U.S. patent application number 13/246567 was filed with the patent office on 2012-01-26 for geothermal energy extraction system and method.
Invention is credited to George L. Danko.
Application Number | 20120018120 13/246567 |
Document ID | / |
Family ID | 45492602 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120018120 |
Kind Code |
A1 |
Danko; George L. |
January 26, 2012 |
GEOTHERMAL ENERGY EXTRACTION SYSTEM AND METHOD
Abstract
The present disclosure provides a system and process for
extracting geothermal energy from a geologic formation. According
to an embodiment of the method, coolant fluid is introduced into a
first location, at a first height in the formation. The geologic
coolant fluid is collected from a second location, at second height
in the geologic formation. The second height is typically lower
than the first height. Geologic coolant fluid introduced into the
first location migrates through the geologic formation to the
second location, extracting heat from the geologic formation.
Pressure is lowered at the second location relative to the
surrounding areas. In a particularly advantageous solution, the
pressure is lowered at the second location using a pump, which is
integrated into an energy conversion implement. The energy
conversion implement is installed at, or close, to the second
location. The pump delivers the cooled geologic coolant fluid to
the first location.
Inventors: |
Danko; George L.; (Reno,
NV) |
Family ID: |
45492602 |
Appl. No.: |
13/246567 |
Filed: |
September 27, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12780538 |
May 14, 2010 |
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13246567 |
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61178237 |
May 14, 2009 |
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61386742 |
Sep 27, 2010 |
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Current U.S.
Class: |
165/45 |
Current CPC
Class: |
Y02E 10/10 20130101;
Y02E 10/14 20130101; F24T 10/20 20180501 |
Class at
Publication: |
165/45 |
International
Class: |
F24J 3/08 20060101
F24J003/08 |
Claims
1. A geothermal energy extraction-method comprising: using a deep
well pump to create a decreased pressure zone; establishing a
converging flow circulation in a hot geologic formation for
convective heat exchange within the decreased pressure zone towards
the pump; circulating collected hot fluid through an
energy-conversion unit for electrical energy production; and
re-injecting cooled coolant fluid into the geologic formation at a
circulation injection point for zero net coolant fluid loss.
2. The method of claim 1, further comprising establishing coolant
fluid flow in a downward direction.
3. The method of claim 1, further comprising engineering the
pressure difference at any elevation between ambient (P.sub.A) and
low (P.sub.L) pressure zones using a pump to overcome an
upward-driving natural buoyancy pressure difference to create a net
downward percolation plume.
4. The method of claim 3, further comprising controlling the
pressure difference between P.sub.A and P.sub.L during performance
of the method in order to balance convective coolant fluid
re-circulation within the natural, ambient hydrologic system in
order to provide a desired degree of pressure-lowering.
5. The method of claim 3, further comprising actively controlling
the pressure difference between P.sub.A and P.sub.L according to a
water level in a suction collection part of a production
borehole.
6. The method of claim 1, wherein the circulation injection point
is selected close to an undisturbed water level in the geologic
formation.
7. The method of claim 1, wherein the pump comprises a submersible,
high temperature pump and the energy conversion unit comprises a
direct energy conversion unit and both the pump and the energy
conversion unit are installed in the production borehole in situ
and cooling power for the energy conversion unit is supplied from
the surface at low temperature.
8. The method of claim 1, wherein the energy conversion unit
comprises a binary-phase heat exchanger comprising a boiler heated
by the hot geothermal fluid, a turbine, a condenser comprising a
heat exchanger cooled by coolant fluid circulation from the
surface, an electrical generator driven by the turbine, and an
electrical transmission line that carries electrical energy from a
production borehole to the surface.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of, and incorporates by
reference, U.S. Provisional Patent Application Ser. No. 61/386,742,
filed Sep. 27, 2010. This application is also a continuation in
part of, and claims the benefit of, U.S. patent application Ser.
No. 12/780,538, filed May 14, 2010, which in turns claims the
benefit of U.S. Provisional Patent Application Ser. No. 61/178,237,
filed May 14, 2009. Each of these applications is incorporated by
reference herein in its entirety.
FIELD
[0002] The present disclosure relates, generally, to a method and a
system for increasing the thermodynamic efficiency of extracting
geothermal energy.
SUMMARY
[0003] One embodiment of the present disclosure provides a
geothermal energy extraction method. According to the method, in a
geologic formation, coolant fluid is introduced into a first
fracture system at a first height. Water is collected from a second
fracture system a second height, which is lower than the first
height. The first and second fracture systems are in fluid
communication. Coolant fluid introduced into the first fracture
system percolates through a geologic formation from the first
fracture system to the second fracture system, collecting heat from
the geologic formation. In some examples, the second fracture
system has a higher temperature than the first fracture system.
[0004] In some implementations, the method includes forming the
first or second fracture system. The method can also include a
third fracture system intermediate, and in fluid communication
with, the first and second fracture systems. In another
implementation, the fracture system may be natural as it often
occurs in fractured or sedimentary rock formations. According to a
particular example, the first, second, or third fracture system is
formed by creating a borehole, creating a pilot hole ahead of the
borehole, and fracturing the geologic formation proximate the pilot
hole.
[0005] In a further implementation, coolant fluid is removed from
the second fracture system. In one example, the coolant fluid is
lifted from the geologic formation. For example, the coolant fluid
can be removed to the surface for energy extraction. Energy can be
extracted at the surface using a heat exchanger. In another
example, energy is extracted in situ and the cooled coolant fluid
is circulated within the geologic formation, such as entering
through the first fracture system. When energy is extracted in
situ, a heat exchanger may be located in a production borehole. Yet
in another implementation, the geologic thermal energy is directly
converted into electrical energy in situ, that is, the hot
geothermal reservoir fluid is circulated in an energy conversions
implement, whereas coolant fluid is delivered underground to
facilitate the energy conversion. The energy conversion implement
may include a heat exchanger as a boiler, a binary liquid steam
Ranke cycle, a turbine, an electrical generator, and a condenser;
which is cooled by the coolant fluid circulation from the
surface
[0006] Energy may be extracted under steady or variable geologic
coolant fluid flow. Pressure control means, such as a pump, can be
used to lower the pressure of the second fracture system below that
of the surrounding area. This can discourage loss of coolant fluid
to the surrounding geologic area by reducing or eliminating the
driving pressure difference.
