Determining the maximum operating pressure of gas storage caverns as function of the horizontal in situ geostatic stress

摘要

The analysis of the geomechanical stability of gas storage salt caverns is greatly dependent uponrnthe magnitudes of the geostatic stresses that exist in the underground formations before the caverns are leached.rnIn the salt formations an isotropic geostatic stress field is typically assumed and this assumption is customarilyrnaccepted as basis of design in determining the maximum operating pressure in gas storage caverns, rangingrncustomarily between 80% and 85% of the vertical lithostatic stress at the depth of the last cemented casingrnshoe. However, a series of hydraulic/hydrofrac tests carried out in the UK’s Triassic salt fields identified that thernconcept of isotropic geostatic stress field was not a valid assumption. The horizontal geostatic stresses, whichrnessentially provide the lateral confinement to the last cemented casing shoe of the cavern’s well, were found to bernlarger than the respective vertical stresses. Geomechanical numerical modelling of an underground gas storagerncavern in Cheshire, was used to investigate whether the maximum operating pressure could be larger than thernapplied limit of 80% of the vertical lithostatic stress. The results have shown that for the assessed maximumrnpressure all principal stress components around the cavern were compressive, i.e. no development of tensilernstresses. Moreover, it was shown that the investigated cavern was able to endure effectively the developed shearrnstresses and was also fully surrounded by a zone of finite thickness where no gas infiltration could occur.