U.S. patent application number 12/670898 was filed with the patent office on 2010-10-07 for device for in situ extraction of a substance comprising hydrocarbons.
Invention is credited to Dirk Diehl, Bernd Wacker.
Application Number | 20100252249 12/670898 |
Document ID | / |
Family ID | 40175964 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100252249 |
Kind Code |
A1 |
Diehl; Dirk ; et
al. |
October 7, 2010 |
DEVICE FOR IN SITU EXTRACTION OF A SUBSTANCE COMPRISING
HYDROCARBONS
Abstract
A device for in situ extraction of a substance including
hydrocarbons while reducing the viscosity thereof from an
underground storage site is provided. The device includes at least
one production pipeline leading out of the storage site, and at
least two electrodes which are inductively and resistively
effective relative to at least parts of the storage site.
Inventors: |
Diehl; Dirk; (Erlangen,
DE) ; Wacker; Bernd; (Herzogenaurach, DE) |
Correspondence
Address: |
SIEMENS CORPORATION;INTELLECTUAL PROPERTY DEPARTMENT
170 WOOD AVENUE SOUTH
ISELIN
NJ
08830
US
|
Family ID: |
40175964 |
Appl. No.: |
12/670898 |
Filed: |
July 31, 2008 |
PCT Filed: |
July 31, 2008 |
PCT NO: |
PCT/EP08/60074 |
371 Date: |
January 27, 2010 |
Current U.S.
Class: |
166/60 |
Current CPC
Class: |
E21B 43/2408 20130101;
E21B 43/305 20130101; E21B 43/2401 20130101 |
Class at
Publication: |
166/60 |
International
Class: |
E21B 36/04 20060101
E21B036/04; E21B 43/24 20060101 E21B043/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2007 |
DE |
10 2007 036 832.3 |
Claims
1.-15. (canceled)
16. A device for in situ extraction of a substance including
hydrocarbons from an underground storage site by reducing the
viscosity of the substance, comprising: a production pipeline
leading out of the storage site; and two electrodes of an electric
heater, the electric heater working inductively and resistively
relative to parts of the storage site.
17. The device as claimed in claim 16, wherein the two electrodes
of the electric heater are formed by essentially perpendicularly
oriented electrodes running at least partially in the storage
site.
18. The device as claimed in claim 16, wherein the two electrodes
of the electric heater are formed by essentially horizontally
oriented electrodes running at least partially in the storage
site.
19. The device as claimed in claim 16, further comprising: a power
source, wherein segments of the electrodes are spaced at a distance
from one another, the distance decreasing steplessly with an
increasing length of the electrodes from the perspective of the
power source.
20. The device as claimed in claim 19, wherein the distance between
the segments of the electrodes decreases linearly.
21. The device as claimed in claim 16, wherein the electrodes are
rod-shaped metal electrical conductors.
22. The device as claimed in claim 16, further comprising: a guide
pipe, the electrodes running coaxially in the guide pipe, wherein
the guide pipe is permeable to liquid for a deposition of a liquid
in parts of the storage site at corresponding subareas running in
the storage site.
23. The device as claimed in claim 19, further comprising: a guide
pipe, the electrodes running coaxially in the guide pipe, wherein
the guide pipe is permeable to liquid for a deposition of a liquid
in parts of the storage site at corresponding subareas running in
the storage sige.
24. The device as claimed in claim 16, wherein the electrodes are
electrically connected to one another by a connecting post at their
end areas remote from the power source.
25. The device as claimed in claim 16, further comprising: an
injection pipeline projecting into the storage site.
26. The device as claimed in claim 23, further comprising: an
injection pipeline projecting into the storage site.
27. The device as claimed in claim 25, wherein the injection
pipeline and the production pipeline have horizontally oriented
pipe sections running essentially parallel to one another in the
storage site, and, when viewed in a cross-section perpendicular to
the injection and production pipeline, the electrodes are disposed
on both sides of the injection and production pipeline.
28. The device as claimed in claim 25, wherein one of the two
electrodes of the electric heater is formed by at least parts of
the injection pipeline and the other electrode is formed by the at
least parts of the production pipeline.
29. The device as claimed in claim 26, wherein one of the two
electrodes of the electric heater is formed by at least parts of
the injection pipeline and the other electrode is formed by the at
least parts of the production pipeline.
30. The device as claimed in claim 25, wherein hot steam is applied
to the injection pipeline and the production pipeline.
