Recent work has shown strong correlations between magnetic susceptibility and key petrophysical parameters such as clay content and fluid permeability. The magnetic measurements in previous work were mainly undertaken at ambient (room temperature) conditions on core samples. The present study involved theoretical modeling and experimentation on the temperature dependence of the magnetic properties (mass magnetization and magnetic susceptibility) of reservoir rocks and minerals over a range of low and high applied fields. It paves the way for correctly interpreting borehole magnetic susceptibility measurements, and accurately predicting petrophysical properties in situ, from a potentially new suite of low and high field borehole magnetic tools.The temperature dependent magnetic susceptibility measurements provide an improved means of quantifying the diamagnetic versus paramagnetic mineral content in reservoir rocks compared to a single ambient measurement. Paramagnetic clays, such as illite, are important in controlling the fluid permeability in many of these samples, and we again saw correlations between the magnetic measurements, paramagnetic clay content, and permeability. We also show how to derive ferrimagnetic mineral hysteresis curves by subtracting the high field magnetic data from the total signal. The resulting magnetic hysteresis curves give important information concerning the content (often only a few ppm, which X-ray diffraction cannot detect), mineralogy, and domain state of the ferrimagnetic particles.

Summary Our recent work on deep tight gas reservoirs containing red and white sandstones (Potter et al. 2009) has suggested that the presence of small amounts of hematite in reservoir samples can have a dramatic effect on permeability. Such conclusions were made using laboratory-based low- and high-field magnetic-susceptibility measurements on reservoir-rock samples and by comparing these measurements with the permeability data. These rapid, nondestructive magnetic measurements have been applied previously in clastic reservoir samples (Potter 2007; (Ivakhnenko 2006; Ivakhnenko and Potter 2008; Potter and Ivakhnenko 2008) and carbonate reservoir samples (Al-Ghamdi 2006; Potter et al. 2011). However, such laboratory-based analyses are not representative of the downhole in-situ conditions, especially in deep gas reservoirs where the temperature can reach quite high values. Typical tight-gas-reservoir depths can reach approximately 4000 m (Abu-Shanab et al. 2005) and 6000 m (Tang et al. 2008), and the equivalent temperatures would measure 131 and 192°C, respectively, if one assumes the normal geothermal gradient (Mayer-Gurr 1976). This paper investigates the in-situ magnetic properties of deep tight gas reservoir samples (containing permeability-controlling reservoir minerals hematite and illite) by means of laboratory experiments to model downhole temperature conditions. We perform magnetic hysteresis measurements at various temperatures in order to identify and quantify mineralogy and model changes in the magnetic behavior of these minerals at in-situ downhole conditions. From these measurements, we are able to show whether the mineralogy or domain state of the permeability-controlling minerals is likely to change with temperature in deep gas reservoirs. These changes can potentially have a major effect on permeability. We also demonstrate that there are strong correlations between core-permeability and magnetic-susceptibility data in these tight-gas-reservoir samples. The permeability is low in red sections of the core wherever there is hematite.

No abstract

A paper by Mi et al. [1] suggested that certain nano-sized hematite (α-Fe 2 O 3 ) particles had diamagnetic properties at room temperature. Since diamagnetic behavior is not a property normally attributed to hematite particles (hematite is generally regarded as a canted antiferromagnetic material at room temperature) we decided to test the validity of the suggestions in [1] by performing magnetic susceptibility and magnetic hysteresis measurements on a series of hematite nanoparticles with average sizes of 8 nm, 30 nm and 40 nm in diameter. We initially considered two possible explanations for the apparent diamagnetic behavior of the nanoparticles in [1]: either 1) the hematite nanoparticles themselves exhibited this unusual diamagnetic behavior, or 2) the diamagnetic response was simply the signal created by a diamagnetic dispersant that was overriding a weak positive magnetic susceptibility signal of the hematite nanoparticles. Our experiments strongly suggested the latter explanation that the apparent "diamagnetic" behavior seen in [1] was caused by a diamagnetic dispersant dominating the magnetic properties of the dispersed hematite nanoparticles.

No abstract