Thermal extraction system and method

The Thermal Extraction System and Method utilizes a geothermal heat source to provide thermal energy at the surface by drilling a bore hole into the geothermal heat source and equipping the bore hole with a casing and multiple tubing strings. A cavern is formed in the geothermal heat source, such as a brine cavern. One of the tubing strings is a production tubing string used to produce hot brine at the surface and has a low thermal conductivity fluid surrounding the production tubing string which may be adjusted to maintain an interface with brine from the cavern at varying levels around the production tubing string and thereby provides temperature regulation of the brine. This invention contemplates the production of brine at the surface which may be used in heat transfer applications or processed as feed stock for salt production with unsaturated brine returned to the brine cavern where it becomes saturated. The system and method may be used with an immiscible heat exchange fluid provided to absorb heat in the cavern from the mineral deposit and circulated to the surface through a heat exchanger and returned to the cavern.

BACKGROUND OF THE INVENTION 
This invention relates to an improved thermal extraction system and method 
for underground heat sources and in particularly to thermal extraction 
from solid masses of crystal or rock salt or similar minerals. 
It is known that highly heat conductive minerals such as rock salt and 
quartzite and the like sometimes occur in geological forms, such are known 
as spines or spires or domes or veins, and comprise relatively solid 
masses extended upward towards the earth's surface. These rock salt or 
mineral domes are within reach of modern drilling equipment. 
The occurrence of this salt dome phenomena is fully described in U.S. Pat. 
Nos. 3,676,078 (Jacoby I) and 3,864,917 (Jacoby II) of Charles H. Jacoby 
and assigned to International Salt Company. In the background discussion, 
Jacoby I states that the expense and difficulties in attempting bore hole 
operations through ordinarily encountered geological structures to reach 
such depths as would encounter rock formations competent to support a 
constant and sufficiently high temperature heat extraction operation have 
proved prohibitive, and cites examples of such prior proposals in U.S. 
Pat. Nos. 2,461,449; 3,140,986; 3,274,769; and 3,363,664. 
Jacoby I describes a method of solution mining and recrystalization 
recovery of solid sodium chloride from underground formations which 
utilize the heat conductivity of underground salt spires or domes or the 
like. Jacoby I penetrates a salt dome with a well and by utilizing water 
dissolves sufficient salt in the salt dome to form a cavern from which the 
brine is withdrawn. A second well is drilled for the purpose of solution 
mining salt. The first well is used as a heat source to heat a heat 
exchange fluid, such as any inert (to salt) gas or liquid. The heat 
exchange fluid is circulated from the heat reservoir through the 
evaporator and back to the heat reservoir which provide a source of heat 
for the evaporator. Brine from the solution mining well is pumped into the 
evaporator where the brine solution is vaporized in the evaporator with 
the salt being precipitated. The water vapor is condensed and returned to 
the salt cavern for further extraction of salt. 
Jacoby I has several drawbacks, first at the depths where the relatively 
high temperatures exist, the rock salt will behave plastically. Unless a 
counter pressure is maintained in the cavern, the normal pressure, due to 
the weight of the salt and its overburden, is sufficient to cause the 
plastic salt to flow and thereby close the cavern (see U.S. Pat. No. 
4,052,857, discussed later herein). Jacoby I also, requires mining, for 
each salt dome cavern in operation as a heat reservoir, of relatively 
large amounts of salt. 
Jacoby II discloses a geothermal energy system, wherein a rock salt deposit 
or dome is penetrated by a bore hole to a suitable depth and the deposit 
is solution mined of the salt to provide a cavern of prescribed shape and 
dimension in the salt mass as a heat reservoir for the system. The heat 
reservoir is emptied of the salt solution and flushed with heat exchange 
fluid. 
The heat reservoir of Jacoby II must be maintained as a highly controlled 
volume heat reservoir and the flow rate of the heat exchange fluid must be 
closely monitored to maintain a desired temperature. A pump is utilized to 
maintain the flow of the heat exchange fluid through the heat reservoir 
and the flow rate and/or temperatures are monitored to maintain the 
desired temperature. 