31. The device as claimed in claim 26, wherein hot steam is applied
to the injection pipeline and the production pipeline.
32. The device as claimed in claim 30, wherein the hot steam is
provided with an electrolyte to increase its electrical
conductivity.
33. The device as claimed in claim 31, wherein the hot steam is
provided with a salt to increase its electrical conductivity.
34. The device as claimed in claim 16, wherein the electric heater
is an alternating current heater.
35. The device as claimed in claim 16, wherein a frequency in the
range of 1 Hz and 200 kHz and a voltage in the range from 100 V to
10 kV is provided for the electric heater.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2008/060074 filed Jul. 31, 2008, and claims
the benefit thereof. The International Application claims the
benefits of German Application No. 10 2007 036 832.3 DE filed Aug.
3, 2007. All of the applications are incorporated by reference
herein in their entirety.
FIELD OF INVENTION
[0002] The invention relates to a device for in situ extraction of
a substance comprising hydrocarbons. The substance comprising
hydrocarbons is conveyed from an underground storage site while
reducing the viscosity thereof. The device comprises at least one
production pipeline leading out of the storage site. Such devices
for conveying substances comprising hydrocarbons are for example
known from "Steam-Injection Strategy and Energetics of
Steam-Assisted Gravity Drainage" by I. D. Gates, 2005, SPE
International Thermal Operations and Heavy Oil Symposium, Calgary,
Canada, Nov. 1-3, 2005.
BACKGROUND OF INVENTION
[0003] Large parts of the world's oil reserves are in the form of
oil sands. Oil sand is a mixture of rock, clay, sand, water and
bitumen or other heavy oils. In the following, references to
bitumen, which occurs with a typical viscosity of API 5.degree. to
15.degree. depending on the storage site, also include heavy and
very heavy oils or long-chain hydrocarbons generally. The bitumen
can be converted into synthetic crude oil by means of further
process steps. Oil sands in some cases are found in geological
strata at low depth, which are accessible to open-cast mining.
However, large deposits of oil sands also exist which cannot be
accessed by open-cast mining. Typically in situ extraction takes
place at depths of 60 m or more, since in this situation it no
longer seems profitable to remove the overburden.
[0004] A method typically used to extract such deposits is "Steam
Assisted Gravity Drainage" (SAGD), in which the bitumen occurring
in a storage site is heated by hot steam and the storage site is
rendered more permeable by the steam pressure. In this way the
viscosity of the bitumen is reduced, so that it can be conveyed in
liquid form, as well as more quickly, from the storage site. The
change in the viscosity of the bitumen is caused by an increase in
temperature. For this purpose hot steam is compressed through
pipelines into the underground storage site, so that the storage
site is heated, and at the same time an overpressure builds up in
the storage site. Liquid bitumen is conveyed through a further pipe
to the surface by the overpressure prevailing in the storage site
(cf. also undergraduate seminar on topics relating to
"Unconventional hydrocarbons" on the subject of "Heavy oils and
ultra-heavy oils" by J. Seim, Freiberg, Germany, January 2001).
[0005] To improve the fluidity of the bitumen the hot steam can be
mixed with a solvent. The pipelines for the injection of the hot
steam, or of the mixture of hot steam and solvent, are essentially
laid in parallel to one another, running horizontally within the
storage site. The injection pipeline and production pipeline are
typically about 5 m to 10 m apart from one another in the vertical
direction. However, the distance between the injection and the
production pipeline depends on the thickness of the storage site.
In the horizontal direction the pipes extend for a length of
between several hundred meters and a few kilometers within the
storage site.
[0006] Before bitumen actually starts being conveyed out of the
storage site, it first has to be heated, in order to reduce the
viscosity of the bitumen present in the sand or rock. During the
heating phase, to ensure rapid heating of the storage site, hot
steam is applied to both the injection pipeline and the production
pipeline for a period of approximately 3 months. At the end of the
heating phase, the viscosity of the bitumen in the storage site is
such that when further hot steam is applied to the injection
pipeline, resulting in overpressure in the storage site, liquid
bitumen can be conveyed out of the production pipeline to the
surface. When sufficient pressure builds up the installation of
jacking oil pumps to bring the bitumen/water emulsion up can be
dispensed with.
