GB2460550A

GB2460550A - Gas storage in salt caverns

Info

Publication number: GB2460550A
Application number: GB0909735A
Authority: GB
Inventors: Andrew Hindle
Original assignee: PORTLAND GAS PLC
Current assignee: PORTLAND GAS PLC
Priority date: 2008-06-06
Filing date: 2009-06-05
Publication date: 2009-12-09
Also published as: GB0909735D0; GB0810365D0

Abstract

A gas storage method, and associated apparatus, features the storage of gas, such as natural gas to be used as commercial and domestic fuel, in an underground or sub surface salt 10 cavern 9. A fluid 12, which may be a cushion fluid 12, is pumped in and out of the cavern 9 to maintain a balance of pressure, during periods of differing demand for the gas. The fluid 12 may be a saline solution, which may be pumped to and from a porous aquifer 22, or surface supply of water (22', fig.3).

Description

I

GAS STORAGE METHOD AND SYSTEM

FIELD OF THE INVENTION

The present invention relates to an improved method and system for the storage of gas within salt caverns and, in particular, for the storage of natural gas in such caverns.

BACKGROUND OF THE INVENTION

Sufficient reserves of natural gas are often stored in a particular locality to meet a peak demand for gas in the locality such as that occurring during a prolonged period of cold weather. This is particularly true in the absence of a source of gas in the locality. For any gas storage system it is desirable that gas is stored or supplied at a pressure that is maintained within acceptable limits. To accomplish this, conventional gas storage systems require that a reserve of gas known as cushion gas" remains in the gas store to maintain the gas pressure above an acceptable minimum threshold level.

SUMMARY OF THE INVENTION

: According to a first aspect of the present invention there is provided a gas storage " 20 method comprising: *:*:: storing a gas in a sub-surface salt cavern; retaining a reserve of gas in the cavern during a first period; *:*::* moving fluid into the cavern and removing the reserve of gas from the cavern *:*. to maintain a pressure of the gas in the cavern during a second period; and replacing the reserve of gas in the cavern and moving fluid out of the cavern to maintain the pressure of the gas in the cavern once the second period has passed.

A gas demand level during the first period may be lower than the gas demand level during the second period. For example, the gas demand level during the first period may be less than a predicted maximum gas demand level and the gas demand level during the second period may be greater than the predicted maximum gas demand level.

Moving a fluid into or out of the cavern for compensation of the gas pressure has the advantage that the reserve of gas, which may be known as "cushion gas" and which is routinely maintained in the cavern under normal gas demand levels, may be removed from the cavern in periods of acute gas demand. When the demand for gas falls, a new reserve of cushion gas' may be stored in the cavern and the fluid moved out of the cavern. Thus, the effective storage capacity of the cavern is increased and the cushion gas can be viewed as a "strategic" asset available at times of acute need for the supply of gas to markets.

Preferably, the method comprises maintaining the pressure of the gas in the cavern during the second period between a pie-determined lower gas pressure limit and a pre-determined upper gas pressure limit. The pie-determined upper and lower gas pressure limits may, for example, comprise acceptable upper and lower gas pressure limits.

The fluid may be water having a salinity below a saturation level. In particular, the fluid may be seawater or freshwater. Since seawater and freshwater *,. : are two of the most abundant fluids on earth, it is desirable to use such fluids for gas

S

pressure compensation. When salt from the cavern is at least partially soluble in the *:*:: fluid, however, some dissolution of the salt cavern is to be expected when the fluid is moved into and/or out of the cavern. The cavern must, therefore, be designed to account for such dissolution. * S.

*:*. Alternatively, the fluid may be water that is completely saturated with salt.

The fluid may, for example, be brine. This has the advantage that dissolution of the salt cavern should be minimised.

