GB2067276A

GB2067276A - Process for the Development of Underground Store for Fluid

Info

Publication number: GB2067276A
Application number: GB8100673A
Authority: GB
Original assignee: GEOSTOCK
Current assignee: GEOSTOCK
Priority date: 1980-01-11
Filing date: 1981-01-09
Publication date: 1981-07-22
Also published as: CA1178811A; SE428200C; JPS56101494A; CH651523A5; SE428200B; ES498414A0; GB2067276B; ES8204047A1; SE8008893L; FR2473618B1; FR2473618A1

Abstract

A method of storing fluid products in underground cavities in water- saturated permeable rock strata which avoids any leakage of stored fluid into the strata as a result of flow of water towards the cavities requires the cavities to be subject at their roof to a hydrostatic load which is greater than the sum of the fluid storage pressure and a shape parameter dependent on the geometry of the cavity. The shape parameter may be determined theoretically or experimentally on a model. <IMAGE>

Description

SPECIFICATION Process for the Development of Underground Store for Fluid The present invention concerns the technique of underground storage of liquid and gaseous products, especially compressed liquefiable gases and, in particular, hydrocarbons.

Development of this kind of store involves hollowing out one or more cavities of any suitable shapes and dimensions with a view to the intended application and the nature of the terrain, or preparing existing natural or artificial voids, such as mines and quarries. The store is situated at a certain depth below the level of the watertable in a more or less permeable, water-saturated terrain.

Figure 1 diagrammatically represents such a store in vertical section across the terrain. Line 1 represents the soil level, line 2 the level of the water-table, with a noticeable depression 2' above the storage cavity 3. Line 4 diagrammatically indicates a possible very permeable zone- natural or artificially developed by means of boreholes, which can be replenished-situated above the store, in which the capacity of the bed is found to be clearly restored. Line 4' diagrammatically indicates long distance sources of supply for the bed. The store contains a liquefied gas 5, with gas 6 above the surface 5' of the liquid. There is water 7 in the lower part, generally in a draining-well 8.

It has been hitherto accepted that a height H between the top of the cavity and the bottom of the water-table, at least equal to the pressure P of the gas in the cavity, expressed as water level, was required for preventing percolation of the product stored into the terrain, i.e. for preventing leakage. The difference was regarded as a safety margin.

It has been recently realised-see, in particular, the paper of B. Aberg at Meeting 4 of ROCKSTORE, Stockholm, from 5th to 8th September 1 977-that the maximum pressure that can be tolerated in a cavity-above which there is leakage-is lower than the hydrostatic load H on the roof of the store.

This problem is also mentioned in other documents:- Swiss Patent Specification 404 548 described the development of an artificial water-table and indicates that the pressure of the water must be higher than that in the store, and no more.

United States Patent Specification 2 661 062 indicates that the pressure of the fluid has to be kept lower than that of the water.

United States Patent Specification 2 976 691 lays down hydrostatic considerations for an open air store, in which it is accepted that the product stored penetrates into the wall, which is contrary to the object of the present invention.

In order to ensure tightness, a solution might consist in having a large safety margin. First, however, this does not give absolute certainty and causes an increase in the costs of development and application of storage. From the economic angle, it is important to have a minimum safety margin.

The Applicants have carried out measurements on a mock-up and carried out tests on models.

They have observed that for a store to be tight at a given pressure P, the hydraulic load H had to be greater than P+F, F being called a shape parameter.

Other aspects of the invention will become clear in the course of the following description, given without impiying any limitation, with regard to the drawings attached hereto, in which: Figure 1 represents an underground store in diagrammatic section; Figure 2 is a graph showing the maximum tolerable pressure Pmax in relation to the water level H; and Figures 3 and 4 respectively represent in elevation and in section, along the line IV-lV of Figure 3, an experimental device for the simulated hydraulic measurement of the shape factor.

Reference will now be made to Figure 2, which is a graph showing the maximum tolerable pressure Pmax in relation to the height H for a cavity in the shape of a horizontal cylinder, assumed to be infinite-circular section---of 10 m radius, situated in an infinite homogeneous rock massif, having a very permeable surface. By carrying out measurements with different heights H, it is possible to trace the curve shown, which, in this case, deviates by up to 28% from the bisecting line, which would correspond to Pmax=H.

The Applicants have carried out tests with cavities of different shapes and variable number, conditions having different limits, in the presence or otherwise of water-curtains,-natural or artificial-above or between the cavities, having different capacities and different products stored.

