Abstract:

The paper discusses the hydrogeological design aspects and subsequent water curtain tunnel design of unlined rock caverns for the storage of hydrocarbons including the investigations and analysis necessary to prevent any flow of contaminants into the subsurface. The basic principle of such storage in unlined rock caverns is the hydraulic confinement. Thus, the rock caverns are planned at a depth such that there is sufficient hydrostatic pressure to counter the vapour pressure of the stored product. A water curtain system above the cavern ensures that the water flow is directed into the cavern, which prevents leakage of products into the surrounding ground. The paper presents the interaction between cavern units under different operating conditions. The influence of vertical water curtain system and presence of a circular tunnel between the cavern units is also presented. The design of water curtain system based on geological and hydrogeological considerations is also investigated in detail during the study. The occurrence of various joint sets in rock mass along with their orientation is found to have a significant impact.

1.0 INTRODUCTION

The ideas of protecting hydrocarbon products in underground storages have been in practice for a long time. Even during the First World War such ideas were practiced. However, it was only after the development of techniques of building caverns without any wall protection in the 1950"s, that the underground storage of crude oil, oil products and LPG became commercially attractive. The first mined cavern of only 3000 m3 capacity was completed in 1950 in a shale formation in Texas. The storage of hydrocarbons in today"s world is ensured by the application of a hydrodynamic criterion. The cavity is excavated, according to characteristics of the stored product, at a depth such that all the flows in the rock mass are directed towards the cavity so that any outward migration of the product is prevented. Therefore a storage system must be situated so deep below the natural ground water level that the hydrostatic pressure of the ground water exceeds the vapour pressure of the gas/vapour at its maximum temperature in the reservoir. However, strictly speaking in order to prevent any leakage of gas it is not sufficient to keep only the hydrostatic pressure of water above the vapour pressure of the gas, but a sufficiently strong flow of water in the downward direction in the fractures above the storage is also required. In 1971, a large scale storage plant for propane and butane was excavated in limestone in France, which for the first time used water curtain tunnel above the roof of the caverns in order to control the ground water pressure above storage system. It is well understood at this point of time that rock storage design is a very specific task as this is neither a mine nor a civil engineering structure. However, progress made by mining and civil engineers, and rock mechanics researchers is considered to be duly appreciable (Duffaut, 2004).

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