Preferably, the cavern is designed so as to withstand a predetermined number of gas reserve removal/replacement cycles without any significant degradation in a gas storage capability of the cavern. A gas reserve removal/replacement cycle comprises the steps of moving fluid into the cavern and moving the reserve of gas out of the cavern so as to maintain the pressure in the cavern within pre-determined limits, moving the reserve of gas back into the cavern and moving fluid out of the cavern so as to maintain pressure in the cavern within the pre-determined limits. To preserve the integrity of the cavern, the cavern should be proportioned and/or located so as to allow a predetermined degree of dissolution.

Since the cavern is only designed to tolerate a finite number of gas reserve removal/replacement cycles, it is, however, envisaged that the reserve of gas, would only be used, and the cavern pressure maintained within acceptable limits using such a fluid in this way, on an infrequent basis.

Preferably, the fluid is transferred to/from the cavern from/to a reservoir of fluid. The reservoir of fluid may, in particular, comprise a body of fresh water such as a lake or a body of salt water such as the sea.

Alternatively, the reservoir of fluid comprises fluid stored in at least a portion of a porous sub-surface layer.

Preferably, the gas is transferred directly into or out of a gas-containing portion of the cavern and the fluid is transferred directly into or out of a fluid- : containing portion of the cavern. Thus, intermixing of the gas and the fluid is minimised.

*:*:,* More preferably, the gas enters the cavern through a gas pipe that terminates at a position towards an uppermost point of the gas-containing portion of the cavern * and the fluid enters the cavern through a fluid pipe that terminates at a position S. * : towards a lowermost point of a fluid-containing portion of the cavern. Due to the relative densities of the gas and the fluid, the gas will naturally reside above the fluid in the cavern so that this arrangement avoids gas having direct contact with the fluid as the gas passes into the cavern or the fluid having direct contact with the gas as the fluid passes into the cavern.

Preferably, the gas comprises natural gas.

According to a second aspect of the present invention there is provided a gas storage system configured to implement the gas storage method of the first aspect of the present invention.

Preferably, the gas storage system comprises a fluid pump and a gas compression plant in which the fluid pump is operable so as to move the fluid into the cavern and the gas compression plant is operable so as to move the gas into the cavern.

The gas storage system may comprise a gas shutoff valve and a fluid shut-off valve in which the gas shut-off valve is operable so as to control a flow of gas into or out of the cavern and the fluid shut-off valve is operable so as to control a flow of fluid into or out of the cavern.

The gas storage system may comprise a sub-surface salt cavern, a gas conduit and a fluid conduit in which the gas conduit and the fluid conduit extend from ground-level to the cavern.

The gas conduit may comprise a gas pipe located within a first borehole and the fluid conduit may comprise a fluid pipe located within a second borehole. * * * ** S S.,.

BRIEF DESCRIPTION OF THE DRAWINGS

**5.* The present invention will be further described by way of non-limiting example only with reference to the following figures of which: * Figure 1 schematically illustrates a gas distribution system; *..: Figure 2 schematically illustrates a first embodiment of a gas storage system; and Figure 3 schematically illustrates a second embodiment of a gas storage system

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 shows a gas distribution system comprising gas sources 2 and 3 connected to gas sinks 4, 5 and 6 via a gas distribution network 7. The gas sinks 4, and 6 may, for example, comprise industrial or domestic premises where gas is consumed by combustion. At least one of the gas sources 2 and/or 3 may be remotely located from the gas sinks 4, 5 and 6. At least one of the gas sources 2 and/or 3 may, for example, be located in a foreign country such that the supply of gas from the gas sources 2 and/or 3 to the gas sinks 4, 5 and 6 involves importation of the gas. To allow for a mismatch between the rate of gas supply from gas sources 2 and 3 and the rate of gas usage at the gas sinks 4, 5 and 6, a gas storage system 8 may also be connected to the gas distribution network 7. The gas storage system 8 is capable of storing gas from the gas distribution network 7 and subsequently supplying the stored gas to the gas distribution network 7.

A first embodiment of the gas storage system 8 is shown in Figure 2. The gas storage system 8 comprises a gas storage cavern 9 formed in a salt medium 10.

