatures and in the relatively near-atmospheric pressure 
environment of the surface metal value extraction plant, typically-used 
lixiviants boil off. Consequently, the systems of the prior art require 
that lixiviant be constantly replaced in the mine system at the injection 
holes, or, alternatively, a surface cooling plant is needed for reducing 
the pregnant leach liquor temperature to the temperatures of the order of 
40.degree. C. prior to the metal value extraction processing. The cost of 
either alternative detracts substantially from the many favorable economic 
factors associated with in situ mining. 
Furthermore, it is well known that stainless steel tubing strings are well 
suited, particularly in terms of convenience and ease of use, for 
providing the production hole conduit for removing the pregnant leach 
liquor from the leaching interval. However, such tubing strings are not 
generally used due to their relatively high cost, particularly in deep 
mine environments. Tubing strings made of less expensive material such as 
fiberglass reinforced plastic (FRP) are typically used. While such tubing 
strings are relatively inconvenient and difficult to handle, the 
considerable saving offsets the difficulty factor in terms of economic 
operating conditions for in situ mines using conventional technology. 
It is an object of the present invention to provide sufficiently effective 
downhole heat exchangers for the production holes of in situ minefields to 
eliminate, or substantially reduce, the requirement for surface heat 
exchangers prior to ion exchange processing for metal value extraction. 
It is a further object of the present invention to provide downhole heat 
exchangers having relatively high thermal conductivity, metallic tubing 
strings for the production holes of in-situ minefields wherein the heat 
exchangers offset the economic disadvantage normally involved in using 
such tubing strings in the production holes. 
SUMMARY OF THE INVENTION 
According to the present invention, each injection hole in an in-situ 
minefield is accompanied by at least one nearby production hole extending 
to and including the leaching interval. Each production hole includes a 
peripherally disposed casing and a central lixiviant-return, or 
production, tubing string, each extending to the leaching interval. A 
packer is disposed at the uppermost portion of the leaching interval and 
isolates the region within the production hole in the leaching interval 
from the region within the casing and exterior to the production string, 
while coupling the former region to the region interior to the production 
string. 
In addition, a circulation tubing string, having a diameter greater than 
the lixiviant-return string and less than the production hole casing, is 
maintained in an arrangement substantially concentric with the production 
tubing string. Near the leaching interval, a fluid coupler is provided to 
establish a fluid flow path between the outer and inner annular 
cross-section regions, respectively between the hole casing and the 
circulation string, and between the circulation string and the production 
string. A cooling liquid is pumped from the surface to the coupler by way 
of the inner annular cross-section region and then through the coupler and 
back to the surface by way of the outer annular cross-section region. By 
controlling the temperature and flow rate of the cooling fluid in relation 
to the temperature and flow rate of the production hole lixiviant, and 
also in relation to the surface area and heat transfer coefficient of the 
circulation tubing string, heat is transferred from the lixiviant across 
the production tubing string to the cooling fluid so that the temperature 
of the pregnant leach liquor emerging from the production hole may be 
maintained at a predetermined value. With suitable control, this value may 
be selected so that the surface cooling plant for the pregnant leach 
liquor, which has been typically required for other in situ mining 
systems, is not required. Accordingly, the pregnant leach liquor may be 
pumped directly from the production holes to the metal extraction plant 
for processing. 
The elimination of or reduction in cooling capacity of the requirement for 
the surface leach liquor cooling plant substantially reduces the minefield 
cost to an extent which may permit the use of stainless steel tubing for 
the production hole lixiviant-return tubing string, thereby substantially 
easing the level of difficulty in establishing and maintaining an in situ 
minefield. For example, the use of stainless steel, as opposed to FRP, in 
some embodiments, permits a substantial improvement in heat transfer 
between the production string lixiviant and cooling fluid since typically 
used FRP tubing is characterized by a heat transfer coefficient of 10 
Btu/hr.degree.Fft.sup.2 while stainless steel tubing is characterized by a 
heat transfer coefficient of 100 Btu/hr.degree.Fft.sup.2.

In the drawing, a production hole 10 is shown extending from the surface 12 
to the lowermost limit of a leaching interval indicated by the arrow 14. 
As in the incorporated reference, the production hole 10 is lined with a 
casing 20 extending from the surface to the uppermost limit of the 
leaching interval 14. The casing 20 is cemented to the surrounding rock 
formation as indicated by cement 22. A conventional packer assembly 24 is 
shown to define the uppermost limit of the leaching interval 14. A 
production string 26 extends through the production hole 10 so that a 
stinger 28 at its lowermost end is seated in a seating nipple 30 and 
coupling assembly 32 affixed within hole 10 by the packer 24. A pump 36 is 
connected to the other end of the coupling assembly 32 within the leaching 
interval. The pump 36 is adapted to return pregnant leach liquor which 
enters the portion of hole 10 within the leach interval 14, by way of the 
production string and a valve 38 at the surface to a metal extraction 
plant (not shown in the Figure). It will be understood that the packer 24 
also provides passage for an electrical power cable 40 extending from a 
surface power source to the pump 36. With this configuration, a packer 
effectively isolates the region exterior to production string 26 from the 
region of production hole 10 within the leaching interval 14. As thus far 
described, the production hole configuration is considered to be within 
the teaching set forth by the incorporated reference, and the devices and 
assemblies described in that reference may be utilized for the 
corresponding devices and assemblies in the present embodiment. 
