System modelling and testing of a magnetic susceptibility device and effect of downhole in-situ temperatures on magnetic susceptibility
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Magnetic techniques have so far been under exploited in the petroleum industry. Recently scientists have published some results on laboratory based magnetic susceptibility data for reservoir rock minerals which show strong correlations with key petrophysical parameters including permeability. However, such laboratory based experiments are not representative of in-situ reservoir conditions. Part of this thesis investigates the in-situ magnetic properties of reservoir rocks and minerals at reservoir temperatures via laboratory experiments to model downhole conditions. The shapes of hysteresis curves at various temperatures and the shapes of temperature dependent susceptibility (TDS) curves for individual minerals (paramagnetic, diamagnetic, ferromagnetic) were used to identify the type of magnetic minerals present, their concentrations, phase transitions and changes in mineralogy. For paramagnetic minerals, magnetic susceptibility decreases with an increase in temperature, whereas diamagnetic susceptibility is shown to be independent of temperature. Ferromagnetic substances can show a variety of thermomagnetic changes depending on the type of ferromagnetic minerals present. Changes in the domain state of ferromagnetic carriers with temperature were also observed, which either resulted from the formation of new ferromagnetic components and/or variations in grain size. Theoretical and experimental data on the thermomagnetic properties of reservoir rock minerals are also presented. A downhole sonde capable of taking magnetic susceptibility measurements in-situ downhole has been designed and tested in this thesis. Since the laboratory based methods have demonstrated correlations between the magnetically derived mineral contents and petrophysical parameters, the downhole magnetic susceptibility sonde can potentially provide in-situ predictions of these parameters. Model boreholes were prepared in the laboratory using powder samples and by drilling through solid blocks of rock. Tests were performed on these model boreholes using the magnetic susceptibility sonde.