[0007] Coolant fluid can be removed from second fracture system
using any suitable techniques. One technique is pumping. Another
suitable technique is air lifting. When pumping is used, in one
example compressed air is injected to the high pressure side of the
pump, contributing to air lifting. In another example when pumping
is used, compressed air is introduced into the body of the pump.
The compressed air expands in the pump, contributing to pump
cooling. In another example, pumping is used to lower the pressure
of the second fracture system, which can increase the flow of
coolant fluid into the second fracture system from the first
fracture system. In yet another example, a pump is located at a
higher level than the bottom of the second fracture system. This
can aid in opening fractures within the first or second fracture
systems.
[0008] In another implementation, coolant fluid is provided to the
first fracture system. In one example, the coolant fluid is
supplied by an injection borehole. In another example, the coolant
fluid is supplied from a natural geologic feature, such as a
fracture or fault to/in a hydrothermal reservoir.
[0009] According to another embodiment, which may be combined with
the above embodiment, fluid within first and/or second fracture
systems is periodically evaporated in order to increase the active
surface area of the first or second fracture systems. In one
example, evaporation is achieved by lowering the pressure of the
first or second fracture systems below the saturated or dissolution
pressure limit of the coolant fluid. When the pressure is suitably
lowered, a phase change is induced and liquid phase coolant is
purged from fractures by the gas phase in one cycle. The pressure
can then be raised, or allowed to rise, above the saturated or
dissolution limit to deliver liquid phase coolant to the fractures
in another cycle. Liquid phase and gas phase cycles are alternated,
in one embodiment, in order to increase heat extraction, such as by
contacting a greater surface area of the fracture system. Between
cycles, a vigorous fluid flow field and latent heat exchange, in
addition to sensible heat exchange, is created between the coolant
fluid and the fracture system (rock). The implementation can be
used to promote delivery of coolant fluid to stagnant, hard to
reach fracture areas.
[0010] In another embodiment, the present disclosure provides a
geothermal energy extraction system at least partially located in a
geologic formation. The system includes a first fracture system at
a first height and a second fracture system at a second height. The
second height is lower than the first height. The first and second
fracture systems are in fluid communication. The system includes a
fluid transport means, such as a pump or gaseous fluid source, such
as an, air source, for gaseous lifting, such as air lifting, of
coolant fluid. An energy recovery unit, such as a heat exchanger,
is also included in the system. A coolant fluid source is in fluid
communication with the first fracture system.
[0011] In operation, coolant fluid is provided to the first
fracture system from the coolant fluid source. The coolant fluid
travels through the first and second fracture systems. Coolant
fluid is transported from the second fracture system to the energy
recovery unit using the fluid transport means.
[0012] In some implementations, the system includes a gaseous fluid
source, such as a compressed air source. In one example, the
gaseous fluid source is coupled to the high pressure side of a
pump, wherein it can be used to aid gaseous lifting of the coolant
fluid from the second fracture system. In another example, the
gaseous fluid source is coupled to the body of a pump, where it can
be used to help cool the pump.
[0013] In another implementation, the system includes a third
fracture system intermediate and in fluid communication with the
first and second fracture systems.
[0014] In a particular example, the coolant fluid source includes
an injection borehole. In another example, the coolant fluid source
includes a natural geologic feature, such as a fracture or fault
in/to a hydrothermal reservoir.
[0015] In a further example, the heat exchanger is located in a
production well. In a yet further example, the pump is located
higher that the bottom of the second fracture system, where is can
aid in opening fractures in the first or second fracture
systems.
[0016] In a further example, the geologic coolant fluid circulates
through a heat exchanger in one side whereas the other side of the
heat exchanger is a boiler for a binary steam cycle. The binary
steam cycle powers a turbine which turns an electrical generator.
Another heat exchanger is included to condense the returning
steam.
[0017] In a further example, the entire power version implement is
installed in the hot reservoir area underground. In this case the
hot geologic fluid is not lifted to the surface, therefore, heat
and energy can be reduced. The temperature of the coolant fluid for
energy conversion, used in the heat exchanger of the condenser, is
closer to the temperature of the surrounding rock along the
circulation pathway to the surface, reducing parasite heat exchange
and energy loss relative to the known solutions in which the hot
geologic coolant fluid is circulated.
[0018] There are additional features and advantages of the subject
matter described herein. They will become apparent as this
specification proceeds.
[0019] In this regard, it is to be understood that this is a
summary of varying aspects of the subject matter described herein.
The various features described in this section and below for
various embodiments may be used in combination or separately. Any
particular embodiment need not provide all features noted above,
nor solve all problems or address all issues in the prior art noted
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic diagram of a method of producing a
multilayered fracture system according to an embodiment of the
present disclosure, and of the fracture system formed thereby.
[0021] FIG. 2 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure
having an injection borehole.
[0022] FIG. 3 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure
not having an injection borehole.
[0023] FIG. 4 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure
showing pressure fields within the system.
[0024] FIG. 5 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure
where the system/process use increased injection pressure showing
pressure fields within the system.
[0025] FIG. 6 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure
where the system/process lacks an injection borehole showing
pressure fields within the system.
[0026] FIG. 7 is a flowchart of a method of extracting geothermal
energy according to an embodiment of the present disclosure.
[0027] FIG. 8 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure in
a formation having sufficiently high permeability and employing a
pump to create a pressure differential.
[0028] FIG. 9 is a schematic diagram of an energy extraction system
and process according to an embodiment of the present disclosure in
a formation having sufficiently high permeability, employing a pump
to create a pressure differential, and having coolant delivery
boreholes and collection boreholes.