Further, Jacoby II discloses a self moving heat exchange system with the 
hot heat exchange fluid withdrawn near the top of the cavern and the 
relatively cold fluid returned near the bottom of the cavern. Because the 
return fluid is cold having its heat extracted in a heat exchanger it is 
heavier than the relatively hotter heat exchange fluid. The cold heat 
exchange fluid being returned near the bottom of the cavern will cause the 
hotter fluid near the top of the cavern to rise and flow up the tubing and 
through the surface heat exchange device where its heat is extracted and 
then is returned as relatively cold fluid to the bottom of the cavern. 
Jacoby II restricts the heat exchange fluid as inert to the heat 
reservoir, since it must maintain the precise volume to control the 
temperature of the heat exchange fluid. The same plasticity problem 
encountered by Jacoby I is also present in Jacoby II. 
Another Patent, U.S. Pat. No. 3,348,883, of Charles H. Jacoby, assigned to 
International Salt Company (Jacoby III), describes a method of mining and 
beneficiation of salt from a salt dome in which he discusses conveying the 
brine from a brine cavern in a salt dome in heat-insulated form upwardly 
to the point of discharge of the brine into an evaporator. Jacoby III 
suggests a suitable arrangement for insulating the brine upflow, is by 
covering the production tubing with heat insulating material or enclosing 
the production tubing within a larger conduit and suggest filling this 
conduit with dead air or vacuum. Of course, the conduit must be sealed to 
the brine production tubing to avoid contact with the brine in the brine 
cavern. Such arrangements are subject to various casualties such as 
leakage of seals for the dead air or vacuum, as well as, saturation of the 
insulating material with brine. 
In another context a heat pump using a deep well for a heat source is 
disclosed in U.S. Pat. No. 2,461,449 of Marvin Smith, assignor to Muncie 
Gear Works, Inc. Smith describes the use of a conduit around a production 
tubing from the deep well to provide a surrounding airspace throughout its 
length. This casing is closed at each end and is secured to the production 
conduit by welding or another suitable manner. This arrangement suffers 
the same casualties as Jacoby III. 
U.S. Pat. No. 4,052,857, assigned to Dow Chemical Company (Dow) describes a 
geothermal energy extraction process utilizing a tubing or pipe closed at 
one end and preferably pointed, weighted with removable weights, and sunk 
into the salt dome at a depth where the salt exhibits plasticity. Once the 
first pipe is installed and the weights removed a second open-ended, 
insulated pipe is inserted into the first pipe to provide a double heat 
exchanger. In this operation thermal energy is extracted from the salt by 
passing a heat exchange fluid either down the second pipe and up through 
the annulus between the first and second pipe or vice versa. Dow 
encounters limitation in sinking the closed tubing in the plastic salt 
because to overcome the salt density Dow uses uranium dioxide to obtain 
sufficient weight to overcome this high density. Dow process is further 
limited due to overburden between the top of the salt dome and the surface 
of the earth which dissipate heat from the well. Special note is made in 
the Dow Patent that the thermal conductivity of the overburden to that of 
the rock salt is inversely related to the insulating properties of the 
overburden, and suggests that an anhydrite caprock above a salt dome 
possesses the best insulating property. 
U.S. Pat. No. 4,512,156 of Nagase discloses an apparatus and method for 
using terrestrial heat to increase the temperature of a liquid which 
comprises a pipe buried in the earth in the region of high subterranean 
heat with a second pipe telescoped therein. The first pipe has a digging 
head on it's lower end and the second pipe is open at its lower end to 
communicate with the first pipe. The pipes are insulated with thermal 
insulation material on the outer surface of the inner and outer pipes. 
Water or other heat exchange fluid is pumped down the annulus between the 
pipes and withdrawn through the inner pipe for transfer to surface heat 
exchangers for use as a source of heat energy. 
U.S. Pat. No. 3,862,545 of Ellis discloses a process for using energy from 
a hot brine well to operate a steam turbine for electric power generation. 
Ellis utilizes brine from a geothermal well as a source for recovering 
thermal energy by vaporizing the hot brine and using the steam generated 
to operate a turbine. 