The salt medium 10 may, for example, comprise a sub-surface layer of rock salt.

The cavern 9 contains a gas 11 and a fluid 12. A first borehole 13 extends from ground-level and enters the cavern 9 at a first aperture 14. In addition, a second : borehole 15 extends from ground-level and enters the cavern 9 at a second aperture p...

16. A gas pipe 18 for the transfer of gas to\from the cavern 9 passes through the P..... first borehole 13, enters the cavern 9 through the first aperture 14 and connects the cavern 9 to the gas distribution network 7. A fluid pipe 20 for the transfer of fluid to\from the cavern 9 passes through the second borehole 15, enters the cavern 9 through the second aperture 16 and connects the cavern to a fluid reservoir 22.

In the embodiment shown in Figure 2, the fluid reservoir 22 comprises at least a portion of a sub-surface layer of porous rock known as a porous aquifer. The porous aquifer is capable of storing relatively high volumes of a fluid. The porous aquifer 22 may be located adjacent to or remotely from the cavern 9. In the embodiment shown in Figure 2, for example, the porous aquifer 22 is located remotely from the second borehole 15 and is connected to the cavern 9 via a ground-level section 24 of the fluid pipe 20. In addition, a fluid reservoir pump 25 may be used to pump the fluid 12 to or from the fluid reservoir 22. The fluid reservoir pump 25 may, for example, be located downhole between ground-level and the porous aq uifier.

The gas storage system of Figure 2 also comprises a gas compression plant 26 for pumping the gas 11 into the cavern 9 and a fluid pump 28 for pumping the fluid 12 into the cavern 9. The gas compression plant 26 is capable of controlling the pressure of the gas 11 and may be capable of controlling the water content and temperature of the gas 11. Similarly, the fluid pump 28 may be capable of controlling the composition and temperature of the fluid 12. Although the gas compression plant 26 and the fluid pump 28 are shown as being located at ground level in Figure 2, it should be understood that the gas compression plant 26 may be located anywhere along the gas pipe 18, and the fluid pump 28 may be located anywhere along the fluid pipe 20. The gas storage system 8 further comprises a gas shut-off valve 34 for the control of gas flow along the gas pipe 18 between the gas compression plant 26 and the cavern 9 and a fluid shut-off valve 36 for the control of fluid flow along the : fluid pipe 20 between the cavern 9 and the fluid pump 28. The gas shut-off valve 34 * IL. 20 and the fluid shut-off valve 36 may form part of a wellhead structure (not shown) *...:. In use, the fluid 12 is transferred into or out of the cavern 9 so as to maintain the pressure of the gas 11 stored in the cavern 9 within acceptable limits. The gas 11 stored in the cavern may, for example, be a reserve of gas known as "cushion gas". Compensation of the gas pressure may be complete such that the gas pressure before the transfer of the fluid 12 remains substantially unchanged after the transfer of the fluid 12. Alternatively, compensation of the gas pressure may be partial, in which case the gas pressure remains between a predetermined upper pressure limit and a predetermined lower pressure limit. When the gas 11 comprises a reserve of "cushion gas" which is to be removed during, for example, a period of acute gas demand, the fluid pump 28 moves the fluid into the cavern 9 and, in response, gas is moved out of the cavern 9 to maintain the pressure of the gas in the cavern within acceptable limits. Gas may be moved out of the cavern 9 at the same time as the fluid is moved into the cavity or at some later time depending on when the gas shut-off valve 34 is opened. Similarly, when the reserve of cushion gas" is to be replaced at, for example, the end of the period of acute gas demand, the gas compression plant 26 moves gas into the cavern 9 and, in response, the fluid is moved out of the cavern 9 to maintain the pressure of the gas in the cavern within acceptable limits. The fluid may move out of the cavern 9 at the same time as the gas is moved into the cavity or at some later time depending on when the fluid shut-off valve 36 is opened.