SUMMARY OF INVENTION
[0007] The SAGD procedure currently practiced, as has been roughly
outlined, exhibits various technical problems. Firstly, hot steam
can escape from the storage site via channels or porous rock strata
present in the storage site, thereby reducing the thermal energy
introduced into the storage site. Excessive pressures in the
storage site can result in soil distortions on the surface
(blow-out), especially if the overburden is not very thick. A
further problem is "fingering" within the reservoir, whereby steam
breakout (steam short-circuit) occurs generally at the start or end
of the horizontal part of the parallel steam injection or
production pipeline, when the steam seeks out a preferred
communicating path between both pipelines and undesired pressure is
dissipated, with the injected steam condensing and being conveyed
through the production pipeline as water, so that less steam and
thus thermal energy is supplied to the storage site and the
effectiveness of the process is dramatically reduced. Pressure and
temperature within the storage site, for the rapid heating thereof,
cannot therefore be increased arbitrarily, depending on the storage
site conditions. Large quantities of fresh water are required for
the SAGD procedure. The quantity of water required is measured on
the basis of the "steam to oil ratio" (SOR). Strict environmental
regulations in the production areas call for a reduction in the
"steam to oil ratio", in order to protect the above-ground and
underground stores of water.
[0008] An object of the present invention is to specify a device
for in situ extraction of a substance comprising hydrocarbons, in
particular a device for conveying heavy oils or bitumen out of an
oil sand storage site, which is an improvement over the prior art
at least in that a reduced heating phase prior to the start of the
production phase can be achieved.
[0009] The object is achieved with a device as claimed in the
independent claim. Developments are specified in the dependent
claims.
[0010] The invention is based on the consideration of using an
electric heater, which works both inductively and resistively
relative to at least parts of the storage site, for the rapid
heating thereof. The storage site itself works as a resistive
(non-inductive) resistor relative to the heater's at least two
electrodes. At the same time the storage site is also heated
inductively by the electric heater.
[0011] According to the invention a device is specified for in situ
extraction of a substance comprising hydrocarbons while reducing
the viscosity thereof from an underground storage site, said device
containing at least one production pipeline leading out of the
storage site. The device further has at least two electrodes, which
work inductively and resistively relative to at least parts of the
storage site as an electric heater. Advantageously the heating time
for the reservoir containing the substance comprising hydrocarbons
can be reduced with the inventive device. Compared to devices as
known from the prior art, the "steam to oil ratio" can be
reduced.
[0012] Advantageous embodiments of the inventive device for in situ
extraction of a substance comprising hydrocarbons emerge from the
claims dependent on the independent claim, it being possible to
combine the embodiment as claimed in the independent claim with the
features of one dependent claim, and preferably with those of
several dependent claims. Accordingly the inventive device for
extraction of a substance comprising hydrocarbons can additionally
have the following features:
[0013] The two electrodes of the electric heater can be formed by
essentially perpendicularly oriented electric conductors running at
least in part in the storage site. A perpendicular borehole does
not require a great deal of drilling. Thus electric conductors
which work inductively and resistively relative to at least parts
of the storage site can be introduced into the storage site simply
and effectively. This device is especially advantageous if it has
to be assumed that the permeability diminishes at increasing depth,
or the permeability is non-homogeneous in a horizontal direction,
in other words that a non-homogeneous and if appropriate
anisotropic storage site is present in respect of permeability
and/or porosity.
[0014] The at least two electrodes of the electric heater can be
formed by essentially horizontally oriented electric conductors
running at least in part within the storage site. With electric
conductors which run horizontally within the storage site, a large
part of the storage site can be heated both resistively and
inductively by electricity.
[0015] The electrodes can be rod-shaped metal conductors.
Rod-shaped metal conductors are especially simple and cheap.
[0016] The space between at least segments of the electrodes can
decrease as the length of the electrodes increases, from the
perspective of a power source. The reduction in distance can in
particular be stepless. In particular the distance between the
segments of the electrodes can decrease linearly. By changing the
distance between the electrodes it is possible to keep the voltage
drop constant over the length of the electrodes. This voltage drop
is determined by the electrical resistance of the electrodes
themselves, plus the electrical resistance of the soil
correspondingly present between the electrodes. In this way it is
advantageously possible to prevent the entire heat output of the
electrodes dropping away at an area in the soil close to the power
source.