[0029] FIG. 10 is a schematic diagram of an energy extraction
system and process according to an embodiment of the present
disclosure in a formation having sufficiently high permeability and
employing a pump to create a pressure differential, the pump being
at a comparatively higher elevation.
[0030] FIG. 11(A) is a schematic diagram of an energy extraction
system and process according to an embodiment of the present
disclosure in a formation having sufficiently high permeability and
employing a pump to create a pressure differential in a confined
aquifer bounded by two impermeable layers. FIG. 11(B) illustrates
the pressure distribution of the system of FIG. 11(A).
[0031] FIG. 12(A) shows a cross sectional and 12(B) shows a plan
view a schematic diagram of an energy extraction system and process
according to an embodiment of the present disclosure in a formation
having sufficiently high permeability and employing a pump to
create a pressure differential in an unconfined aquifer with
multiple boreholes.
[0032] FIG. 13 is a schematic diagram of an energy extraction
system and process according to an embodiment of the present
disclosure in a confined permeable formation used for carbon
dioxide sequestration and having an injection borehole, a
production borehole, and using a pump for carbon dioxide
circulation.
[0033] FIG. 14 is a block diagram illustrating energy flow between
thermal energy from a geothermal reservoir and electrical energy
using a circulating conductive geologic coolant.
[0034] FIG. 15 is a block diagram illustrating energy flow between
thermal energy from a geothermal reservoir and electrical energy
according to an embodiment of the present disclosure, using a
circulating conductive geologic coolant that is transmitted to a
surface installation.
[0035] FIG. 16 is a block diagram of an energy extraction system
and process according to an embodiment of the present disclosure
having a direct energy conversion unit.
[0036] FIG. 17 is a block diagram of a direct energy conversion
unit according to an embodiment of the present disclosure.
DETAILED DESCRIPTION
[0037] Unless otherwise explained, all technical and scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which this disclosure belongs.
In case of conflict, the present specification, including
explanations of terms, will control. The singular terms "a," "an,"
and "the" include plural referents unless context clearly indicates
otherwise. Similarly, the word "or" is intended to include "and"
unless the context clearly indicates otherwise. The term
"comprising" means "including;" hence, "comprising A or B" means
including A or B, as well as A and B together. All numerical ranges
given herein include all values, including end points (unless
specifically excluded) and any and all intermediate ranges between
the endpoints.
[0038] Geologic coolant fluid, as used in the present disclosure,
may be a liquid, has, or a mixture of liquid and gas, or a liquid
with disclosed gas. In a specific example, the liquid is water. In
further examples, the gas is air, carbon dioxide, or the vapor or
steam from evaporated liquid. In a more specific example, the
liquid is water and the gas is air, carbon dioxide, or water vapor
or steam.
[0039] Phase changes due to pressure changes may be in the form of
evaporation and/or condensation or solution and/or dissolution.
[0040] Latent heat refers to energy change due to phase change in
general, including latent heat for evaporation/condensation and
latent heat for solution/dissolution.
[0041] Although methods and materials similar or equivalent to
those described herein can be used in the practice or testing of
the present disclosure, suitable methods and materials are
described herein. The disclosed materials, methods, and examples
are illustrative only and not intended to be limiting.
[0042] A new geothermal energy extraction system is described with
an improved subsurface heat exchanging process. The new system
increases the output temperature of the geologic cooling fluid used
for energy recovery, and this way increases the efficiency of the
power system with an increased thermodynamic efficiency for a given
geologic setting. The solutions described in the disclosure
pertain, in particular examples, to a conventional hydrothermal
system with low hydrologic permeability and low natural flow rates,
or a system involving the geothermal energy of dry hot rock which
does not have a substantial natural fluid flow system, or a
high-permeability, e.g. sedimentary rock formation which cannot be
utilized with conventional geothermal recovery methods and systems
due to unacceptable fluid and energy loss.
[0043] Natural geothermal energy systems with low permeability and
water flow rates in large and hot rock areas have similar problems
to those in deep geological settings with dry hot rock with regard
to energy recovery. They both need convective fluid flows to be
established in order to exploit significant amounts of thermal
energy. They both need an artificial fracture system in the rock to
be established for the facilitation of convective fluid flows.
[0044] Enhanced geothermal systems often rely on the principle of
hydro-fracturing of dry rock mass deep underground. The fracture
system at any location around the borehole typically depends upon
the in situ stress field of the rock mass and the hydraulic
pressure applied to the fracturing process. Since hydrostatic
pressure greatly varies with elevation, more control can be
exercised when the fracturing along the borehole is created
sequentially, one fracture network system at one elevation at a
time.
[0045] In the following description, it is assumed that a
sufficiently long section of a deep borehole stretching across a
hot geologic area can be fractured at various depths. It is further
assumed that the fractured zone around the borehole can extend in
radial direction into the rock mass several tens or hundreds of
meter. A particularly advantageous sequence of fracture network
creation is described along a deep borehole drilled in dry hot rock
or hot rock in a hydrothermal field with low permeability, such
permeability insufficient for creating convective flows.
[0046] The process of generating multiple levels of fracture
network around a borehole is described with the use of FIG. 1. When
the depth of borehole 1 reaches the desired depth at which useful
temperature is found for geothermal energy extraction, the drilling
is stopped and a smaller diameter pilot hole 10 is drilled ahead of
the larger-diameter borehole 1 to some depth, e.g., 300 m, to
create fractures starting at that depth. The pilot hole 10 is used
to create a fracture network around it. In some examples, the
fracture network is substantially spherical or ellipsoidal in
shape.