The gas storage system 8 further comprises a first gas safety valve 37 located along the gas pipe 18 between the gas shut-off valve 34 and the cavern 9. In addition, the gas storage system 8 further comprises a second gas safety valve 38 located along the fluid pipe 20 between the fluid shut-off valve 36 and the cavern 9.

The gas safety valves 37 and 38 are located below ground level and can be activated in an emergency to isolate the gas 11 stored in the cavern 9 The gas safety valve ie 38 can be activated in the event that gas enters the fluid pipe 20.

In general, when the gas 11 is being removed or replaced, the cavern 9 will comprise a gas-containing portion disposed above a fluid-containing portion. The gas pipe 18 enters the gas-containing portion of the cavern 9 while the fluid pipe 20 enters the fluid-containing portion of the cavern 9. As shown in the embodiment of Figure 2, the gas pipe 18 terminates at a position towards an uppermost point of the gas-containing portion of the cavern 9 and the fluid pipe 20 terminates at a position towards a lowermost point of the fluid-containing portion of the cavern 9. This arrangement serves to minimise the degree of intermixing between the gas and the fluid.

It should be understood that when transferring the fluid 12 into and/or out of the cavern 9, some dissolution of the cavern 9 will occur if the material from which the cavern 9 is formed is at least partially soluble in the fluid 12. To at least partially suppress such dissolution of the cavern, therefore, the fluid 12 may be chosen so that the cavern material is substantially insoluble in the fluid. The fluid 12 may, for example, be brine that is completely saturated with salt so as to minimise dissolution of the salt cavern 9 thus maximising the lifetime of the cavern 9 and the lifetime of the gas storage system 8. Since salt caverns such as cavern 9 are generally formed by dissolving the salt 10 using water that is not saturated with salt, the brine may be introduced into the cavern 9 when the cavern 9 is formed. Alternatively, water may be injected into a sub-surface layer of rock salt such as the sub-surface layer of rock salt 10 to provide a further cavern containing brine. Such a further cavern may be known as a donator cavern' and constitutes a reservoir of brine. In a further alternative, the brine may reside naturally in the fluid reservoir 22.

In the event that a supply of brine is not available, a fluid in which the salt 10 is at least partially soluble may be used. When using such a fluid, however, the cavern 9 should be designed so as to maintain the cavern volume within a pre-defined final volume. In particular, the mechanical integrity of the cavern 9 should be * * S preserved by choosing the size, geometry, and/or location of the cavern 9 within the 20 rock salt layer 10 so as to preserve a gas storage capability of the cavern 9 for a * (S predetermined number of "cushion gas" removal/replacement cycles. It is envisaged, however, that maintaining the pressure of the gas within acceptable limits * * using such a fluid would not be routinely used for gas storage. Instead, the "cushion gas" would only be withdrawn and a fluid injected, on an infrequent basis (e.g. in periods of extreme gas demand). Examples of such fluids in which the salt 10 is at least partially soluble include seawater and freshwater. Such fluids may be stored in the fluid reservoir 22.

A second embodiment of a gas storage system is shown in Figure 3. Many of the features of the gas storage system of Figure 3 are identical to features of the gas storage system of Figure 2 and are identified with the same reference numerals but suffixed "". In the embodiment of Figure 3, fluid from a fluid reservoir 22' may be used to maintain the gas pressure within acceptable limits. In contrast to the reservoir 22 of the gas storage system of Figure 2, the reservoir 22' is formed at ground-level. The fluid 12' stored in reservoir 22' may, for example, be brine, seawater or freshwater. The reservoir 22' may be located inland or may be the sea itself. The ground-level section of fluid pipe 24' may comprise a separate first pipe along which the pump 25' is located to take fluid from the reservoir 22' to the cavern 9', and a separate second pipe to take fluid from the cavern 9' to the reservoir 22'.