[0017] The electrodes can run coaxially in a guide pipe, said guide
pipe being permeable to liquid for the selective deposition of a
liquid in parts of the storage site at the corresponding subareas
running in the storage site. Thanks to the guide pipe the storage
site can be supplied with liquid in selective areas, as a result of
which the electrical conductivity of the storage site can be
affected. In this way the conductivity of the storage site can be
ensured, so that the electric heater will operate fault-free.
[0018] The device can have an injection pipeline projecting into
the storage site. In that the device has both an injection pipeline
and a production pipeline, the storage site can be heated on the
one hand by means of the electric heater and on the other hand by
for example a steam-assisted heating procedure. Both procedures can
interwork in synergy.
[0019] The electrodes can be connected to one another electrically
at their end regions remote from the power source by a connecting
post. Thanks to such an electrical connection the operational
reliability of the electric heater can be improved.
[0020] The injection pipeline and the production pipeline can have
pipe sections running in the storage site essentially parallel to
one another, oriented essentially horizontally. Seen in
cross-section perpendicular to the injection and production
pipeline, the electrodes of the electric heater are disposed on
both sides of the injection and production pipeline. Thanks to an
arrangement of the electrodes of the electric heater on both sides
of the injection and production pipeline, the volume of the storage
site, which extends in a horizontal direction between the injection
and production pipelines, can in particular be heated by the
electric heater. Particularly advantageously, a larger volume of
the storage site can be extracted in this way. SAGD procedures
known from the prior art achieve an extraction of between 40 and
60% of the bitumen present in the storage site. According to the
embodiment described above, extraction levels of over 70% appear
possible.
[0021] At least two of the electrodes of the electric heater can be
formed by at least parts of the injection pipeline or the
production pipeline. In that the electric heater is formed by at
least parts of the injection or production pipeline, it is possible
to cut down on additional material for the electric heater; another
borehole is thus likewise dispensed with. Such a design of the
electric heater is hence especially advantageous.
[0022] Hot steam can be applied to the injection pipeline and the
production pipeline. If an underground storage site is exploited in
accordance with an SAGD procedure or a similar or related
procedure, hot steam is applied to pipelines typically present in
the storage site. The combination of such a hot-steam-based
procedure with an electric heating procedure is especially
advantageous, since as a result of the hot steam additional water
is introduced into the storage site, which increases the electrical
conductivity of the storage site. A particular electrical
conductivity is necessary in order to implement an inductive and
resistive electric heating procedure. Thanks to the synergetic
combination of a hot steam procedure and an electric heating
procedure the efficiency of the combined procedure can be increased
over and above that of both individual procedures.
[0023] The hot steam can be enriched with an electrolyte,
preferably with salt. The electrical conductivity of the steam is
increased in this way. The effectiveness of an inductive and
resistive electric heater relative to at least parts of a storage
site essentially depends on the electrical conductivity of the
storage site. In that the hot steam introduced into the storage
site by the injection and/or production pipeline is additionally
provided with minerals, preferably with salt, the electrical
conductivity of the storage site can be selectively adjusted and if
appropriate increased.
[0024] The electric heater can be an alternating current heater. An
alternating current heater prevents ions from migrating within the
storage site. Advantageously carbonization or salt encrustation of
the injection and/or production pipeline can be prevented in this
way.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Further advantageous embodiments of the inventive device
emerge from the claims not addressed above and in particular from
the drawing explained below. The drawing shows preferred
embodiments of the inventive device in a highly diagrammatic
illustration. It shows:
[0026] FIG. 1 an outline sketch of a device for in situ extraction
of a substance comprising hydrocarbons from an underground storage
site,
[0027] FIG. 2 a cross-section through the extraction area of such a
storage site,
[0028] FIGS. 3 and 4 a device for in situ extraction of a substance
comprising hydrocarbons with an electric heater
[0029] and
[0030] FIG. 5 a further possible embodiment of such a device.
[0031] Corresponding parts in the figures are in each case provided
with the same reference characters. Parts not explained in greater
detail are generally known prior art.
DETAILED DESCRIPTION OF INVENTION
[0032] FIG. 1 shows a diagrammatic illustration of a device for in
situ extraction of a substance comprising hydrocarbons from an
underground storage site 100 while reducing the viscosity thereof.
Such a device can for example be a device for extraction of bitumen
from an oil sand deposit. An injection pipeline 101 runs from the
earth's surface into the storage site 100. A production pipeline
102 leads out of the storage site 100 to the earth's surface.
