The use of magnetic hysteresis and remanence measurements for rapidly and non-destructively characterizing reservoir rocks and fluids

This paper details how magnetic hysteresis and remanence measurements can provide improved, rapid, non-destructive characterization of multiple mineral and fluid components in reservoir samples. This extends previous work (Potter, 2007), which only considered low field magnetic susceptibility. Low-field results alone can sometimes be influenced by small amounts of ferro- or ferrimagnetic minerals that can obscure the major diamagnetic matrix minerals and paramagnetic clays. The advantage of hysteresis measurements is that, by acquiring magnetic susceptibility data over a range of low and high applied fields, they can identify ferro- or ferrimagnetic components at low fields and highlight the combined effect of the diamagnetic and paramagnetic components at high fields. Diamagnetic components, including matrix minerals such as quartz or calcite, or most reservoir fluids, give straight lines with negative slope on hysteresis plots, whilst paramagnetic components, such as permeability controlling clay minerals such as illite give straight lines with positive slope. Mixtures of diamagnetic and paramagnetic components exhibit a slope that is dependent upon the mixture composition. Characteristic "kinks" or "loops" at relatively low fields can identify extremely small concentrations of ferro- or ferrimagnetic minerals (such as magnetite) that may not be detected by XRD measurements. Furthermore, remanence measurements can independently identify the remanence carrying ferro- or ferrimagnetic particles without any influence from the diamagnetic or paramagnetic components (which do not acquire a remanence). The techniques have allowed one to distinguish (i) subtle variations in clastic reservoir samples, (ii) different turbidite types, (iii) different carbonate types, and (iv) different reservoir fluids such as formation waters and crude oils. © 2008 Society of Petrophysicists and Well Log Analysts. All rights reserved.