The use of solution-mined caverns for oil storage poses different design considerations than typically considered when using solution mining for mineral extraction only. The process nature of solution mining requires that the design be considered as a system with all system design aspects integrated to obtain optimum cost, schedule, and operational performance. Such optimization requires the use of cost-effective investigative programs combined with state-of-the-art predictive analyses and the use of tradeoff analyses to integrate system design aspects. Possible investigation programs are reviewed briefly relative to system design aspects and various means of performing tradeoff analyses are discussed. The combined use of cost benefit studies, risk evaluations, and formal matrix evaluation procedures are reviewed and an example of the use of an overall matrix evaluation of the optimum number of wells to be used for a given site-specific situation is presented as an illustration of the method.
The use of solution-mined caverns in salt domes for the storage of crude oil represents an important use of underground space as demands for strategic petroleum reserves increase. While solution mining technology is well developed for the mining of minerals such as salt and potash, the creation of underground caverns to be specifically used for oil storage poses new and different problems for the designer. Where the traditional emphasis of solution-mining technology was on the solutioning process itself to maximize mineral extraction, the entire solution-mining system must be considered for the creation of underground storage space where containment is more important than extraction. There are many factors which require consideration in the design of the solutioning, water handling and oil handling facilities which comprise the solution-mining system used for oil storage.
System aspects which are typically the most important are:
geometrical aspects of solutioning relative to possible directional preference due to bedding planes and the existence of zones of high and low solubility;
local cavern stability if solutioning does not provide the desired cylindrical shape;
the number of wells which should be used to develop the caverns considering the need for sonar measurements to examine cavern shape, the possibility of controlling solutioning direction and fluid handling and redundancy requirements of the system;
the reliability of water supply and brine disposal systems; and
whether to use a leach-fill or leach-then-fill method of cavern development.
A solution-mined cavern should be planned to optimize the entire system because of its process nature. Once the solutioning process commences, any failure of a system component can lead to failure of the entire system. This is aggrevated by the fact that solutioning continues in an essentially uncontrolled manner where the system is down until the entire cavern is filled with saturated brine. This can lead to significant distortions in actual cavern shape compared with the design cavern shape.
Each of the above aspects of a solution-mining system used for oil storage is usually understood individually; however, to minimize risks at a reasonable cost each individual aspect must be integrated to achieve optimum system performance.