Finally, since the caverns 9 and 9' shown in Figures 2 and 3 respectively may be formed by the injection and withdrawal of a fluid in which the cavern material is at least partially soluble such as seawater or freshwater, maintaining the gas pressure within acceptable limits using such fluids may be achieved using at least some of the same infrastructure used to create the caverns 9 and 9' in the first instance. The cavern 9 or 9' may, for example, be formed by injecting and subsequently withdrawing such a fluid through the first borehole 13 and/or the second borehole 15. * S S... * S. * . . * .. * *S * . . * S.

SI S * * * * S.

Claims

CLAIMS1. A gas storage method comprising: storing a gas in a sub-surface salt cavern; retaining a reserve of gas in the cavern during a first period; moving fluid into the cavern and removing the reserve of gas from the cavern to maintain a pressure of the gas in the cavern during a second period; and replacing the reserve of gas in the cavern and moving fluid out of the cavern to maintain the pressure of the gas in the cavern once the second period has passed.
2. The gas storage method of claim 1 in which a gas demand level during the first period is lower than the gas demand level during the second period.
3. The gas storage method of claim 2 in which the gas demand level during the first period is less than a predicted maximum gas demand level and the gas demand level during the second period is greater than the predicted maximum gas demand level. *..*
4. The gas storage method of any of the preceding claims comprising * 20 maintaining the pressure of the gas in the cavern during the second period between a pre-determined lower gas pressure limit and a pre-determined upper gas pressure limit. * ** * * * * �S *. *

* .:
5. The gas storage method of any of the preceding claims in which the fluid comprises water having a salinity below a saturation level.
6. The gas storage method of claim 5 in which the fluid comprises brine, seawater or freshwater.
7. The gas storage method of any of the preceding claims comprising removing the reserve of gas from the cavern in response to moving the fluid into the cavern.
8. The gas storage method of claim 7 comprising removing the reserve of gas from the cavern at the same time as or at a later time after moving the fluid into the cavern.
9. The gas storage method of any of the preceding claims comprising moving fluid from the cavern in response to replacing the reserve of gas in the cavern.
10. The gas storage method of claim 9 comprising moving fluid from the cavern at the same time as or at a later time after replacing the reserve of gas in the cavern.
11. The gas storage method of any of the preceding claims comprising configuring the cavern so as to withstand a predetermined number of gas reserve removal and replacement cycles without any significant degradation in a gas storage capability of the cavern. S... * S *.
12. The gas storage method of claim 11 comprising proportioning or locating the cavern so as to allow a predetermined degree of dissolution of the cavity. * S.
13. The gas storage method of any of the preceding claims comprising moving fluid between the cavern and a reservoir of fluid.
14. The gas storage method of claim 13 comprising moving fluid between the cavern and a body of fresh water or a body of salt water.
15. The gas storage method of claim 13 comprising moving fluid between the cavern and a reservoir of fluid stored in at least a portion of a porous sub-surface layer.
16. The gas storage method of any of the preceding claims comprising moving gas out of or into a gas-containing portion of the cavern and moving fluid into or out of a fluid-containing portion of the cavern.
17. The gas storage method of any of the preceding claims in which the gas comprises natural gas.
18. A gas storage system configured to implement the gas storage method of any of the preceding claims.
19. The gas storage system of claim 18 comprising a gas compression plant and a fluid pump in which the gas compression plant is operable so as to move gas into the cavern and the fluid pump is operable so as to move fluid into the cavern.
20. The gas storage system of claim 18 or 19 comprising a gas shut-off valve and a fluid shut-off valve in which the gas shut-off valve is operable so as to control a flow *:*::* 20 of gas into or out of the cavern and the fluid shut-off valve is operable so as to control a flow of fluid into or out of the cavern. * **
21. The gas storage system of any of claims 18 to 20 comprising a sub-surface salt cavern, a gas conduit and a fluid conduit in which the gas conduit and the fluid conduit extend from ground-level to the cavern.
22. The gas storage system of claim 21 in which the gas conduit comprises a gas pipe located within a first borehole and the fluid conduit comprises a fluid pipe located within a second borehole. * ..* * * * ** * * .*S * * S... * .. * * S * .* * .* * * S * S. S. S * S S * *5