SUMMARY OF THE INVENTION 
The present invention contemplates heat extraction from a salt dome by 
drilling a bore hole into the top of a salt dome and forming a brine 
cavern by solution mining or the like. The bore hole is equipped with a 
surface casing and three telescoping tubing strings, an inner string, 
outer string and a central string. Non-heat conducting centralizers are 
provided between the tubing strings and the casing to prevent contact 
between the various tubing strings and casing. The annulus between the 
outer surface casing and the outer tubing string is open to the salt 
cavern; and the annulus between the inner tubing string and the central 
tubing is open to the salt cavern. These two annuli are pressurized by 
surface compressor or source of compressed air or inert gas which provide 
insulation for the outer and inner strings of tubing. Immiscible heat 
exchange fluid is pumped down the inner tubing string which extends down 
near the bottom of the salt cavern and as the heat exchange fluid 
progresses towards the top of the cavern it absorbs heat and then is 
produced through the annulus between the outer tubing string and the 
central tubing string. The compressed air or inert gas may be maintained 
at different pressures to permit heat exchange fluid to enter the annuli 
and interface the air or inert gas insulator at varying levels which 
exposes the heat exchange fluid to the overburden temperature such that 
the temperature of the heat exchange fluid can be controlled by varying 
the pressure of the air or inert gas. 
Neither Jacoby III, nor Smith disclose a system of regulating the 
temperature of the production stream of fluid, such as brine, by 
surrounding the brine production tubing with an insulating tubing forming 
an annulus with the production tubing in which the level of brine entering 
the annulus may be varied throughout it's length to increase or decrease 
the insulating jacket surrounding the production tubing which allows 
regulation of the temperature of the production stream of fluid. Thus, 
Jacoby III and Smith both lack the capability of regulating the 
temperature as disclosed by Applicant. 
Dow describes the use of insulated pipe in a sealed heat exchanger which is 
designed to insulate the primary fluid rising through the insulated inner 
pipe. The overburden surrounding the Dow heat exchanger would impact the 
heat recovery. On the other hand Applicant utilizes a low thermal 
conductivity fluid as insulation between the overburden surrounding the 
casing of the bore hole and the outer most string of tubing as well as the 
annulus between the inner tubing string and the central tubing string. 
Additionally centralizers can be used to maintain spacing between the 
tubing strings and the casing. 
Further, Applicant's invention avoids the problems associated with 
maintaining insulation exposure to brine or to the heat exchange fluid 
which may increase the conductivity of the insulation, and thus defeat its 
purpose. Moreover, Applicant's invention provides an adjustable 
arrangement where the temperature of the heat exchange fluid may be 
gradually increased by increasing the air or inert gas pressure in the 
annuli to lower the interface between the air and heat exchange fluid. 
Such temperature regulation is often required in start-up operations such 
as large steam turbines or pumps. 
In another arrangement of Applicant's invention a casing and two tubing 
strings telescoped inside the casing extend into the salt dome. The inner 
tubing string and the outer tubing string may have non-heat conducting 
centralizers to maintain separation therebetween. The outer tubing string 
which is open to the salt cavern is supplied with compressed air or inert 
gas, or other low thermal conductivity fluid, thus surrounding the inner 
tubing string with insulation. In this arrangement cool brine is pumped in 
the annulus between the casing and the outer tubing string and into the 
top of the salt cavern, while at the same time hot brine is pumped from 
the bottom of the salt cavern through the inner tubing string, thus 
providing a source of hot brine at the surface. The temperature of the 
cold and hot brine are adjusted by raising or lowering the interface 
between the brine and the compressed air or inert gas in the outer tubing 
string. 
Instead of using telescoping tubing strings, two separate wells could be 
drilled in the salt dome and similarly insulated using compressed air or 
gas with the cool brine being pumped down one tubing string and the hot 
brine being pumped up the other tubing string. Insulation of the cold 
brine would be optional. Likewise, the temperature of the hot brine could 
be controlled by the compressed air insulation technique. 
