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Heater experiment: rock and bentonite thermo-hydro-mechanical (THM) processes in the near field

Deliverables

For the geophysical (seismic) exploration of boreholes, the sensors have to be guided along the borehole. Normally, the equipment is mounted on a support that fits into a singular borehole diameter only. The cost-efficient planning of the drilling campaign (T43) made it necessary to restrict the number of boreholes. Hence, there was the need to adapt from diameter 86 mm to 131 mm while working in situ. The prototype was tested by BGR during the investigations of HE task T43. It has been in use for other seismic investigations since.
Water and gases such as carbon dioxide, hydro carbons, hydrogen, and hydrogen sulphide may be released from the Opalinus Clay at ambient and elevated temperatures. The generated and released volatile components were determined at the existing physico-chemical conditions (temperature, humidity, pore pressure, mineralogical composition, etc.) of the test field, as a function of time. In an additional laboratory programme, the gas generation and gas release was determined as a function of temperature from samples of the heated and unheated areas of the test field. These samples were taken by drilling before the dismantling of the heater. The measurements on gas release in the test field indicated that already at mine temperature (14 °C) the hydrocarbons within the clay become oxidised, resulting in an increase of carbon dioxide and a decrease of oxygen. The release of helium originates from the a-decay of the uranium within the clay. The fluctuation on the gas composition in the different boreholes results from the inhomogeneities of the Opalinus clay. The laboratory measurements at 100 °C indicated that mainly carbon dioxide and small amounts of hydro carbons are released at elevated temperature. The investigation of samples taken from the test field after heater shut-down indicated that within the heating period of 18 months up to 50 °C, no significant alteration of the Opalinus clay with regard to gas and water content or mineralogical composition was observed. For much longer time periods within the temperature range up to 90 °C alteration cannot be excluded.
The objectives of this work were to assess: Thermally induced hydraulic and mechanical stress phenomena in the rock mass, such as the evolution of pore pressure, compressive stress, and shear stress. Emphasis will be on stress-permeability relationships (normal and shear stresses/shear displacements) to extend capabilities of the MHERLIN code. The modelling work involved: Implementing heat balance, thermal coupling with solid and fluid phases, code verification, 2D predictive modelling of pore water and stress evolution in the rock mass during saturation, heating and cooling phases, 2D scoping calculations and sensitivity studies in the rock mass adjacent to a heat source, anisotropy of material properties (mechanical behaviour, hydraulic and thermal conductivity) on the evolution of pore pressure and stress, interpretation of selected data in 2D/3D (distribution of temperature, pore pressure, water saturation, and stress in the rock mass), analysis of parameter uncertainty, determination of domains of critical stress.
The geoelectric measurements show a homogeneous distribution of the resitivity within the Opalinus clay of the test field, which indicates full water saturation. Only close to the floor the resitivity is comparatively high as a result of micro fissures and drying. Within the investigation period no desaturation as a result of elevated temperature was observed. The investigation of samples taken from the test field after heater shut-down indicated that within the heating period of 18 months up to 50 °C, no significant alteration of the Opalinus clay with regard to water content was observed. For much longer time periods within the temperature range up to 90°C alteration cannot be excluded.
This task's objective is to define the condition of all instruments and sensors recovered from the dismantled section of the test, in order to check the validity of the data provided during the test, to be able to correct the existing data base if necessary and to determine causes of failure in the case of faulty sensors to document sensor reliability and improvements required for further tests. The scope of the work will involve check and re-calibration of the sensors recovered from the dismantled section (about 34 units in the bentonite of BHE-0, those retrievable from BHE-3 and BHE-4 plus the heater equipment/connectors, seals). The analyses include: Visual analysis, Seal checks, Cable checks, Failure analysis of faulty sensors, Laboratory calibration of functioning sensors and corrosion Inspection/characterization.
The three objectives of this work are: - Obtain representative samples of rock and buffer, - Select instrument components for characterization, - Document sample and exposed surrounding environment conditions. The work involves: - Removal/modification of monitoring/control systems to maintain continuity of remaining in situ-sensors, - Removal/sampling of surface materials and concrete surrounding the steel liner, - Obtaining representative samples of borehole sand, backfill, bentonite, liner, and instruments while dismantling, - Document activities, samples, and setting conditions (photographs, video clips), - Securing the excavation, borhole vicinity, and remaining instrumentation for continued monitoring, and - Maintaining a project activity record. Results are summarized in the final report of the HE project.
The BGR liner stores core samples in gastight condition under external pressure so that specimen for geomechanic laboratory tests can be transported and stored with the least possible decay of the sample at a moderate cost of storage with easy handling. The BGR liner has been constructed as a cost effective replacement of ANDRA's T-cell. A number of prototypes has been produced and they are currently in use. They were first put into use for the sampling during the drilling campaign for T43 that preceded the dismantling process. BGR is willing to market the design in case of interest by others.
The most significant result is a validated model for predicting the THM processes in a clay buffer and its interaction with the host rock, to be used in the performance assessment exercises for HLW disposal in clayey formations. The initial numerical model got additional developments/modifications for taking into account the specific characteristics of the host rock formation. The final adjustment of the model was done by adjusting rock and buffer parameters based on laboratory results, and by comparisons between the predictions made, and the test data recorded.
Simple and easy as well as inexpensive storage of clay specimen is intended by sealing the sample mechanically into a can. The concept was realized by co-operation with a SME (Lanico-Maschinenbau) who produces can making machinery. The canned clay cores could be put under pressure, as well as in the BGR liner.
In laboratory tests, the Opalinus Clay was investigated to characterize the anisotropic elastic (transverse isotropic) and thermal expansion properties. The failure and post-failure behaviour was described by determining the Mohr-Coulomb parameters. The data can be used for constitutive equations e.g. in finite element programs.
The objective of this work is to determine the mechanical, hydraulic, and geochemical characteristics of the bentonite blocks from different parts of the buffer including the joints and validate model predictions when variations in the material properties have been defined. The work in this task includes laboratory tests on samples taken during the dismantling operation. The following parameters have been determined: - Water content and bulk density, - Permeability, - Geotechnical (i.e. thermal, swelling, strength, and deformation), geochemical, and mineralogical alteration, - Pore water chemistry alteration, - Effects of corrosion products. With these results, we quantify and spatially define the progression of the geochemical, textural, micro/macro structural transformation in the buffer.
The report (deliverable) on the geomechanical characterization of the rock mass around the heater described: The applied methods, the information achieved, The relevance of the results for the characterization of the EDZ around the heater, The experiment-induced changes. Static and dynamic moduli were derived from velocity measurements and dilatometer tests. Video inspection and caliper measurements give information on the overall stability of the boreholes and the existence of open fractures. All results were analysed and compared, to get reliable information on the geomechanical state of the rock mass close to the heater after the experiment. The results enable the optimization of future measurements.
As in the case of the bentonite buffer material, the objective is to determine for the host rock material (Opalinus Clay) the mechanical, hydraulic, and geochemical characteristics. Laboratory tests were performed on samples taken in the pre-heating as well as the post-heating stage. The results of the analysis quantify and spatially define the geochemical, textural, micro/macro structural transformations in the host rock.