The Applicants have then accepted-- and the process according to the invention is based on this-that the difference, F=H-P, depends on the combination of these parameters. For a given product stored, F is a function of the shape of the cavities and of their environment. The term, shape parameter, has been kept for this factor.

In accordance with the present invention, the depth is now being defined by the relationship: H=Pmax+F+S in which P is the maximum pressure in storage operation, F the shape parameter, which may be determined by coordinate geometry calculation, numerical model, simulated model or by test on a mock-up, and S is the actual safety margin, in which case the simulated model may be, for example, hydraulic or electric.

In practice, the shape and the number of the cavities, their depth and the geometry of possible water curtains, serving to replenish the bed, are defined, on the one hand, in relation to these tightness criteria, while maintaining the safety margin required in relation to future conditions of application, and, on the other hand, on the basis of the geotechnological and economic requirements and of the facilities for development.

An optimum design arises from this.

Thus an object of the invention is a process enabling liquid, liquefied or gaseous products to be stored with or without pressure in underground cavities, specially created for this purpose, or in existing natural or artificial voids, developed in water-saturated, permeable rock massifs, so as to avoid any leakage of stored product into the massif, owing to a permanent flow of water towards the cavities, which is ensured by observing the following condition:- H > P+F H: hydrostatic load on the roof of the cavities.

P: storage pressure.

F: shape parameter.

F is determined theoretically-coordinate geometry calculation, numerical or simulated model- and/or experimentally-test on mock up- and, for a given product stored, depends on the geometry of the cavities and of their environment-in particular, the possible presence of water curtains.

This condition is applied with a certain safety margin, chosen with regard to the conditions of storage application and to the hydrogeological setting.

This kind of condition enables an optimum design to be chosen for a store consisting of cavities to be formed, absence of leakages being ensured, or the maximum tolerable pressure to be defined in an existing void, converted into a store.

It makes it also possible to judge the need or otherwise of providing water-curtains, working out their dimensions and defining the hydraulic potential to be given to them.

Figures 3 and 4 represent a simulated hydraulic device. A thin sheet of liquid, having a thickness of half a millimetre, for example of water, preferably coloured, is used, arranged between two transparent plates 11, 12, made of glass or plastics. It is known that, owing to the capillarity phenomenon, a device of this kind is equivalent to a permeable system. The circumference is closed on the sides 13, 14 and possibly also at the base 15, so as to represent a tight deep layer. It is also possible to let water pass through a filter, limiting the flow rate. A hopper 17 is provided on the upper part.A profile, corresponding to the section of the intended cavity, for example circular, is cut into the two walls 11, 12 and a pocket 18 is arranged in it, having a corresponding shape, in which pressure of gas, for example of air, is maintained by a connection 1 9.

In the absence of pressure, water runs into the, pocket 18, from where it is drawn off by a drainage 20. This is shown in the left-hand half of Figure 3. In order to determine the shape factor, the pressure in the pocket 18 is allowed to rise gradually, until bubbles are liberated on top between the two sheets 11 and 12. This is shown in the right-hand half of Figure 3. The pressure, at which bubbles appear, is noted and it is observed that there is a difference from the water level H, between the upper level of the water and the high point of the pocket. This difference is the shape parameter, which is evidenced in pressure. By repeating the experiment with cavities having different sections, different parameters are found-for example, rhomb or square with the angle facing upwards or horizontal rectangle etc.

Knowledge of this kind of parameter affords a considerable advantage in this technique, both from the point of view of safety and from the economic point of view; there is a guarantee of a store without danger at the lowest cost.

Claims

1. Process enabling liquid, liquefied or gaseous products to be stored with or without pressure in underground cavities, specially created for this purpose, or in existing natural or artificial voids, developed in water-saturated, permeable rocks massifs, so as to avoid any leakage of stored product into the massif, owing to a permanent flow of water towards the cavities, which is ensured by observing the following condition:- H > P+F H: hydrostatic load on the roof of the cavities.

P: storage pressure.

F: shape parameter.

F is determined theoretically-coordinate geometry calculation, numerical or simulated model-and/or experimentally-test on mock upland for a given product stored, depends on the geometry of the cavities and of their environment-in particular, the possible presence of water-curtains.

2. Process enabling liquid, liquefied or gaseous products to be stored in underground cavities, substantially as hereinbefore described with reference to the accompanying drawings.