Alternatively, a bore hole could be drilled in the salt dome and equipped 
with a casing and two side by side tubing strings; one tubing string 
extending to the bottom of the casing and the other tubing string 
extending beyond the casing a substantial distance. A brine cavern would 
be formed by solution mining. In operation the casing is pressurized with 
a source of air or inert gas surrounding the side by side tubing strings 
and interfaced with the brine near the bottom of the casing. Hot brine is 
pumped to the surface through the tubing string at the top of the brine 
cavern for thermal recovery or salt production and the spent brine is 
reinjected through the tubing string extended near the bottom of the brine 
cavern. In this arrangement the side by side tubing strings are insulated 
from each other and the overburden. 
In any of the three next foregoing arrangements, hot brine is provided at 
the surface and the temperature can be regulated through the raising or 
lowering of the interface between the brine in the salt cavern and the air 
or inert gas insulation. Hot brine could be used in numerous applications 
since it is supplied to the surface at a temperature hotter than most 
brine wells because of the insulation provided by air or inert gas or 
other low thermal conductivity fluid. Thus, the hot brine could be used as 
the feed stock in a salt manufacturing process, such as by the use of 
evaporators and the condensate from the evaporators could be returned to 
the brine cavern to produce more brine. 
Also, hot brine at the surface could be used in a number of heat exchanger 
operations that are impervious to salt corrosion.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now particularly to FIG. 1. A salt dome generally referred to as 
10 is penetrated by bore hole 12. Bore hole 12 is equipped with casing 14, 
secured in bore hole 12 with cement 15, and tubing strings 16, 18, and 20. 
Each of the tubing strings 16, 18, 20, are equipped with non-heat 
conducting centralizers 39 to prevent the tubing strings and the casing 
from touching each other and thus lose insulation therebetween. Initially 
water may be injected through tubing string 16 to dissolve the salt in 
salt dome 10 and brine withdrawn through tubing string 18 to form brine 
cavern 19. Brine cavern 19 is enlarged to exceed the depth of tubing 
string 20. A sufficient quantity of brine is removed to permit circulation 
of an immiscible heat exchange fluid in sufficient quantity to absorb the 
heat from brine cavern 19 in salt dome 10. The produced brine may be 
disposed of or suitably used for other purposes. At the surface heat 
exchanger 25 is connected to tubing string 16 by pipe 26 and pipe 28 
connects the heat exchanger to pump 30. Pump 30 is connected by pipe 31 to 
tubing string 20. A source of compressed air or inert gas 35 is connected 
to tubing string 18 by pipe 36 and casing 14 by pipe 38. Cool heat 
exchange fluid is injected through tubing string 20 (depicted by a series 
of the letter C) at the bottom of brine cavern 19. As the immiscible heat 
exchange fluid migrates towards the top of brine cavern 19, it absorbs 
heat, and this hot heat exchange fluid (depicted by a series of the letter 
H) is supplied to heat exchanger 25 through pipe 26 connected to tubing 
string 16. The return line 28 from heat exchanger 25 is connected to pump 
30 and pump 30 is connected to tubing string 20 through line 31. The 
circulation of the cold heat exchange fluid through tubing string 20 to 
the bottom of brine cavern 19 moves the heat exchange fluid near the top 
of brine cavern 19, which has been heated to the temperature of brine 
cavern 19, through tubing string 16, back through pipe 26 to the input of 
heat exchanger 25. 
In order to maintain the heat exchange fluid at a desired temperature 
casing 14 and tubing string 18, which are open to the top of brine cavern 
19 are pressurized by source 35 of air or inert gas through pipes 36 and 
38 such that the interface between the heat exchange fluid flowing into 
casing 14 and tubing string 18 are maintained at or near the bottom of 
casing 14 and tubing string 18 to provide maximum insulation and the 
highest temperature. The temperature of the heat exchange fluid reaching 
the surface may be controlled by decreasing the pressure to allow the heat 
exchange fluid interface to be further up in casing 14 and tubing string 
18, thus the natural cooling effect of the overburden would reduce the 
temperature of the heat exchange fluid. Without the air or inert gas 
providing insulation the temperature of the heat exchange fluid reaching 
the surface would be low and the process would be uneconomical. It should 
be understood that brine alone could be used as the heat exchange fluid in 
applications where brine is suitable. 
