Abstract
The presence of evaporitic formations in sedimentary basins, often dominated by the salt mineral halite, is of great influence on the structural style developed during tectonic events. On a somewhat smaller scale, salt rocks often host a variety of deep solution mined caverns, which are increasingly finding use for strategic
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storage of energy resources in the form of gaseous or liquid fuels and as vessels for off-peak energy storage in compressed gas. This is in addition to the use of conventionally mined galleries for the long term storage of hazardous waste materials. The low permeability of most salt rocks, with weak rheological behaviour and tendency to self-seal, has attracted engineers to use salt formations to host such projects. The Zechstein salt deposits in the Netherlands exist in the form of original beds and as migrated salt domes/pillows that may reach the kilometre scale. Some contain magnesium rich salts which are solution mined as an economic resource. Caverns constructed within these more soluble and weaker materials pose additional challenges for long term storage management. Careful management of fluid filled caverns requires a full knowledge of the rheological and transport properties of the host salt formations for a safe operation into the future. This thesis addresses a number of deficiencies in the current knowledge of salt mechanical behaviour regarding the creep of halite and its mixtures with magnesium salts. (1) The pressure sensitivity of creep was investigated, using dry synthetic halite rock. The rate controlling mechanism in the temperature range 22-350 oC was found to change from dislocation glide to dislocation climb. The experimental results allow quantification of the activation volume for creep. (2) The transition from creep governed by grain size sensitive (GSI) dislocation mechanisms to creep controlled by grain size sensitive (GSS) solution-precipitation mechanisms was studied on the basis of multi-step deformation experiments with constant strain rate parts and stress relaxation. Stress exponents n of conventional creep laws changed from ~10 at relatively high stress and strain rate to ~1 at the end of the relaxation steps. (3) The rheology of the magnesium bearing salts carnallite and bischofite was also investigated by means of multistep experiments including relaxation. Special attention was given to the mechanical behaviour of mixtures of these salts with halite. Both bischofite and carnallite, tested at real in situ conditions of temperature 70 oC and pressure 40 MPa, showed a change from GSI creep to GSS creep comparable to what was found for wet halite. n-values changed from ~5 to ~1. Composite GSI-GSS flow laws for bischofite and carnallite were established. (4) The effects of compositional layering on permeable transport in the excavation damage zones around mined cavities were investigated by argon gas permeability, using natural layered material from mines in China, to better understand effects of bedding orientation relative to deformational stresses expected in cavity and gallery walls at depth. Microstructural investigation revealed that local dilatancy occurred at interfaces, which must have given rice to a local increase in permeability.
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