Description of work
Task 1: In-situ experiment at Mt Terri
Although the SELFRAC experiments 1 and 2 at Mont Terri clearly demonstrated the self-sealing capacity of the Opalinus Clay, the understanding of the underlying governing processes is still incomplete. The proposed work will help to provide the information and data for a basic understanding of the observed phenomena and add information on the influence of temperature increase on the self-sealing process. The results of the proposed experiment will provide important information and data for one of the proposed benchmark tests in WP 5.2.
The in-situ experiment of Task 1 in WP 4.1 has two phases and is planned as an extension of the SELFRAC 2 experiment at Mont Terri. Emphasis will be on a detailed understanding of failure mechanisms around circular cavities and the self-sealing processes as a result of mechanical / thermal load.
In phase A, the water flow paths along the EDZ and the seal sections of a borehole shall be visualised. For this purpose the existing SELFRAC 2 test will be exhumed. During the SELFRAC 2 test the self-sealing of the EDZ has been observed under isothermal conditions for several years. Before overcoring of the borehole, the EDZ will be impregnated with dyed or fluorescent resin. The resin traces are expected to image those flow paths along the damage zone which have not been completely resealed. Key aspects of interest are the orientations, the trace lengths and the interconnectivity of the individual flow paths. The visualisation of the flow patterns, together with the comprehensive hydro-mechanical data base from the SELFRAC 2 test, will serve as an input for the verification of conceptual understanding of self-sealing mechanisms in Opalinus Clay.
In phase B a setup similar to the SELFRAC 2 test will be designed, installed and tested. The new equipment will allow to investigate the evolution of the hydraulic conductivity of the EDZ and the packer seals also at elevated temperatures (up to 90°C). Hydraulic tests are planned before, during and after stepwise heating of the host rock. Subsequent the thermally influenced EDZ around the heater will be sampled – depending on borehole conditions - with a sidetrack. The analysis of the sample for EDZ structures and flow paths will follow the strategy of Phase A.
The two phases of the experimental procedure comprise the following steps:
- Detailed work programme
- Laboratory trial experiments for EDZ impregnation
- Impregnation of the existing SELFRAC test interval with a tracer / marker (dyed, fluorescent epoxy resin)
- Overcoring of the SELFRAC double packer configuration
- Analysis of the recovered core and characterisation of the EDZ along the seal section
- Elaboration of a conceptual model of the key self-sealing processes
- Detailed design of a new SELFRAC experiment (non-isothermal)
- Site preparation and instrumentation
- Self-sealing experiments at elevated temperature
- Impregnation of the test interval with a tracer / marker (dyed, fluorescent epoxy resin)
- Sampling of the EDZ around the heater through a deflected drill hole.
- Analysis of the recovered sample and characterisation of the EDZ along the seal section.
- Interpretation of results and modelling
Task 2: In-Situ experiment at Mol
The ATLAS (Admissible Thermal Loading for Argillaceous Storage) experiment (small scale heater test) realised in the underground laboratory HADES of MOL will be re-activated to have a better assessment of the THM characteristics of Boom Clay.The current set-up of ATLAS was installed in 1992 and consists of a horizontal central heater borehole (drilled 19 m deep) and two nearly symmetrical instrumentation boreholes, which were installed at a short distance of the heater source (one is 1.5 m apart and other one 1.2m apart).
Some extensions to the current set-up will be made to improve the scientific output of this heater test:
- additional temperature sensors in the central borehole will be incorporated to get a more accurate picture of the source term (heater), this is of importance to have a good control of the boundary condition of the test.
- a new piezometer in the horizontal plane at a distance of 2.7 m from the heater borehole will be installed to obtain measurements over a larger range.
- an additional borehole for temperature sensors in the vertical plane will be installed to get a better picture of the possible thermal anisotropy
- At the end phase of test, the clay will be overheated up to drastic observations in pore pressure, stresses, etc. Permeability measurement through piezometers will be made at different stages of the test.
- Additional sampling borehole will be drilled after test..
Combining with previous experiment results, the reactivation and extension of ATLAS ( ATLAS III) will allow to:
- confirm/verifiy the actual thermal characteristics of the Boom clay as well as its evolution with temperature
- in comparison with previous experiments (ATLAS I & II), ATLAS III provides opportunity to
- evaluate the sealing process around heater borehole (EDZ influence)
- have a good assessment of the initial condition before heating
- estimate the ultimate thermal limit that Boom clay can support
provide a good link with large scale Praclay heater test : up-scaling, thermal source control, etc.