Devices are conceivable in which a plurality of injection pipelines
101 and a plurality of production pipelines 102 are used to convey
the substance comprising hydrocarbons from the underground storage
site 100. The storage site 100 can in particular be an oil sand
deposit or an oil shale deposit, from which bitumen or other heavy
oils can be extracted.
[0033] To be able to extract the substance comprising hydrocarbons
from the storage site 100, the viscosity of the substance
comprising hydrocarbons present in the storage site 100 must be
reduced. For this purpose at least parts of the storage site 100
must be heated. To heat the storage site 100 hot steam is applied
to the injection pipeline and the production pipeline 102. After
being heated for a period of typically 3 months the viscosity of
the substance comprising hydrocarbons present in the storage site
100 has decreased so much as a result of the increased temperature
that the substance has become free-flowing. If water steam is now
further applied to the injection pipeline 101, then as a result of
the overpressure present in the storage site or at least parts of
the storage site 100 a substance comprising hydrocarbons can be
conveyed through the production pipeline 102 to the surface. To
maintain the temperature needed for the substance comprising
hydrocarbons to flow freely, hot steam is again applied to the
injection pipeline 101. Once the extracted substance comprising
hydrocarbons arrives at the surface it can be subjected to further
processing steps, so that synthetic crude oil can be obtained.
[0034] FIG. 2 shows a cross-section through the storage site 100,
in which two injection pipelines 101, 101' and two production
pipelines 102, 102' run. The hot steam exiting from the injection
pipelines 101, 101' is shown diagrammatically by arrows. The hot
steam foams steam chambers 201, 201' within the storage site 100,
from which the substance comprising hydrocarbons which has become
free-flowing can be conveyed. A dead volume 202, in which the
viscosity of the substance comprising hydrocarbons present in the
storage site 100 is too high to enable it to be conveyed, extends
in a horizontal direction between the steam chambers 201, 201'. The
dead volume 202 of the storage site 100 cannot be extracted.
[0035] FIG. 3 shows a device for in situ extraction of a substance
comprising hydrocarbons from a storage site 100, which in addition
to two injection pipelines 101, 101', which have pipeline sections
running horizontally in the storage site 100, has two production
pipelines 102, 102'. The device for in situ extraction of a
substance comprising hydrocarbons furthermore has two electrodes
301, 301', which run in perpendicular boreholes within the storage
site 100. The electrodes 301, 301' work as an inductive and
resistive electric heater relative to at least parts of the storage
site 100. Determined by the conductivity of at least parts of the
storage site 100, especially of the dead volume 202, this can be
heated resistively by current flowing between the two electrodes
301, 301'. The resistive heating effect is indicated by arrows
and/or lines within the storage site 100. At the same time the
electrodes 301, 301' work in the volumes 303, 303' as inductive
heaters. The electrodes 301, 301' can be insulated in respect of
the soil by means of electrical insulators 304, 304' in areas in
which these run outside the actual storage site 100. In this way
the resistive heat output can be introduced in defined areas of the
storage site 100. The electrodes 301, 301' can in particular be
rod-shaped metal conductors made of an especially highly conductive
metal.
[0036] The device shown in FIG. 3 for in situ extraction of a
substance comprising hydrocarbons can in particular be operated
such that the storage site 100 is heated both by means of hot steam
and also by means of the electric heater. In the heating phase of
the storage site 100 hot steam can thus be applied both to the
injection pipelines 101, 101' and to the production pipelines 102,
102', and in addition the volume to be conveyed out of the
substance comprising hydrocarbons, especially the dead volume 202,
can be heated by means of the electric heater via the electrodes
301, 301'. In this way a shorter heating time can be achieved than
if the storage site 100 is merely heated by means of the electric
heater or by means of steam being applied. The electric heater can
in particular be an alternating current heater, as a result of
which no migration of ions occurs within the storage site 100.