[0047] For fracturing, the pilot hole section 10 is sealed at its
collar with pressure seal 15. Hydro-pressure is generated around it
by pumping a sufficient flow rate of water into it through the
fracturing pressure pipe 14. A fracture network system 2 is created
to a desired depth or degree, such as having a radial diameter of
about 300 m. Orientations of the fractures typically depend on the
in situ stress field and the planes of weakness of the rock
formation. The development of the fractures in a radial direction
typically results from the fracturing process, and the network
occurs with a random structure in spite of dominant fracture
planes. However, in some embodiments, the fracture system is
developed in a more controlled manner, such as by adjusting the
applied hydraulic pressure or the pumping flow rate, which can
influence fracturing velocity. In yet further examples, the
fracture apertures are treated to help them remain open, such as by
pumping in solid particles, e.g., screened sand or gravel, called
propants.
[0048] Multiple branches of the fracture network 2a and 2b may be
created along the first pilot hole section 30, as shown in FIG. 1.
The fracture network system is, in some implementations, assessed
and completed after sufficient fracture volume and apertures are
established. Assessment may be made by suitable methods, including
measuring the volume of the fracturing fluid pumped into the
fracture network; pumping and extracting tracer elements and
observing the variation of concentration with time; micro-seismic
wave reflection measurements, and combinations thereof.
[0049] Once the fracture system is created at one depth area, the
fracturing pressure pipe 14 is extracted, and the pilot hole 30 is
re-drilled, removing pressure seal 15 at that depth, and extending
borehole 1 to a greater depth below the area of fracture network
2b. At this second depth, shown in FIG. 1, the fracturing process
is repeated by means of drilling a pilot hole 30a; insertion of
fracturing pressure pipe 14; and creating a pressure seal 15a; and
creating a new fracture network system 3, with possible fracture
network branches 3a and 3b. The assessment of the fracture network
system 3, 3a, and 3b is conducted again and re-fracturing is
continued until satisfaction. Fracturing is completed when
hydrologic flow connection between branches of fracture network 2,
2a, and 2b and branches of fracture network 3, 3a, and 3b has been
established and positively confirmed by assessment means.
[0050] Once the fracture system is completed at the second depth
area, the fracturing pressure pipe 14 is extracted, and the pilot
hole 30a is re-drilled, removing pressure seal 15a at that depth,
and extending borehole 1 to a greater depth below the area of
fracture network 3b. At this third depth, shown in FIG. 1, the
fracturing process is repeated by means of drilling a pilot hole
30b; insertion of fracturing pressure pipe 14; and creating a
pressure seal 15b; and creating a new fracture network system 4,
with possible fracture network branches 4a and 4b. The assessment
of the fracture network system 4, 4a and 4b is conducted again and
re-fracturing is continued until satisfaction. Fracturing is
completed when hydrologic flow connection between branches of
fracture network 3, 3a, and 3b and branches of fracture network 4,
4a, and 4b has been established and positively confirmed by
assessment means.
[0051] Once the fracture system is completed at the third depth
area, the process may continue to even greater depths. For example,
it may be advantageous to fracture the rock mass around a borehole
from 4,000 m depth down to 6,000 m depth, or even deeper. Shallower
depths can be sufficient in a natural geothermal area. The fracture
system will be a high-permeability fracture network around borehole
1, forming fractured zone 6, capable of carrying convective cooling
fluid for geothermal energy extraction from the top of fractured
zone 6 to its bottom. The advantage of creating a multiple-level
fracture network system through a series of pilot holes is that the
process does not interfere with the completion of borehole 1 and
the placement of its liner in long, constant-diameter sections.
[0052] The completion of the geothermal energy extraction system in
FIG. 2 involves several more steps. A cooling fluid delivery
fracture network system 8 is created within the fractured zone 6.
This can be done from borehole 1, effectively designating the first
fractured system 2 as delivery fracture network 8. The fracture
network system 8 is also connected to a second borehole, borehole
7, for injecting geothermal cooling fluid 7a if a hot dry rock area
is used. Optionally, delivery fracture network 8 may be created
from borehole 7 in a similar manner described regarding fracture
creation around borehole 1.
[0053] As shown in FIG. 3, injection borehole 7 is omitted in some
configurations, such as if the hot rock area is connected to a
natural hydrothermal system. For example, injection borehole 7 may
be replaced by a major natural fracture or fault in a geologic
system of a hydrothermal reservoir. In this case, injection fluid
7a is percolation fluid flow from the natural system.
[0054] Referring primarily to FIG. 2, although a similar
system/process takes place in FIG. 3 (with the exception of the
injection borehole 7), collection fracture network 5 is created at
the bottom of fractured rock zone 6, carrying hot fluid flow 10 to
borehole 1 which is used as a production borehole to lift hot fluid
for power generation. This can be done from borehole 1, effectively
designating the last fracture network system (such as system 4 in
the rightmost panel of FIG. 1) as collection fracture network
5.
[0055] Multiple levels of fracture network systems around borehole
1 can be created by other ways. For example, borehole 1 can first
be drilled, observed for assessing geology, lithography, fracture
and/or faults, and completed with casing. Fractured zone 6 can then
be created as a next step around borehole 1 over the desired length
by sequentially, level-by-level (1) jet-drilling through the
casing; (2) sealing of a section around the jet-drilled holes; and
(3) hydro-fracturing at each level. According to an embodiment of
the present disclosure, the access holes in the casing drilled for
fracturing are sealed off, except for those connected to injection
fractures 8 and collection fractures 5. Sealing may be accomplished
by inserting another liner or casing into borehole 1 or by other
means.
[0056] FIG. 2 illustrates the connected fracture network system in
a simplified fashion, depicting the delivery fracture network 7,
transmission fracture network branches 2, 3 and 4 as examples for a
possible solution with a large-enough vertical extent, and the
collection fracture network 5 around borehole 1 all within
fractured rock zone 6. FIG. 2 also shows pump 12 with its delivery
pipe to carry upward the hot fluid.
[0057] At least one delivery fracture system 8 for the introduction
of cooling fluid 7a at a higher elevation and one collection
fracture system 5 at a lower elevation are used according to an
embodiment of the present disclosure. The two fracture systems are
connected as an integral flow system in fractured zone 6. An
arbitrary number of intermittent, transmission fracture network
systems may be inserted between the delivery and the collection
fracture systems to increase the volume and surface area along the
working length of borehole 1 for heat exchange.
