GMT - A Large Scale In-Situ Test of the Gas Migration Properties of Engineered Barriers
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Figure 1. The GMT Concept
The components of the field experiment (see Figure 1) are: a silo cavern 4.5 m deep and 4.0 m in diameter, a concrete silo (height of 2.5 m, external diameter of 2.5 m) embedded in the cavern, the vent on the top of the concrete silo, the bentonite/sand backfill around the silo (20 % wt. bentonite and 80 % wt. sand) and, finally, the normal backfill in the upper part of the cavern [2]. Gas will be injected at the center of the concrete silo and the gas breakthrough and migration through the engineered barriers and the adjacent geosphere will be monitored. A total of approximately 250 sensors will be embedded in the engineered barriers and special tracers will be utilized that allow visualization of the gas flow paths. Two of the main objectives of GMT are: • •
Assess the function of the engineered barrier system (EBS) and adjacent geosphere as a total system with respect to repository-generated gas migration. Evaluate models (conceptual and numerical) applicable to gas migration through barriers under realistic in-situ conditions.
OVERVIEW OF ACCOMPLISHMENTS TO DATE The GMT project is structured in sequential Phases with a parallel laboratory program. An overview of the different Phases and the accomplishments to date is given in Table I. Table I: GMT Phases and Activities Completed Phase 1. Detailed planning/ Site Selection Completed in FY1997 2. Excavation/ Site characterization Completed in FY1998
Description of main activities - Experiment Planning (design calculations, study of the gas vent structure and material, operational safety considerations) - Planning and drilling of investigation boreholes - Site selection - Layout finalization and excavation planning - Excavation (in two steps: access drift and silo cavern) - Preliminary field tests for characterization of access drift and silo cavern (for the silo, measurements performed before and after excavation) - Laboratory program: Design/Instrumentation tests
3. Gas Migration Experiment: Silo cavern tests
- Test of geosphere adjacent to silo cavern (water/ gas) - Estimation of geosphere parameters - Finalization of in-situ gas test planning: instrumentation, gas injection rates, pressure etc. - Laboratory program: i) Support for the design of EBS (saturation system, emplacement procedures, instrumentation) On-going to be completed in Fall 2000 ii) Estimation of parameters and data for the final design (2-phase flow, rock-mechanics, cement / bentonite interaction data)
4. Gas Migration Experiment: EBS with vent To be initiated in Fall 2000
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Emplacement of EBS Saturation of EBS Water/gas pressure test Dismantling Data interpretation/input for next Phase
In summary, the following subtasks of GMT have been completed, in accordance to the master-plan: i) all excavations; ii) all boreholes; iii) hydrogeological and gas testing of the nearfield geosphere; iv) geological and geo-mechanical characterization; iv) instrumentation of all the boreholes and the initiation of the long-term monitoring of hydraulic pressures; v) laboratory
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