Recent Developments in Geotechnical Design of Natural Gas Storage Cavities Regarding Physical Modelling as Well as Numerical Simulation

Abstract

Salt cavities for storage of natural gas in bedded or domal salt structures are an important element of today’s and tomorrow’s energy supply management. In Germany the mechanical design of salt cavities has a history of more than 35 years. Based on laboratory investigations, physical modelling, analytical or much more importantly numerical simulations and last but not least field experiments and operational experience, knowledge about salt mechanics and salt cavern load bearing behaviour has increased significantly. This scientific-technical improvement corresponds very well with the development of individual cavern sizes

starting at a volume of 10,000 m³ and rising to a Million m³ of natural geometric storage with optimal geotectonic conditions.

The presentation first gives an overview on the development of salt cavity design and an insight into today’s state of the art designs. Special items include the geo-mechanical characteristics of storage cavities and principle safety demands for their design as well as recent design concepts and a way to provide geotechnical proof of safety with specialized criteria. Furthermore, the main aspects of lab testing and physical modelling are considered. Here damage to salt rock and its numerical characterization are of significant importance for further development. Based on numerical simulations using commercial or specialized software, safety analysis and cavern design, with respect to site specific defined criteria, lead to admissible values for cavern configuration and operation parameters.

Additionally, based on decades of existing experience, a method for designing salt caverns for high frequency cycling of storage gas is introduced using long – or short-term historic operational data. Material parameters and the deformation behaviour are adjusted to cavern convergence data from sonar surveys with reference to multicyclic laboratory tests of rock salt cores. Using the novel Lux/Wolters constitutive material law, where rock salt is treated as “material with memory” by a modified Lubby2 approach, the real behaviour of rock salt deformation can be reliably described. For multicyclic storage operations additional thermal stresses have to be taken into account for analysis of cavern integrity. A numerical thermodynamic simulation model is used to forecast related temperature changes. Cavern design in this case is based on thermo-mechanic coupled simulations.