[0058] The geothermal energy extraction system according to an
embodiment of the present disclosure operates on a reversed flow
principle relative to known solutions. The flow direction of the
convective cooling fluid is from top to bottom, allowing for
increasing the temperature of the cooling fluid along its flow path
as it reaches hotter rock areas at increasing depths. This way, the
heat extracted from the rock mass will be delivered at the highest
possible temperature, allowing the thermodynamic efficiency of the
energy conversion to be increased from thermal energy to mechanical
energy in the turbine system. Heat is typically more valuable at
higher temperature than at lower temperature, increasing the
thermodynamic efficiency. If the flow direction of the hydrothermal
fluid is in the direction of the geothermal gradient, the exit
temperature is increased, and the entropy is decreased, relative to
other flow directions. Therefore, the geothermal extraction
principle according to an embodiment of the present disclosure can
be more efficient that prior techniques. This increased efficiency
is due, in some implementations, by the enhanced convective heat
exchanging system and the extraction of the counter-current
percolation flow relative to natural, buoyancy-driven circulation
directions.
[0059] In typical existing systems, the geothermal fluid flow
direction is upward, reaching rock areas at lower temperature as
the cooling fluid collects thermal energy, impeding the overall
thermal efficiency, and increasing entropy. A system according to
an embodiment of the present disclosure allows for converting a
larger portion of the gross thermal energy lifted from the ground
into usable mechanical or electrical energy. Buoyancy-driven,
small-scale flow eddies moving upward may develop within certain
areas in fractured zone 6. Natural, thermal siphon-type circulation
loops involving vertical upward flows locally may also be formed in
certain areas. These circulation loops will not impede, but rather
further enhance local convective heat transport within the overall,
dominantly downward flow direction for convective thermal energy
transport.
[0060] Downward coolant flow encounters hotter rock and removes
heat from it, while increasing its own temperature and energy
content. At the same time, the fracture aperture opens up due to
thermal contraction of the rock under cooling. In comparison, in
case of upward coolant flow direction in the current practices, the
hot fluid may encounter colder rock, effectively losing its cooling
ability, and making the heat exchange surface inefficient. The
fracture aperture may tend to close under heating by the hotter
fluid, increasing the pressure loss and the required pumping
pressure, and/or decreasing the flow velocity.
[0061] The flow system is explained with the use of FIG. 2.
Injection fluid 7a enters the fractured zone 6 of the hot rock area
as induced percolation 9, originating from injection borehole 7
through injection fracture network 8, in fractured, enhanced dry
hot rock application; or from natural percolation through fractures
and faults in fractured, enhanced hydrothermal reservoir
application. The induced percolation continues throughout the
vertical extent of the fractured zone 6 until it reaches the
collection fracture network 5 and flows back as hot fluid flow 10
into the low pressure chamber 11 of the bottom of production
borehole 1. Pressure lowering is accomplished by a suitable fluid
extraction means for removing coolant fluid from the geologic
formation, such as employing a pump 12 or applying gaseous lifting,
such as air lifting, which elevates fluid from a well by mixing
well fluid with compressed air bubbles. FIG. 2 shows pump 12 with
the suction side connected to the low pressure chamber 11 and a
delivery pipe which carries the hot reservoir fluid to the surface.
Heat and pressure insulation 13 is used to fill the annulus between
the delivery pipe and borehole 1.
FIG. 4 provides another embodiment of a flow system for an enhanced
hydrothermal reservoir with a major fracture or fault. Injection or
natural groundwater fluid 7a enters the fractured zone 6 of the hot
rock area as induced percolation 9, originating from a fracture or
fault and replacing borehole 7 in FIG. 2, through a fracture
network 8 which, in one example, is created from borehole 1. The
induced percolation continues throughout the vertical extent of the
fractured zone 6 until it reaches the collection fracture network 5
and flows back as hot fluid flow 10 into the low pressure chamber
11 of the bottom of production borehole 1. Pressure lowering is
accomplished by employing a pump 12 or applying air lifting,
elevating fluid from a well by mixing well fluid with compressed
air bubbles. FIG. 3 shows pump 12, with the suction side connected
to the low pressure chamber 11 and a delivery pipe which carries
the hot reservoir fluid to the surface. Heat and pressure
insulation 13 is used to fill the annulus between the delivery pipe
and borehole 1.
[0062] FIG. 4 shows the main elements of the geothermal energy
extraction system as well as the outside hydrostatic pressure field
16 within the outside of the fractured zone and the inside pressure
field 17 within the inside of the fractured zone. As shown, inside
pressure field 17 inside the fractured zone is lowered relative to
the in situ pressure due to the friction loss caused by the
downward flow field, placing and keeping the rock mass in fractured
zone 6 under lower pressure, i.e., depression, relative to the
surrounding rock formation. This advantageous pressure field can be
maintained due to the placement of the pump at the lowest
elevation, provided that induced percolation 9 does not require
increased pressure relative to the in situ hydrostatic pressure.
This case is shown in FIG. 4 with matching pressure lines 16 and 17
at the injection elevation 20 at the top of fractured zone 6. It
must be noted that pump delivery pressure line 19 may be increased
arbitrarily to meet the necessary pressure demand to lift the hot
fluid to the surface, i.e., pressure 19 does not have to intersect
pressure lines 16 and 17 at elevation 20. The pressure field
according to FIG. 4 can eliminate or reduce the potential loss of
hot fluids. Pump 12 increases the pressure within its length by a
pressure difference 18 until it reaches delivery pressure in the
delivery pipeline creating an upward flow.
[0063] The fractured zone can be made sufficiently large in volume
(to access stored heat) as well as large in outside surface area
(to access convective-conductive heat from the hot surrounding rock
mass), and therefore, can be designed to extract large amounts of
geothermal energy for sufficiently long time periods without
significant depletion.