[0037] By heating the storage site 100 or at least parts of the
storage site 100 it is possible to reduce the liquid content of the
storage site 100 and thus to reduce the electrical conductivity of
the storage site 100. A decreasing conductivity of the storage site
100 means that the electric heater, in particular the resistive
method of operation, becomes less efficient. To counter this loss
of conductivity, hot steam forced into the storage site 100 can be
enriched with minerals, in particular with salts. Furthermore, by
controlling the enrichment of the hot steam with minerals, in
particular with salts, the conductivity of the storage site 100 can
be selectively adjusted. The minerals or salts are thereby added to
the hot steam after it exits from the steam generator. On the other
hand, when the bitumen is extracted at a location, no conductivity
is required at this location either. In particular the inductive
mode of working means that losses only occur where conductivity
exists, in other words the depth of penetration expands accordingly
and the bitumen present in the case of natural conductivity is
heated and flows down by force of gravity with the naturally
occurring reservoir electrolyte. Naturally initially the loss
mechanism at the electrode will be at its most effective as a
result of resistive as well as inductive working; in other words at
the start of extraction the bitumen is made more free-flowing
there. If the conductivity at the electrode is low, there is no
contact between electrode and reservoir; the inductive mechanism
now comes into its own, as it does not rely on the electrical
contact between electrode and reservoir.
[0038] FIG. 4 shows a device for in situ extraction of a substance
comprising hydrocarbons from a storage site 100. According to the
exemplary embodiment illustrated in FIG. 4 at least parts of the
injection pipelines 100, 101' are used as electrodes of the
electric heater. In particular those parts of the injection
pipelines 100, 101' can be used as electrodes 301, 301' of the
electric heater which run essentially horizontally within the
storage site 100. The parts of the injection pipelines 100, 101'
acting as electrodes 301, 301' heat at least parts of the storage
site 100 both resistively and inductively. In particular the
electrodes 301, 301' heat areas 303, 303' which extend in
cylindrical fashion around the injection pipelines 101, 101'. In
addition to these inductively heated areas 303, 303' the dead
volume 202 is in particular heated in resistive fashion. For the
purpose of the electric heater the injection pipelines 101, 101'
are connected to an alternating current source 302.
[0039] FIG. 5 shows a device for in situ extraction of a substance
comprising hydrocarbons from a storage site 100. The exemplary
embodiment shown in FIG. 5 comprises two production pipelines 102,
102' which lead out of the storage site 100. Two electrodes 301,
301', each of which is disposed in a guide pipe 501, 501', project
into the storage site 100. The guide pipes 501, 501' can be
accessed from the surface and the parts running outside the storage
site or the areas of the storage site 100 to be heated are
additionally electrically insulated against the soil by electrical
insulations 304, 304'. The guide pipes 501, 501' can be accessed
from the earth's surface for a liquid and have areas permeable to
liquid in certain areas within the storage site 100. This can for
example entail porous designs of the pipe walls or openings or
holes.
[0040] In that liquid, preferably a liquid which is enriched with
an electrolyte to improve the conductivity, is selectively
introduced into the storage site 100 or parts of the storage site
100, the electrical conductivity of the storage site 100 can be
selectively adjusted. In this way the functional capability of the
electric heater can be ensured. As a further measure to ensure the
functional capability of the electric heater the ends of the
electrodes 301, 301' which face away from the power source 302 can
be short-circuited using an electrical bridge 502.
[0041] With the exemplary embodiment illustrated in FIG. 5 it is
possible to exploit a bitumen storage site without using hot steam.
The storage site can be heated merely by means of the electric
heater in inductive and resistive fashion, and the liquid bitumen
present in the production pipeline 102, 102' can be recovered by
means of a lifting pump or is conveyed to the surface by natural
geological overpressure. As an option, hot steam can be applied to
the storage site 100 at intervals over the production pipelines
102, 102', so that the pressure within the storage site 100 rises.
The overpressure created in this way can likewise be used to convey
bitumen from the storage site 100.
[0042] The electrodes 301, 301' can continue to run within the
storage site 100 such that the distance between them decreases with
the increasing length of the electrodes 301, from the perspective
of the power source 302. In particular the distance between the
electrodes 301, 301' can decrease linearly. In this way it is
possible to prevent the electric heat output, in particular the
resistive electric heat output from the perspective of the power
source 302, from being introduced into the storage site 100 at the
start of the electrodes 301, 301' or dropping away into this area.
The distance between the electrodes 301, 301' can in particular be
selected such that a continuous heat output over the length of the
electrodes 301, 301', in particular over the length of the subareas
of the electrodes 301, 301' which run within the storage site 100,
can be achieved by taking account of the electrical conductivity of
the storage site 100.
[0043] The distance between the boreholes can in this case be
controlled using generally known measures; for example a
transmitter can be operated in the first borehole, with the drill
head of the second borehole being able to determine the distance
from the first borehole on the basis of this transmitted
signal.
* * * * *