The thermal extraction as depicted in FIG. 2 utilizes two bore holes 41 and 
42, which penetrate salt dome 45 in spaced relationship. Bore hole 41 is 
equipped with casing 47 secured therein by cement 44 and tubing string 48. 
Non-heat conducting centralizers 43 prevent tubing string 48 from touching 
casing 47. Casing 47 penetrates the upper region 49 of brine cavern 50 and 
is supplied with source 51 of compressed air or inert gas. Tubing string 
48 extends to lower region 58 of brine cavern 50. Bore hole 42 is equipped 
with casing 54 secured therein by cement 55 and tubing string 56. Non-heat 
conducting centralizers 53 prevent tubing string 56 from touching casing 
54. Casing 54 penetrates lower region 58 of brine cavern 50 and tubing 
string 56 extends to lower region 59 of brine cavern 50. Optionally casing 
54 is supplied with source 68 of compressed air or inert gas. Brine cavern 
50 in salt dome 45 may be formed in salt dome 45 by injecting fresh water 
through casing 47 in bore hole 41 or casing 54 in bore hole 42, thus 
solution mining the salt. After brine cavern 50 is formed, then tubing 
string 48 is connected by pipe 62 to heat exchanger 64. Heat exchanger 64 
is connected to pump 65 by pipe 66 and pump 65 is connected to tubing 
string 56 by pipe 67. 
In operation, the two well system can operate either with brine or an 
immiscible heat exchanger fluid. In the case of brine, source 51 of 
compressed air or inert gas would be injected through casing 47 to form an 
interface with brine near the bottom of casing 47 in upper region 49 of 
brine cavern 50 and would thus provide insulation for tubing string 48. 
Pump 65 produces brine from region 58 of brine cavern 50 through tubing 
string 48. Since tubing string 48 is insulated by source 51, hot brine is 
provided through pipe 62, through heat exchanger 64 where the heat would 
be extracted for surface uses and the spent brine flows through pipe 66, 
pump 65 and pipe 67 to tubing string 56 where it is pumped to bottom 
region 59 of brine cavern 50. Source 68 of compressed air or inert gas may 
optionally be provided to casing 54 to insulate tubing string 56 from the 
surface down to salt dome 45. This arrangement would prevent the brine in 
tubing string 56 from cooling down to the overburden temperature. 
Also, it should be understood that brine cavern 50 could be shaped to 
provide production of the hot brine from a higher level than the 
reinjection level of the cool brine, thus brine cavern 50 could be shaped 
similar to FIG. 2 of Jacoby II. Of course, the flow of cool brine through 
tubing string 56 in brine cavern 50 to tubing string 48 would provide 
sufficient dwell time for the brine to again reach the temperature of salt 
dome 45. 
It will be understood that the thermal extraction system in FIG. 2 could 
employ immiscible heat exchange fluid by making brine cavern 50 such that 
upper region 49 was substantially above upper region 57 and tubing string 
48 was raised to about the depth of casing 47. 
Referring now to FIG. 3, a salt dome 70 is penetrated by a bore hole 72. 
Bore hole 72 is equipped with casing 73 which is cemented into bore hole 
72 with cement 74. Tubing String 75 is set in casing 73 and tubing string 
77 is telescoped in tubing string 75. By solution mining or other known 
ways brine cavern 78 is formed in salt dome 70, such that brine cavern 78 
extends beyond the bottom of casing 73 to below the bottom of tubing 
string 77. Tubing string 75 is provided with source 79 of compressed air 
or inert gas. Tubing string 77 is connected to heat exchanger 80 by pipe 
81, heat exchanger 80 is connected to pump 82 by pipe 83, and pump 82 is 
connected to casing 73 by pipe 84. 
In operation, pump 82 pumps the brine from the bottom of brine cavern 78 
through tubing string 77, through heat exchanger 80 and returns the brine 
through casing 73 to the top of bring cavern 78. The annulus between 
tubing string 75 and tubing string 77 is pressurized with compressed air 
or inert gas from source 79 such that the brine interface is near the 
bottom of tubing string 75. The pressure from source 79 can be reduced 
such that brine from the top of brine cavern 78 would enter tubing string 
75 and thus have a cooling effect on the brine being pumped through tubing 
string 77 to heat exchanger 80. 