[0064] In some configurations, pump 12 operates at relatively high
temperatures, which can exceed 300.degree. C. In some cases,
according to an embodiment of the present invention, pump 12 is
operated by applying compressed air injection to the high pressure
side of the pump, contributing to air lifting. In another example,
compressed air is delivered to the body of the pump at a higher
pressure than needed for air lifting injection. The compressed air
is expanded to the injection delivery pressure within the casing of
the motor. This way, expansion power, effectively a cooling power,
may be created, which can advantageously decrease the operating
temperature of the pump. Alternatively, in another embodiment of
the present disclosure, the pump is omitted and another technique,
such as air lifting, is used to move upward the hot reservoir
production fluid.
[0065] The elevation of pump 12 may be increased if an increased
injection pressure at elevation 20 is required for maintaining
fracture apertures in an open state; however, opening and
stabilizing fracture apertures by propants may also be carried out,
in addition to or in place of pressure opening. Such a case is
shown in FIG. 5 as an example, delivering an injection pressure
from a high water table head along hydrostatic pressure 16 to the
fractured zone at elevation 20. Note that the hydrostatic head may
be above the surface, not shown. Inside pressure field 17 under
percolation flow 9 starts from hydrostatic pressure 16 at elevation
20 in the example. Suction pressure difference 18a is created by
pump 12 at which the absolute suction pressure at the intake is
lowered to the physical minimum, shown as near zero in the example
for simplicity. During pumping, this pressure must typically be
kept higher than the saturated water pressure at that temperature,
which can be quite high itself. The intake pressure at pump 12 is
decreased by the pressure loss in borehole 1 due to friction, shown
as a gradient difference between line 19 and line 16, as well as
parallel line to it 16a, also depicted for clarity. Pump 12
delivers hot fluid to the surface along pressure line 19a, which
assumes zero pressure at surface elevation. In some
implementations, the hot fluid enters a heat exchanger to remove
its thermal energy and convert it to mechanical and electrical
energy. In a closed control volume in which the coolant fluid may
be considered conserved, the cooled coolant fluid is re-injected
into the circulation system. Such a case may be interpreted by
connecting the end of pressure line 19a at the surface vertically
to the beginning of pressure line 19 at the water table elevation,
assuming that the pressure loss of fluid handling at the power
plant as well as the pressure loss of the discharge of the coolant
fluid to the water table is compensated by the elevation
difference. A total pressure difference 18 typically must be
provided by pump 12 to move hot fluid from the bottom of borehole 1
to the surface, a significant component in the pumping energy
requirement for energy production.
[0066] An advantageous variation of the geothermal energy
extraction system described in the foregoing is discussed with the
use of FIG. 6. In this solution variation, injection borehole 7 is
eliminated, and pump 12 lifts hot fluid from the collection
fracture network system 10 to injection fracture network 8
directly. Pump 12 effectively establishes a thermal fluid
circulation within the fractured rock zone 6, engineered to be
counter-current direction to the direction of natural
re-circulation under buoyancy driving forces. The outside
hydrostatic pressure field 16 within the outside of the fractured
zone and the inside pressure field 17 within the inside of the
fractured zone are both shown to understand the flow system.
[0067] Pump 12 increases the pressure within borehole 1 to delivery
pressure 19, creating an upward flow 20 in borehole 1 which is
cased to prevent radial fluid flow to or from fractured zone 6,
except for the upper section of fractured zone 6 around the fluid
delivery fracture network 8. Along this discharge section 21, the
casing of borehole 1 is screened, or punched, e.g., by jet
drilling. Pressure seal 22 is used to prevent back-flow to the low
pressure chamber 11.
[0068] As shown, the pressure increase 18 across the pump is
engineered to overcome the flow friction and momentum losses as
well as the natural buoyancy pressure differences within the
recirculation system. A standing water level 20 in borehole 1 is
stabilized just above the screened discharge section 21 in order to
maintain cooling fluid delivery to injection fracture network 8 and
percolation flux 9. The energy recovery is accomplished by
evaporative heat exchanger 23, basically a U-tube arrangement with
a downward coolant fluid flow 24 in one leg and upward steam flow
25 in the other leg. This coolant fluid recirculation loop assumes
at least in some examples, that the fractures are kept open without
pressurizing them above the hydrostatic pressure at any location,
e.g., injecting propants during hydrofracturing.
[0069] Another implementation of this variation applies compressed
air injection for cooling and relieving pump 12 from excessive
thermal load. Yet another implementation applies solely compressed
air injection for maintaining recirculation of cooling water
against natural buoyancy driving forces, if water is the coolant
fluid. In this implementation, an air bubble and water mixture in
borehole 1 is created to maintain upward flow and the desired
circulation direction. The air bubble-water mixture at cooled-down
temperature by heat exchanger 23 can still have lower density than
the density of the hottest water in fractured zone 6, therefore, it
can effectively drive the circulation loop in the desired direction
without using pump 12.
[0070] A long section along borehole 1 is used for extracting
thermal energy from the hot fluid by evaporative heat exchanger 23,
as part of a secondary steam cycle that is completed in the power
plant with turbines and condensers. A small amount of fluid can be
replenished to maintain standing water level 20 if loss occurs from
the circulation loop. The hot water is not lifted to the surface in
this implementation. An advantage of this implementation is that
thermal energy loss can be reduced along borehole 1 from the
geothermal energy production level to the power plant level where
the thermal-to-mechanical energy conversion takes place. The
advantage is seen in transporting thermal energy in latent heat
form from a greater depth (at high pressure) to a lower depth (at
lower pressure) in gaseous, compressible physical phase. As
evaporated steam 25 flows upward from heat exchanger 23 which acts
as a boiler, it decompresses and cools, reducing the temperature
difference and resulting heat loss between the carrying pipeline
and the surrounding rock mass. On the other hand, sufficiently high
pressure can be maintained in the steam cycle loop to avoid
condensation of the stem as it flows upward in spite of cooling and
decompressing.