Also illustrated in the drawing is a circulation tubing string 42 which is 
substantially concentrically disposed about the production string 26 and 
extends from the surface to the packer 24, thereby defining a first 
(inner) annular cross-section region between the circulation string 42 and 
the production string 26 and a second (outer) annular cross-section region 
between the casing 20 and circulation string 42. A fluid coupler provides 
a fluid flow path between the first and second annular regions. 
A circulation fluid cooling plant (such as a conventional cooling tower) 
and pump is illustrated by block 60 in the drawing and includes a means to 
inject a circulation fluid into the innermost annular region and extract 
circulation fluid from the outermost annular region by way of conventional 
valve assemblies. 
As illustrated in the drawing, the fluid coupler includes the lowermost 
portion of circulation string 42 having circulation ports 52 and 54, and a 
sleeve member 56 which is adapted for selectively controlled motion along 
circulation string 42 between its illustrated position and the position 
illustrated by the broken line designated by reference numeral 48. With 
the sleeve member 56 in its lowermost position, the ports 52 and 54 are 
full open, thereby permitting maximum fluid coupling between the inner and 
outer annular cross-section regions adjacent to ports 52 and 54. With the 
sleeve member in its uppermost position as indicated by reference numeral 
48, the ports 52 and 54 are full closed, providing substantially no fluid 
coupling through the ports. 
By way of example, the fluid coupler may comprise a conventional 
hydraulically-operated pump-down sleeve device. With such a configuration, 
the production hole configuration may be easily installed with the 
circulation string, pump-down sleeve device, packer and pump being 
initially inserted in the hole as an integral assembly, followed by the 
insertion of the production tube which is stabbed into the seating nipple 
of the packer/pump assembly. Hydraulic pressure in the circulation string 
may then be used both to set the packer and to shear pins in the sleeve 
device so that the sleeve member 56 slides into its illustrated (full 
open) position and the circulation ports 52 and 54 are fully open. 
Alternatively, a conventional knock-down sleeve device may also be 
utilized in place of the pump-down device with this device having the 
further capability of being reset from the surface to block the 
circulation parts, thereby interrupting circulation between the annular 
cross-section regions. Of course the coupler, as described above to 
include the lowermost portion of circulation string 42 and a sleeve 
member, is merely exemplary, and alternative means of providing a fluid 
flow path between the inner and outer annular regions may readily be used 
in keeping with the present invention. For example, the port 52 alone 
provides such a path. 
In operation, as the pump 36 drives pregnant leach liquor by way of the 
production string 26 and the valve 28 to the metal extraction plant, the 
circulation fluid cooling plant and pump 60 drive a cooling fluid by way 
of the inner annular region, the fluid coupler and the outer annular 
region in a circuital path (illustrated by the flow arrows in the annular 
regions in the Figure). The cooling plant and pump 60 maintain the 
temperature and flow rate of the cooling fluid at appropriate values so 
that the heat exchange between the pregnant leach liquor and cooling fluid 
across the production string 26 is sufficient to decrease the temperature 
of the pregnant leach liquor by a predetermined value between the leaching 
interval 14 and the surface 12. 
By way of example, the above configuration may be implemented by drilling a 
105/8 inch production hole to the casing setting depth, i.e., to the 
uppermost portion of the desired leaching interval. A 105/8 inch or 
smaller hole may then be drilled to the lowermost depth of the leaching 
interval to establish the collection region for the pregnant leach liquor 
prior to pumping up to the surface. Following the setting of an 85/8 inch 
casing, a corrosion-resistant REDA pump, duo packer, fluid coupling 
device, and seating nipple and power cable is then run into the production 
hole at the end of a 41/2 inch carbon steel tubing (i.e., the circulation 
string 42). The packer is then set either hydraulically or mechanically, 
and the fluid coupling device set to its operating condition, i.e., with 
the fluid flow ports fully open. At that time, a 27/8 inch stainless steel 
production tubing string having a stinger at the end may be stabbed into 
the seating nipple associated with the pump affixed to the packer. With 
the stainless steel production tubing string, a suitable cooling fluid is 
water mixed with a conventional corrosion inhibitor. 
With the above configuration, the cumulative production tubing string 
surface area for a 25 hole array, 3,000-foot minefield is on the order of 
50,000 square feet. At production hole flow rates of 120 gpm, and with 
30.degree. C. inhibited water cooling fluid at a 720 gpm flow rate, the 
heat exchanger configuration reduces the temperature of the pregnant leach 
liquor from 100.degree. C. at the leaching interval to 40.degree. C. at 
the surface. 
Of course, alternative tubing diameters, flow rates, temperatures, and well 
depth parameters may readily be used in keeping with the present 
invention. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.