Long term stability evaluation of natural gas storage caverns

Underground natural gas storage in salt caverns is the most efficient method of gas storage. Since geological and mining-engineering conditions of Polish rock-salt formations are not favourable, it is essential to specify technical parameters for each cavern independently. Stability assessment of particular cavern should include the following issues: rock salt massif strength ratio at the cavern wall, range of cavern influence and rate of cavern convergence depending on cavern loading scenario and mostly on minimal and maximal storage pressure. The paper presents analysis of the above mentioned aspects on the example of hypothetical cavern, in the case of boundary conditions, similar to typical gas storage cavern being operated in the Mogilno salt dome. The major influence on cavern stability have undefined so far rheological properties of rock salt massif, therefore in the article 4 different sets of Norton creep law parameters used in models have been analyzed. Two variants of model load, corresponding to minimal and maximal interaction of neighbouring caverns have been considered. The calculations included two kinds of cavern loading scenarios: scenario for theoretical purposes, aimed to define minimal storage pressure and scenario typical to storage cavern operation. Due to lack of explicit criteria describing long term stability of rock salt, rock salt massif strength ratio at the cavern wall was estimated on the basis of 3 independent criteria. Analysis of the calculation results showed that greater influence on stress-strain distribution of rock salt massif demonstrate creeping low, than boundary conditions. Besides, the high value of power coefficient describing stress influence in Norton creep law is not safe to estimate strain rate of rock-salt massif and cavern convergence. The calculations pointed out that micro-fracturing process in rock salt massif may occur at pressure value lower than predicted by micro-fracturing tests carried out in boreholes. Independently from the above conclusion, in the case of caverns located in homogenic part of the Mogilno salt dome, it is possible to decrease a minimal storage pressure, regardless of current technical projects. However, it is not clear that this improvement could be favourable from the point of view of effective, long-term storage capacity. Only a detailed analysis of exploitation scenario would give the answer. 100