WP 1.6
To extend the use of renewable energy, it will be necessary to provide possibilities to store energy in order to compensate seasonal as well as daily fluctuations in energy production and consumption. Therefore, mainly two different options for storing energy in the subsoil in terms of the behavior of the adjacent soil will be analyzed extensively. On the one hand the storage of heat in soft rock layers near to the surface will be in the focus (WP 1.6.2) and on the other hand the storage of gas (e.g. compressed air or H2) in rock salt caverns in deep layers is of main interest (WP 1.6.1). Numerical investigation of the behavior of rock salt caverns will be surveyed in WP 2.1 with a modified constitutive model within a FEM-based software. Identification of key parameters in operation phase of salt caverns and best place for locating sensors will be done by sensitivity analysis as a part of WP 3.1.
Geomechanical-seismic behaviour of soft rocks from Northern-Germany due to periodically heating and heat extraction
Seasonal input and extraction of thermal energy within shallow soft rocks will cause periodical changes in their mechanical state (density, fabric). This leads to an alteration of hydraulic property (soil water conductivity) and controls mechanical behaviour (stiffness, strength). Hence, long-term cyclic heating and heat extraction cause an enduring deformation of the subsoil. In the context with Northern-Germanys geologic set-up, which is widely characterized by disturbed stratification as a result of halokinesis and glacial tectonics, subsoil deformations will surely be inhomogeneous, by what civil engineering constructive works could be seriously harmed. Controlling the alteration of the geomechanical state in situ, non-invasive especially seismic methods are suited for long-term monitoring. Against this background the mission of WP 1.6.2 is to parameterize the characteristic thermal-hydraulic-mechanic-seismic (THMS) properties and behaviour of reference soft rocks from Northern-Germany by means of linked THMS-laboratory experiments. Thereby the main focus is to quantify the mechanical response of soft rocks on cyclic heating (Fig. 1).
Figure 1
The leading experiments objective is to allocate soil mechanical parameters fitting constitutive laws for the numerically modelling of ground deformations due to periodically heating and heat extraction. One more task of the laboratory tests is to validate the representation of thermally induced alterations in the mechanical state by the velocity of elastic waves. So that calibration data will be available as a tool for the mechanical interpreting of seismic logs from long-term monitoring of thermal reservoirs. Running THMS tests in the laboratory two experimental plants are available working within the range of temperatures from 10 to 80°C: A Bishop & Wesley test apparatus designed for running triaxial stress paths in general (Fig. 2, left) and an oedemeter device for testing stress conditions at rest in detail (Fig. 2, right). Both plants are in progress.
Figure 2
Behavior of caverns in rock salt under cyclic thermo-mechanical loading
The underground salt caverns which store energy in the form of compressed air and H2 work under fluctuating internal pressures and temperatures. This motivates researchers to perform comprehensive investigations to understand and predict the behavior of rock salt during operation time under cyclic thermo-hydro-mechanical loadings which are caused by the gas injection and withdrawal.
A thermo-hydro-mechanical constitutive model is to be defined in this project to describe the behavior of the rock salt under various loading conditions. Moreover, the effects of cyclic loading, dilatancy-healing process and damage progress are taken into account. In the following, the introduced constitutive model is calibrated using extensive experimental investigations. Having the constitutive model implemented into the finite element code, the operating procedure of a typical salt cavern can be simulated. Subsequently, sensitivity and reliability analysis are performed to identify the optimum operating and monitoring condition of the cavern. The following flow-chart (Fig.3) describes the connections between experimental and numerical parts in this project.
Figure 3
- Experimental investigation
In this research the constitutive behavior of rock salt for alternating mechanical and thermal loading conditions due to periodical charge and discharge of compressed air and H2 in caverns are of main interest. In a newly developed high-pressure triaxial device (Fig. 4), an extensive laboratory testing program will be executed. Results will be used to verify a new constitutive law. The test program will contain isothermal and cyclic mechanical load paths, isobar and cyclic thermal load paths as well as combined cyclic mechanical-thermal load paths. In contrast to conventional triaxial tests, in our device the axial load (σ1) will remain constant whereas the confining pressure (σ3) is variable. Temperature can be controlled independently from load application. The device is designed for maximum loads of 45 MPa and 38 MPa for axial and radial direction, respectively. Temperature can be applied in the range of 20 °C to 80 °C.
Figure 4
- Development of a constitutive model
A thermo-mechanical constitutive model is to be implemented into the finite element code (CODE-BRIGHT) to describe the complex behavior of rock salt under different loading conditions . With this approach it will be possible to consider the operating characteristics that are typical for the compressed air energy storage (CAES) in salt caverns, e.g. creep, compression, extension and thermo-mechanical cyclic loading. Moreover, the employed model has to be able to predict damage progress and failures inside the dilatancy zone. To accomplish this, a viscoelastic-viscoplastic model (LubbyII & Desai) combined with a damaged parameter will be used to explain the material behavior of rock salt.
Fig. 5 (left) shows the yield surface of Desai model. As it can be seen from this figure, the cross section of the surface in π-plane has a triangular shape. Therefore, different responses are obtained for extension and compression stress paths. Fig. 5 (right) represents the evolution of yield surface in I1- space based on a hardening parameter. The dilatancy boundary shown in this figure separates the dilatation zone from compression zone and failure boundary is the ultimate yield surface that can be reached.
Figure 5
- Numerical modeling of caverns and sensitivity analysis
Numerical simulations will be carried out for different, idealized salt cavern geometries, boundary conditions and loading scenarios (cyclic or constant) based on different constitutive models within "CODE-BRIGHT" software (e.g. Fig.5). Using the FEM software in combination with the viscoelastic-viscoplastic model, key parameters for cavern operation and critical location in monitoring phase will be identified within a global sensitivity analysis and reliability analysis. Running the FEM software each time during sensitivity and reliability analysis can be time consuming. Instead, a metamodel or surrogate model will be developed to approximate the behavior of original numerical model accurately.
Figure 6
The works in WP 1.6 are carried out as a contribution and in cooperation with WP1.2 Parameterization of the near-surface bedrock, WP1.4 Investigation and monitoring of shallow heat reservoir, and WP2.4 Impacts of near-surface geothermal energy.
