The overall goal is to be able to predict the occurrence of cavern instability and uncontrolled subsidence (including sinkholes) and to define and supervise cavern risk management protocols ensuring cavern instability and uncontrolled subsidence risks stay at acceptable levels during operation and after abandonment. The specific goals for this KEM project is to improve knowledge on the processes (pereation, hydraulic fracturing and preferential fingering) that occur when brine pressure in the cavern (locally) exceeds the minimum stress in the cavern roof or wall.
The KEM-17 project was commissioned in 2018 to a team led by Dr. Benoit Brouard (Brouard Consulting, France) and consisting of Prof. Pierre Bérest (Ecole Polytechnique, France), Prof. Dr. Boris Kaus & Dr. Tobias Baumann & Dr. Anton Popo (SmartTectonics GmbH, Germany) and Prof. Janos Urai & Dr. Joyce Schmat & Dr. Job Klaver (Microstructures and Pores, GmbH), Germany). The final reports were submitted in November 2019, consisting of a synthetis report and 4 work package reports.
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The overall goal for State Supervision of Mines (SodM) is to be able to predict the occurrence of cavern instability and uncontrolled subsidence (including sinkholes) and to define and supervise cavern risk management protocols ensuring cavern instability and uncontrolled subsidence risks stay at acceptable levels during operation and after abandonment.
The specific goals for this KEM project is to improve knowledge on the processes that occur when brine pressure in the cavern (locally) exceeds the minimum stress in the cavern roof or wall. Currently two end-member processes and an intermediate process are described in the literature and discussed during conferences: permeation, hydraulic fracturing and preferential fingering.
1. Permeation (or percolation): laboratory experiments have shown that brine can be squeezed along salt grain boundaries. It is thought that this permeation process can slowly take place over a large area of for example a cavern roof.
2. Hydraulic fracturing: from the petroleum industry we know rocks can be hydraulically fractured when a fluid exceeds the minimum stress. The hydraulic fracture can lead to a very local and rapid leakage of brine from the cavern.
3. Preferential fingering: it is hypothesized that an intermediate process may occur where fluid leaks through a local, preferential, semi-stable pathway.
We know that the end-members permeation and hydraulic fracture do exist under certain conditions. The specific sub-goals are:
1. To improve knowledge of the micro-mechanisms to determine under what conditions which process takes place/is dominant.
2. To understand how the processes are influenced by local pressure, stress, temperature and salt properties. Also how do local/cavern-scale/salt-dome scale heterogeneities in these parameters affect which process is dominant or the rate of the processes themselves.
RESEARCH REQUEST FULL TEXT
The final report was submitted in November 2019, consisting of a synthetis report.
In addition, 4 work package reports were delivered. Three reports on studies at the micro-, cavern respective dome scale. See: Micro-scale REPORT, Cavern-scale REPORT and Dome-scale REPORT. One report addresses practical measure:
REPORT on Practical Measures
The research question was approached at three different scales: the scale of the grains in the salt formation (micro-scale), the cavern scale, and the salt dome scale. A detailed individual report was prepared for each of the three components of the research, along with a brief report that summarized the findings and conclusions.
The reports on (1) micro-scale, (2) cavern-scale and (3) dome-scale include each:
1. Literature review of already available knowledge on the mechanisms of brine leakage (permeation/hydraulic fracturing/other) when the brine pressure reaches or exceeds the local minimum stress. Include case studies if available. With the micro-scale processes being the most important.
2. Criteria for determining when which leakage mechanism (permeation/hydraulic fracturing/other) is dominant and in whih cases it cannot be determined. Criteria should be a function of salt properties (mechanical, chemical, grain size, heterogeneities), brine pressure, (local minimum) stress and temperature.
3. Report on state of the art and scope for improvement of the salt mining effect modelling approaches and tools for predicting the short (operational) and long term (post abandonment) behaviour of typical caverns in The Netherlands under different cavern pressure strategies, including permeation, fracturing and brine leakage, and salt collapse effects.
The project has resulted in better insight into the potential causes and their role in leakage at microscale, cavern and scales) of salt caverns in The Netherlands. It was concluded that for closed caverns deeper than 1000 meters leakage risks may exist.
Further, a catalogue was produced of required measures by operators and owners of abandoned caverns for different cavern typologies to prohibit any uncontrolled cavern leakage and thereby reduce hydrogeological risks in aquifers and geotechnical risks at the surface.
The potential of catastrophic leakage events can be avoided by either (a) delaying the final plugging of the cavern until closure has occurred (in those cases in which the closure rate is high enough for this to be feasible), or periodically venting brine from the cavern after it has been abandoned, to avoid a pressure build-up that might be large enough to cause fracturing or other types of localised leakage
The results of the KEM-17 project has been evaluated by the KEM scientific expert panel.
The quality of the research in the project was evaluated as excellent. The conclusions and suggestions are right and useful. See also the reaction of the State Supervision of Mines (in Dutch) on the KEM-17 reports .
The research results have been presented in a KEM-DeepNL colloquium October 7, 2021. The presentation can be viewed here.