Gas first fill
The ultimate test
Before gas can be stored in a cavern, the brine it contains must be displaced and replaced by gas. For this purpose, the pumping head is complemented by an additional casing that allows the saturated brine to be removed. The so-called brine displacement string runs through the centre of the gas pumping casing and ends just above the cavern sump. This ensures that as much brine as possible is removed from the cavern. The gas is pumped to the cavern through the annulus between the production casing and the brine displacement string. The injected gas exerts pressure on the brine in the cavern, pumping it
up through the brine displacement string.
The interface between gas and brine continuously lowers during this process until the entire cavern is filled with gas. During this process, the gas and brine volumes are analysed in detail in order to determine the current depth of the gas/brine interface. The values are calibrated when the initial gas filling process has been completed or by measuring the actual depth if required.
The exact current depth of the interface must be known to forecast when the initial gas filling process will be completed. There are two main reasons why this forecast is required: On the one hand, the subsequent work must be coordinated; on the other hand, gas will enter the brine displacement string when the bottom is reached. The rising gas leads to a rapid pressure increase at the cavern head due to its significantly lower density. In order to be able to respond adequately to this pressure increase, it is critical to know when it will occur.
When the gas first fill is completed, the brine displacement string is removed from the cavern using a lock. After that, the full cross-section of the production casing is available for adding or removing gas.
The gas first fill occurs after the excavation and completion of a salt cavern and before subsequent storage operations. This makes it the last step in the construction of a cavern storage facility.
The following technical and economic aspects must be taken into consideration when planning and carrying out the gas first fill:
- Availability of gas (cost, time);
- Transport capacity for removing the brine from the site;
- Results of the calculation of the hydraulic processes with respect to the rock-mechanical limits and cavern completion;
- Planning possible additional measures such as gas or water lifting processes.