[0071] A two-phase coolant fluid system may also be created in
fractured zone 6 by lowering the pressure level by adjusting the
pressure at elevation 20. Pressure lowing at elevation 20 may be
accomplished, for example, by removing liquid from the borehole 1,
or lowering the pressurization of borehole 1 by another pump (not
shown) at another elevation or at the surface. The differential
pressure in the circulation loop is maintained by pump 12. The
absolute pressure in that loop can be independently controlled. For
example, a pressure difference 18 can be carried by adjusting the
working point of pump 12 (for example, changing its speed); whereas
the pressure level at both the intake and delivery points of pump
12 can be simultaneously increased or decreased by adjusting the
pressure at elevation 20.
[0072] If the pressure at any given location is set lower than the
saturated pressure for the coolant fluid at the temperature of the
given location, the result will be evaporation. It may be
advantageous to induce periodic evaporation by decreasing the
pressure level below the saturated pressure limit or dissolution
pressure limit for some time in a purging cycle and allowing the
gas-phase coolant to repel stagnant liquid-phase coolant packets.
During this purging cycle, the coolant circulation is suspended by
stopping pump 12 and avoiding gas-phase digestion. Pressure then
can be increased above the saturated limit or dissolution limit in
another cycle, condensing steam or absorbing the gas-phase
component and flooding the fracture system with liquid-phase
coolant. The two-phase huff-puff operation may alleviate the loss
of active surface area in the heat exchange process between
convective coolant fluid and the fractured rock formation. Pressure
variations in zone 6 may be accomplished by pumping pressure
control from the surface, or pressure control at the subsurface
pump, or a combination of the two.
[0073] In one variation, multiple vertical boreholes with coalesced
fractured zones around them can be used to create an even greater
rock mass volume with convective heat transfer surface area within
it. Boreholes or borehole sections running horizontally and
fractured zones created in between can also be used. In this case,
extraction of the hot fluid is accomplished at a comparatively low
elevation and, therefore, at increased temperature. The fluid
extraction system is then typically engineered to maximize the
temperature of the hot fluid carrying thermal energy; and minimize
the potential heat loss during carrying the hot fluid to the energy
conversion implement.
[0074] A method 200 of extracting geothermal energy according to an
embodiment of the present disclosure is presented in the flowchart
of FIG. 7. According to the method 200, first, second, and/or third
fracture systems are formed in optional steps 204, 208, 212.
According to an implementation of the method 200, steps 204, 208,
212 can involve drilling a borehole in step 216, drilling a pilot
hole ahead of the borehole in step 220, and fracturing around the
pilot hole in step 224.
[0075] Coolant fluid is introduced in the first fracture system in
step 228. In one implementation, coolant fluid is introduced into
the first fracture system through an injection borehole in step
232. In another implementation, coolant fluid is introduced into
the first fracture system through a fault or fracture in step 236,
such as from a natural hydrothermal reservoir.
[0076] Coolant fluid collects in the second fracture system in step
240, optionally after it has passed through a third fracture
system, when present. In step 244, the coolant fluid is removed
from the first fracture system. In one implementation, coolant
fluid is removed by air lifting in step 248. In another
implementation, coolant fluid is removed by a pump in step 252. In
a particular example, compressed air is added to the high side of
the pump in step 256, which can assist with air lifting. Step 260
shows another example where compressed air is added to the body of
the pump, which can aid in cooling the pump during operation.
[0077] Energy is extracted from the collected coolant fluid in step
264. In one implementation, step 268, heat is extracted using a
heat exchanger located at the surface. In another implementation,
step 272, heat is extracted using a heat exchanger located in a
production borehole.
[0078] In optional step 276, cooled collected coolant fluid is
returned to the first fracture system. In another optional step,
280, coolant fluid is cyclically evaporated/liberated and
condensed/dissolved in the first, second, and/or third fracture
systems, such as by cyclically lowering and increasing the pressure
using a pump. Sufficient time periods in the cycle are provided for
pursing stagnant cooling fluid from the fractures during
evaporation or has liberation in one cycle and delivering coolant
fluid during condensation or gas absorption in another cycle.
[0079] An innovative geothermal energy production method and
apparatus are further described from permeable rock formations.
Fractured or sedimentary rocks with moderate permeability of 20 to
90 millidarcy are found abundantly in the earth crust at favorable
depth and temperature, and represent a great capacity for targeted
geothermal energy exploitation.
[0080] Such formations may be found within a natural geothermal
system in areas of favorable groundwater level, hydrologic
conditions, and high geothermal gradient. The natural geothermal
systems may need coolant water injection to increase flow as well
as energy extraction capacity.
[0081] Injection water, however, may escape the flow path towards
production well(s), using conventional techniques; a new solution
for energy-carrying hot water extraction is sought for capturing
the injection water entirely or with minimal loss.
[0082] Another target geologic rock formation for the new solution
is sedimentary rock with sufficient permeability and flow
characteristics under a hydraulic gradient. Some formations around
depleted natural hydrocarbon fields already have enhanced flow
systems from previous production. This would be a desirable target
formation for geothermal energy extraction with the new
technique.
[0083] In general, the innovative geothermal energy extraction
method and apparatus will be applicable broadly to the following
target formations: (1) hot sedimentary rocks with a moderate
permeable range, e.g., of 20 to 90 millidarcy, saturated with
groundwater; (2) fractured rock with moderate permeability within a
natural geothermal area with available groundwater but with low or
no natural flow or circulation; (3) depleted hydrocarbon field with
moderate permeability and sufficient groundwater level but with no
significant flow: and (4) depleted hydrocarbon or another
sedimentary, confined field, used for CO.sub.2 sequestration.
