This research (this research question) focuses on the behavior of salt caverns after closure or abandonment. In abandoned salt caverns the internal pressure can be increased by salt creep and by expansion of brine caused by a rise in temperature. The internal pressure can be reduced by controlled or uncontrolled release of brine from the cavern. The currently identified mechanisms for brine release are:
i) Controlled pressure reduction during brine bleed-off (according to an intended and prescribed procedure).
ii) Uncontrolled pressure reduction by permeation.
iii) Uncontrolled pressure reduction due to the formation of hydraulic fractures.
iv) Uncontrolled pressure reduction through a preferential leak-path.
These mechanisms can occur simultaneously, sequentially, or not at all, depending on the complex interactions among the cavern properties, salt properties and other external controls. Leakage mechanisms (ii), (iii) and (iv) were studied in KEM-171. The study proposed here will focus on mechanism (i), to which we refer in this document as “controlled brine bleed-off” (CBBO).
CBBO will result in a decrease of cavern pressure. This will subsequently affect the state of stress around the cavern to some degree, and at different rates. Basic research questions for CBBO are: how will the stress state change around the cavern or between multiple caverns and what is the potential effect of a changing stress state on cavern integrity (tightness) and stability? A special case is where the abandonment of caverns will be combined with the simultaneous creation or operation of new (storage) caverns in the same salt dome.
SodM would like to understand the effects of CBBO: on subsidence, on stability and on integrity of the cavern. SodM would also like to learn more about what best practices might be: which is better for stability "fast and a lot" or “slow and few" releases? The study will be one of the building blocks that helps SodM to assess the risks of various cavern abandonment scenarios: like hard shut-in, CBBO, controlled high pressure or cavern filling.
Main questionis what happens to the cavern stability and its integrity in the case of CBBO? Sub-questions:
1) How does the stress state around a cavern change during bleed-off? How is this dependent on the bleed-off rate or the convergence rate?
2) How is the cavern stability influenced by these stress changes? Is it dependent on depth or temperature?
3) What are the uncertainties/sensitivities/interactions among various parameters and how can uncertainties be quantified or reduced?
4) Is there a safe/unsafe speed of bleed-off? What are advantages/disadvantages of certain bleed-off cycles?
5) What are additional risks when leakage occurs uncontrolled?
This research project has been commissioned to TNO and the DeepKBB consortium and started in 2024.