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Browsing by Subject "spent nuclear fuel repository"

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  • Karjalainen, Aino (2020)
    NMR Services Australia (NMRSA) Pty Ltd has developed a Borehole Magnetic Resonance (BMR) tool which is based on the principles of nuclear magnetic resonance (NMR). Drillhole NMR tools have been used mostly in sedimentary environments for oil and gas exploration while applications in hard, heterogeneous, crystalline bedrock are still lacking. This study aims to test the BMR method in a hard rock environment, and for determining hydrogeological parameters in the spent nuclear fuel disposal site, the Olkiluoto island. Essentially, the objective is to design an optimal BMR data processing workflow and calibrate the estimated hydrogeological parameters, currently optimized for data from sedimentary environments, to suit the crystalline bedrock. For testing the BMR method in hard, crystalline bedrock, Posiva Oy, the Finnish expert organization responsible of spent nuclear fuel disposal, made test measurements in the drillholes of the spent nuclear fuel repository site, island of Olkiluoto. The collected data was processed with WellCAD software using additional NMR module. The BMR tool derives T2 distribution (representing pore size distribution), total porosity, bound water and moveable water volumes and permeability calculated with two different models. Some processing parameters (main/burst sequence, moving averages, temperature gradient, cutoff values) were tested and adjusted to fit into crystalline bedrock. Magnetizing material of the surface environment strongly disturbed the uppermost ~20.0 m portions of the measurement data. Some noise was encountered also deep in bedrock, which was cut away from the signal. A list of criteria was created for recognizing noise. The BMR data was compared with other drillhole data acquired by Posiva, i.e. fracture and lithology logs, seismic velocities and hydrogeological measurements. It was observed that the T2 distribution and total porosity correlate rather well to logged fractures and seismic velocities. Lithological variations did not correlate to BMR consistently, mostly because of the strong dependency on fracturing. Permeabilities were compared to earlier conducted hydrogeological measurements, with an intention to calibrate the permeability calculation models. However, this proved to be challenging due to the significant differences of the BMR method and conventional hydrogeological measurements. Preferably, the permeability models should be calibrated by laboratory calibration of the drillhole core, and possibly a new permeability model suitable for crystalline bedrock should be created.