Gauging the gas
The thermodynamic behaviour of a gas storage cavern is critical for the quantity of available gas, for the design of the technical systems and for forecasting operating behaviour.
When a cavern is filled, the gas it already contains is compressed, causing its temperature to increase. Initially, the heat generated is largely absorbed by the surrounding salt. As the turnover frequency increases, this effect is reduced and the cavern temperature changes more dynamically, increasing the mechanical loads exerted on the cavern walls. If the gas is withdrawn quickly, it can cool down below the temperature at which gas hydrate forms. This means that the working gas volume is educed by frequent gas withdrawals.
With modern commercial caverns, the significance of this issue increases because they are operated at far higher withdrawal frequencies and higher speeds than seasonal storage facilities.
In order to design a gas storage cavern and its technical facilities and operations, forecasting its thermodynamic behaviour is therefore critical. Key parameters include the following: Depth of the cavern, geometric volume, permissible operational pressure, storage and retrieval rates, gas quality and the thermodynamic history of the cavern during the solution mining process (cooling of the rock by cold water and negative solution heat) and previous gas
The plant’s size is also important for adding hydrate inhibitors, which can decrease the temperature below that at which gas hydrate forms. This prevents the pipes from jamming even with high retrieval rates, thereby increasing the working gas capacity of the cavern.
Taking into account all of the systems involved – cavern gas content, rock conditions near the well and the cavern – and the behaviour of the gas across a wide range of parameters is a fairly complex task. Modern digital systems calibrated in real caverns by means of comprehensive measurements tackle it successfully.