Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns a process for excavating by
dissolution, an underground cavity in a thin salt
layer, for example an underground salt layer.
More particularly, the invention is concerned
especially with obtaining, after excavation, a cavity
for the storage of a fluid, in particular natural gas,
in a salt cavern obtained after the dissolution
process.
In view of the stresses to which the cavity has to be
subjected during its use as an underground gas
storage, the dissolution process has to be controlled
in order to ensure that the final cavity has a
mechanically stable shape.
Although it is relatively easy to control dissolution
when the thickness of the available salt is several
hundred meters thick or so, this operation is more
awkward when the salt is stratified and of more
reduced thickness. In fact, even when the thickness of
the salt available is only a few hundred metres thick,
it becomes necessary to apply specific cavern
dissolution processes to develop cavities whose width
over height ratio is of the order of 1. When the salt
thickness is less than about 100 metres, new salt
dissolution processes are needed. The invention
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concerns a process for developing a tunnel-shaped
cavern in such a thin salt layer.
Certainly, US-A-5 246 273 discloses for the
excavation of a salt layer by dissolution:
- producing an injection duct, an extraction duct, a
void for a communication space which places the
injection and extraction ducts in communication, and
at least one void for a blind tunnel such that:
. the blind tunnel extends between an
open end and a closed end; and
. the blind tunnel communicates via
its open end with the communication
space;
- then injeating via the injection duct a salt
solvent into the communication space in order to
excavate the cavity by dissolution of the salt on
contact with the solvent; and
- then extracting via the extraction duct the brine
formed by the dissolution of the salt.
In this document, US-A-5 246 273, the communication
space and the blind tunnel are excavated as an
extension of each other, sloping downwards from the
injection shaft such that the closed end of the
injection shaft is lower than its open end. The blind
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tunnel is an optional sump designed to recover the
insoluble elements deposited therein.
Consequently, the technique applied in US-A-5 246 273
is costly since, related to the volume of the final
cavity with a mechanically stable shape, the number
of production operations is relatively high (in
particular producing the ducts and the void for the
communication space).
In order to overcome these problems, the invention
proposes that, to excavate the cavity, the blind
tunnel also be excavated by circulating the solvent
in this tunnel, making the solvent pass into the
tunnel via its open end from the communication space
and recovering the resultant brine so that it can be
extracted therefrom.
As will be appreciated, a blind tunnel is a tunnel
which is distinct from the communication space and
has a single end communicating with said space.
The advantage of this solution is in particular that
it permits a varied range of cavity forms and allows
the cavern to be worked in numerous directions.
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Consequently this solution enables the volume of the
final cavity to be substantially increased without
having to move the injection or extraction ducts,
whilst preserving a certain degree of mechanical
stability. The cost of the excavation operation
related to the volume of the cavity is therefore
reduced.
It is thus highly advantageous to increase the number
of blind tunnel voids connected to the communication
space.
This solution is all the more noteworthy since it has
an unexpected effect. Indeed, a blind tunnel a priori
does not encourage the intake and circulation of the
solvent, even less so the mixing of the solvent and
dissolution of the salt. However, this is what happens
in practice.
Throughout the description the term "void" designates
the initial state of a space or tunnel (before the
solvent dissolves the salt). It could correspond to a
preliminary borehole.
According to one preferred embodiment of the
invention, in order to improve dissolution in the
blind tunnel, the closed end thereof is disposed at a
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level which is higher than or substantially the same
as that of the open end.
Since the density of the brine is higher than that of
the solvent, when the closed end of the blind tunnel
is disposed at a lower level than that of the open
end, the brine tends to stagnate at the closed end of
the blind tunnel. Therefore the solvent no longer
circulates in the blind tunnel and the excavation
thereof by dissolution tends to stop. This phenomenon
is all the more notable, the more pronounced the slope
and the longer the blind tunnel.
In order to improve further the dissolution action in
the blind tunnel, at the point of connection between
the communication space void and the blind tunnel void
(which point could a priori be disposed anywhere along
the communication space void between the two ducts),
the open end of the blind tunnel void may be produced
such that it overhangs the communication space.
A variant likewise enabling the dissolution action in
the blind tunnel to be improved may involve:
- forming the open end of the blind tunnel void in the
vicinity of the injection duct;
- circulating the solvent in the injection duct and
making it emerge from this duct via an end which forms
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the point of injection into the cavity; and
- forming the blind tunnel void such that the part
with its open end overhangs the injection point.
According to another embodiment of the invention, in
order to improve further the dissolution in the blind
tunnel:
- the injection and extraction ducts are produced at
a spacing from each other;
- an elongate part is provided in the communication
space void; and
- the void for the blind tunnel and the elongate part
of the communication space void are produced
substantially as an extension of each other, and the
injection duct is produced between the blind tunnel
void and the elongate part of the communication space.
On the other hand, if the cost of producing a cavity
is to be reduced or the range of forms produced
increased, the injection and extraction ducts may be
disposed substantially coaxially, such that one of
these ducts is located in the centre and is surrounded
by the other, at least over part of its axial length.
In this case only one hole has to be excavated in
order to produce the two ducts.
The invention will be understood more clearly from the
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following description of preferred embodiments which
is given by way of example only and with reference to
the appended drawings, wherein:
~igure 1 shows in section a first form of a
cavity void made in a salt layer;
~igure 2 shows in section a first cavity
form obtained from the void of Figure 1
after excavation by dissolution of
some of the salt present in the salt
layer;
~igure 3 shows in section a second cavity form
obtained after excavation by
dissolution of some of the salt
present in the salt layer; and
~igure 4 shows in section a third cavity form
obtained after excavation by
dissolution of some of the salt
present in the salt layer.