[0084] A formation with sufficiently high permeability and ground
water level is shown in FIG. 8. Induced percolation is created by
delivering geologic cooling fluid that returns from the power
conversion unit 40, which is situated on the surface in this
example. Induced percolation from the geologic cooling fluid 7a
enters through natural or engineered channels, such as perforated
borehole section 41 and heats up as it encounters hot rocks of
increasing temperature with depth. The natural or enhanced,
permeable rock 42 of sufficiently high hydraulic permeability is
cooled by convection of gravitationally-driven percolation 9,
providing calorimetric heat from the percolated zone 43 within the
percolation plume. The flow zone 42 is under negative gage pressure
relative to the ambient in situ hydrostatic pressure at the outside
hydrostatic pressure field 16 at any vertical location, forming an
inside pressure field 17. The suction pressure difference 18 is
created by pump 12, which is installed deep in the low pressure
chamber 11. The pressure difference 8 is created by pump 12
gradually decreases with elevation within the powered pressure zone
46, which coincides with the percolation zone. At the delivery
point, of the recalculating geologic cooling fluid 7a, that is, at
the injection elevation 20, the pressure field is balanced with the
ambient hydrostatic pressure in an ideal geologic formation with
sufficiently high permeability. The arrangement includes active
control for adjusting pump pressure difference AP according to
maintaining a sufficient water level in the suction collection part
of production borehole 1, and preventing a dry run for pump 12.
On-off, or variable-speed control of pump 12 is useable for this
task.
[0085] FIG. 9 shows an example with coolant delivery boreholes 44
to deliver geologic cooling fluid 7a farther, and to increase the
lateral size of the percolation zone 43. Collection boreholes 45
are also added to widen the area of influence of the suction
pressure and to achieve a larger lowered pressure zone 46.
Otherwise, the working principle is the same as described for FIG.
8.
[0086] FIG. 10 shows the effects of elevating the location of pump
12, which may be accomplished when the ambient hydrostatic pressure
is high, such as corresponding to a high water table level 47. As
shown, the total pressure difference 18 of the pump gradually
decreases with depth, gradually diminishing the suction pressure
difference 18a. This, in turn, shrinks the lateral size of the
lowered pressure zone and risks the loss of the circulating
geologic cooling fluid. This is why pumped systems with high
suction elevation tend to lose coolant fluid, a disadvantage.
[0087] Other examples are variations of the concept of creating a
lowered pressure zone by a pump and establishing a re-circulatory
geologic heat exchanger system in situ. These variations are shown
in FIGS. 11 through 13
[0088] FIG. 11(A) shows a single borehole application example of
the flow collection concept using pressure lowering by pump 12 in a
confined aquifer bounded by two impermeable layers 48. Coolant
delivery boreholes 49 as well as coolant collection boreholes 45
are shown, both applied for circulation and capacity increase. The
coolant fluid flow direction 24 is downward and converging towards
the low pressure side of the borehole under the pressure
distribution shown in FIG. 11(B). Three pressures, A, B, and C are
shown at three different vertical cross sections, as marked in FIG.
11(A): A-A, B-B, and C-C.
[0089] FIG. 12 shows an application variation of the geologic
cooling fluid collection system using pressure-lowering in a
horizontal arrangement in an unconfined aquifer using multiple
boreholes. FIG. 12(A) is a cross-sectional view, while FIG. 12(B)
is a plan view of the arrangement. Injection boreholes 49 deliver
geologic cooling fluid 7a into the lowered pressure zone 46. Two
production boreholes 50 are shown in the example with their area of
influence zone 51 in the extraction of geologic cooling fluid 7a.
It may be advantageous to re-inject the geologic coolant fluid
downstream from the production borehole 50 as opposed to the
upstream position as shown in FIG. 12, in case of horizontal
groundwater flow in the formation.
[0090] FIG. 13 illustrates a multi-borehole design in a confined
permeable formation used for CO.sub.2 sequestration where one
borehole serves as an injection well 50 and the other serves as a
return production borehole 51. In between the boreholes, permeable
rock 42 serves as both a CO.sub.2 storage volume as well as a
geologic heat exchanger with a lowered pressure zone 46, using pump
12 for controlled coolant CO.sub.2 circulation.
[0091] FIG. 14 shows the conventional energy flow from thermal
energy exploited from geothermal reservoir 53 to electrical energy
56 with the use of the convective geologic coolant fluid
circulation 54 in between. In conventional applications, the
convective geologic coolant fluid circulation 54 transports energy
along the production borehole where energy loss tends to be
encountered as heat and pressure loss, a disadvantage.
[0092] FIG. 15 is an improved arrangement in which the geologic
coolant fluid circulation 54 transmits energy to transmission
coolant fluid 58 through heat exchanger 57 which is part of the
equipment installed in borehole 50 including pump 12 in previous
figures. The transmission coolant fluid 58 is transmitted to the
surface installation 60. Such transmission may be accomplished by a
single-phase or binary, secondary fluid circulation loop connected
to energy conversion unit 55.
[0093] FIG. 16 is a compact arrangement in which subsurface
installation 59 includes the geothermal reservoir 53, a geologic
coolant fluid circulation pump 12, and a direct energy conversion
unit 61 which supplies electrical energy 56 through an electrical
power connection to the surface. The necessary energy conversion
coolant fluid circulation unit 62 for the direct energy conversion
unit 61 is powered by surface installation 60. Since cold coolant
fluid is circulated along the long production borehole, the thermal
energy loss is minimized, an advantage.
[0094] The direct energy conversion unit 61 may be realized by the
application of conventional components, shown in FIG. 17, including
heat exchanger 57, heating binary transmission coolant fluid 58, a
turbine 63, condenser 64, which is cooled by energy conversion
coolant fluid circulation unit 62 delivered from the surface.
Turbine 63 drives generator 65 to produce electrical energy 56.
[0095] It is to be understood that the above discussion provides a
detailed description of various embodiments. The above descriptions
will enable those of ordinary skill in the art to make and use the
disclosed embodiments, and to make departures from the particular
examples described above to provide embodiments of the methods and
apparatuses constructed in accordance with the present
disclosure.
[0096] The embodiments are illustrative, and not intended to limit
the scope of the present disclosure. The scope of the present
disclosure is rather to be determined by the scope of the claims as
issued and equivalents thereto.
* * * * *