ABSTRACT
The prediction of cavern convergence that results from the creep of salt rock is an essential information for the operators of gas storage caverns, since convergence has direct influence on the performance of the storage. Therefore, storage operators rely on a continuous analysis of operational data and in-situ measurements. The case study presented is special since
the cavern is only partially filled with gas and is operated below the maximum cavern pressure
the cavern interval covers two lithologically very different salt rocks
the fraction of insoluble components (and the sump volume) is large
the site is characterized by an anisotropic regional stress field
The history-matching of sonar surveys indicates breakouts at the cavern contour in a preferred direction and convergence perpendicular to that, which leads to increasing ovalisation of cross-sections. At the same time, the sump level has substantially risen during storage operation while the usable open (gas and brine-filled) volume changed by convergence due to salt creep.
An approach is presented that integrates geological, thermodynamical, and rock-mechanical analyses of observed – and partly counteracting – effects. Thereby, the focus was on an estimate of the in-situ creep response with regard to the operational conditions as well as to the different depth sections of the cavern. It is shown that this approach provides a practical tool for the prediction of cavern convergence for the specific cavern considered as well as for other sites.
