THE USE OF MAGNETIC HYSTERESIS AND REMANENCE MEASUREMENTS IN RAPIDLY AND NONDESTRUCTIVELY CHARACTERISING RESERVOIR ROCKS AND FLUIDS

摘要

This paper details how magnetic hysteresis and remanence measurements can be used for improved, rapid, non-destructive characterisation of multiple mineral (and fluid) components in reservoir samples. This extends previous work (Potter, 2005), which only considered low field magnetic susceptibility measurements. The advantage of the new hysteresis measurements is that they can identify multiple mineral components in the same sample, by acquiring data at a range of low and high applied fields. Plots of applied magnetic field versus magnetisation, where the slope represents the magnetic susceptibility, provide a universal template upon which any reservoir rock or fluid can be characterised. Pure diamagnetic components (matrix minerals such as quartz and calcite, or reservoir fluids such as crude oils and formation waters) are characterised by straight lines with negative slope. In contrast, the pure paramagnetic components (permeability controlling clays such as illite and chlorite) give straight lines with positive slope. Mixtures of diamagnetic and paramagnetic minerals can be theoretically modelled and compared with experimental results on the plots. The presence of characteristic “kinks” or hysteresis “loops” at relatively low fields enable very small concentrations of ferro- or ferrimagnetic minerals (such as magnetite) to be rapidly identified. The magnetic measurements provide a rapid, sensitive complement to XRD measurements. The presence of multiple components (diamagnetic, paramagnetic and ferrimagnetic) in the same sample can be recognised by distinctive changes in the slope of the hysteresis curves as a function of applied field. Furthermore, measurements of isothermal remanent magnetisation (IRM) can give independent complementary information regarding the remanence carrying ferrimagnetic particles without any influence from the diamagnetic or paramagnetic components (which do not acquire a remanence). The hysteresis and remanence measurements have allowed very sensitive characterisation of different reservoir rock and fluid types to be made. For instance, subtle variations from clean sand to slightly muddy sand in clastic reservoirs can be easily identified and quantified. The techniques have also enabled different turbidite types to be distinguished in a single well. Different types of carbonate can also be readily distinguished. In addition, different reservoir fluids (formation waters, crude oils) can be distinguished.