Referring now to FIG. 4, a salt dome 100 is penetrated by bore hole 102. 
Bore hole 102 is equipped with a casing 103 cemented into the bore hole 
102 by cement 104. Tubing String 105 is set in casing 103 and extends near 
the bottom of casing 103. A second tubing string 106 is set in casing 103 
and extends substantially beyond the bottom of casing 103. By solution 
mining or other means, brine cavern 110 is formed in salt dome 100 such 
that the brine cavern extends beyond the bottom of casing 103 to below the 
bottom of tubing string 106. Tubing string 105 and tubing string 106 are 
spaced apart from each other and casing 103 by non-heat conducting spacers 
112 which allow the passage of fluid therethrough. Casing 103 is provided 
with a source 114 of compressed air or inert gas. Tubing string 105 is 
connected to heat exchanger 115 by pipe 116. Heat exchanger 115 is 
connected to pump 117 by pipe 118, and pump 117 is connected to tubing 
string 106 by pipe 119. 
In operation, pump 117 pumps hot brine (indicated by a series of the letter 
H) from the top of brine cavern 110, through tubing string 105, through 
pipe 116, through heat exchanger 115, through pipe 118, and returns the 
cool brine (indicated by a series of the letter C) to the bottom of brine 
cavern 110 through tubing string 106. The casing 103 is pressurized with 
compressed air or inert gas from source 114 such that the brine interface 
is near the bottom of casing 103. The pressure from source 114 can be 
reduced such that the brine in the top of brine cavern 110 would rise in 
the casing 103, and thus, have the effect of exposing hot brine to the 
overburden surrounding the casing 103, and thus, have a cooling effect on 
the hot brine being pumped through the tubing string 105 to the heat 
exchanger 115. 
It should be understood and appreciated that the direction of the flow of 
the brine could be reversed with hot brine from the bottom of brine cavern 
110 being pumped through tubing string 106, through heat exchanger 115, 
and returned to the top of brine cavern 110 through tubing string 105 
which would permit even cooler brine from brine cavern 110 to interface 
with the compressed air or inert gas from source 114 in the casing 103. 
In a typical example, a salt dome with it's top at 3,300 feet below ground 
surface is penetrated with a bore hole and equipped with a casing and a 
pair of telescoping tubing strings. A cavern is formed in the salt dome in 
any suitable manner to contain brine. At the depth of the salt dome, brine 
in the cavern is at a temperature of 160.degree. F. The inner tubing 
string extends near the bottom of the brine in the cavern and the casing 
and outer tubing string extend into the brine near the top of the cavern. 
The casing and the pair of telescoping tubing strings are open to the 
brine in the cavern. The inner tubing string is used to produce the brine 
and is insulated by pressurizing the outer tubing string with air at 
approximately 1,400 p.s.i. which holds the brine/air interface in the 
outer tubing string near the top of the salt dome. Upon production, the 
brine, being 160.degree. F. in the cavern, reaches the surface at 
approximately the same temperature. By reducing the pressure to 750 p.s.i. 
the brine/air interface in the outer tubing string is approximately 1,650 
feet below the surface and the temperature of the brine reaching the 
surface is 120.degree. F. 
It will be understood that Applicant's invention is illustrated and 
disclosed with respect to the preferred embodiment being a salt dome with 
a brine or heat exchange cavern formed therein from which thermal 
extraction takes place by circulating from the brine cavern either brine 
or an immiscible heat exchange fluid which is at the temperature of the 
salt dome, passing the brine or heat transfer fluid through suitable 
surface equipment to extract heat, and reinjecting the brine or heat 
transfer fluid into the brine cavern. It will be understood that other 
mineral sources may be utilized such as a quartzite as discussed in Jacoby 
II. Furthermore, Applicant's invention is readily adaptable to mineral 
production, such as salt. 
It will be appreciated that the foregoing disclosure is of the preferred 
embodiments of the invention and many widely different embodiments of the 
invention may be made without departing from the scope of the invention 
disclosed herein. Therefore, the scope of the invention is only limited as 
indicated in the appended claims.