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Figure 1 shows a salt layer 1 in stratified form and
comprised between two layers of other minerals
present in the ground 10. An injection duct 16 and an
extraction duct 18 are disposed in two shafts, after
excavation thereof, substantially vertically between
ground level and a cavity void 1a excavated in the
salt layer 1.
These ducts have one end 16a, 18b located at ground
level and one end 16b, 18a located in the cavity void
1a. The ends 16b, 18a located in the cavity void 1a
are connected by a communication space void 4. In
this case, the communication space void 4 has been
drilled proceeding from the duct 18. The drilling
axis 13 is shown in broken lines. The diameter of the
void is approximately 6 cm and preferably less than
10 cm.
The communication space void 4 has an elongate,
substantially rectilinear and horizontal part 4c
between an end part 4b, connected to the extraction
duct 18, and a further end part 4a surrounding the
injection duct 16 over part of its length, in the
vicinity of its end 16b which forms a point of
injection into the cavity.
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A void 2 for a blind tunnel is produced by extending
the elongate part 4c of the communication space void
4 beyond the injection duct 16. This blind tunnel
void 2 therefore comprises a closed end 2b and an
open end 2a communicating with the communication
space void 4. The open end 2a overhangs the injection
point 16b, such that the open end 2a is disposed in
the vicinity of the injection point 16b and the lower
part of the open end 2a is higher than the injection
point 16b.
Here, the void 4c of the elongate part of the
communication space and the blind tunnel void 2 are
substantially horizontal. They could also be slightly
inclined. In this case, owing to the higher density
of the brine relative to the water injected, the open
end of the blind holes must of necessity be lower
than the closed end and, likewise, the elongate part
of the communication space must be lower at the
extraction duct end than at the injection point end.
In Figure 2 a solvent (in this case water) is
injected as shown by the arrow 17 via the end 16a of
the injection duct 16. The water emerges from the
duct 16 at the injection point 16b where it is
injected into the end 14a of the communication space
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1 0
as illustrated by the arrows 15a. This communication
space, like the cavity as a whole, is entirely or at
least almost entirely filled with water and brine.
Since the water injected is less dense than the
brine, it rises to the upper part of the cavity lla.
The water is injected at a pressure which is slightly
greater than the pressure prevailing in the cavity.
The injected water circulates from the end 14a of the
communication space towards the elongate part 14c of
the communication space 14 and the blind tunnel 12.
The water is not introduced owing to the pressure at
which it is injected into the blind tunnel but under
the effect of a flow created by the dissolution of
the salt.
As illustrated by the arrows 15c, 15e, the water
filling this cavity excavates the salt layer by
dissolving the salt such that, after excavation, the
communication space and blind tunnel voids 4, 2 form
the communication space 14 and the blind tunnel 12,
of which the widths, lengths and heights are greater
than their respective void versions. The salt-charged
water forming brine then circulates in the lower
parts of the communication space 14 and of the blind
tunnel 12, as shown by the arrows 15d, 15f.
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The brine passes from the blind tunnel 12 towards the
elongate horizontal part 14c of the communication
space 14, such that the brine formed in the blind
tunnel is recovered in the communication space so as
to be extracted therefrom.
By applying an injection pressure which is greater
than the extraction pressure, the brine circulates
from the elongate part 14c of the communication space
14 towards the duct 18 via the end 14b of the
communication space 14, as shown by the arrow 15b.
The blind tunnel 12 communicates with the
communication space 14 solely via the open end, which
enables the blind tunnel to be flooded with water by
introducing water thereinto and recovering the brine
formed by dissolution of the salt in the blind
tunnel.
In Figure 3 the parts corresponding to those in
Figure 2 are denoted by the same number increased by
10. This Figure essentially differs from Figure 2 in
that the injection duct 26 and extraction duct 28 are
coaxial. Since the injection duct 26 descends the
furthest in the cavity 21a, it is located on the
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interior and the extraction duct 28 is located on the
exterior. The extraction duct 28 surrounds the
injection duct over the major part of its length.
Owing to this particular configuration, the
communication space here no longer has a horizontal
elongate part and is produced entirely around the
injection duct 26 between the ends 26b, 28a of the
injection and extraction ducts. The communication
space void can easily be drilled at the same time as
the extraction duct is drilled. The axis 23 of the
blind tunnel void is shown in broken lines.
Water and hence brine are circulated by means of a
compression pump 30 which injects water under
pressure into the injection duct 26 as indicated by
the arrow 27.
In Figure 4 the parts corresponding to those in
Figure 3 are denoted by the same number increased by
10. This Figure essentially differs from the
preceding Figure in that a second blind tunnel void
43 is provided. This Figure illustrates the
production of multiple blind tunnels 32, 42 in fluid
communication with the communication space 34 via
their open ends 32a, 42a.
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The water injected via the injection point 36b rises
since it is less dense than the brine present in the
cavity and since the injection point is located below
the open ends 32a, 42a of the blind holes. The water
is then distributed between the various blind tunnels
and excavates them, becoming charged with salt as it
does so. The resultant brine then tends to descend in
the end 34a of the communication space, as
illustrated by the arrows 35c. In practice, the flow
35a of injected water guides the brine towards the
end 38a of the extraction duct 38, as illustrated by
the arrows 35b. The brine is then extracted via the
extraction duct 38.
In this Figure 4 the water and hence the brine are
circulated by means of a suction pump 40 which draws
in the brine by means of the extraction duct 38, as
illustrated by the arrow 39.
It will be appreciated that the invention is in no
way restricted to the embodiments described above.
The injection and extraction ducts could be inverted,
for example, without thereby modifying the invention.
