Quantifying the effects of core cleaning, core flooding and fines migration using sensitive magnetic techniques: implications for permeability determination and formation damage

Magnetic techniques have recently provided a rapid, non-destructive means of quantifying mineral content, particularly permeability-controlling clays, at several core scales (Ivakhnenko and Potter, 2006; Ivakhnenko and Potter, 2008; Potter, 2007; Potter and Ivakhnenko, 2008; Potter et al., 2009; Potter et al., 2011). The correlation of magnetically derived clay content with key petrophysical parameters has led to improved prediction of permeability and SCAL properties. Magnetic techniques also offer a unique means of quantifying the effects of core cleaning. Some preliminary results at low field were reported by Potter et al. (2004). The present study investigates the effects of hot soxhlet cleaning, by monitoring the low field magnetic susceptibility of the core before and after cleaning, on a series of well characterized samples. The work demonstrates that there is a reduction in paramagnetic illite clay content after cleaning, with intrinsically higher permeability samples exhibiting the largest reductions. Importantly, the results question the reliability of some permeability measurements made on cleaned (particularly intrinsically higher permeability) core samples, which undoubtedly overestimate the permeability of the uncleaned core with the original clay and other mineral content. Our techniques suggest a means of correcting for the effect of clay and other minerals removal during core cleaning. Magnetic measurements taken before and after cleaning could uniquely allow one to estimate the permeability of the uncleaned sample with the original mineral content.
We also detail sensitive magnetic techniques for quantifying fines migration and removal during core flooding experiments. A specially constructed coil has been used to measure the low field magnetic susceptibility at various locations along the core both before and after core flooding experiments. Results show that the low field magnetic susceptibility decreased after the experiments, implying that small concentrations of a mineral (or minerals) with positive magnetic susceptibility had been washed out from the samples. Hysteresis measurements of the samples suggest (by analysing the low and high field parts of the hysteresis curves) that paramagnetic clay, and possibly some ferrimagnetic iron oxides, have been removed from the samples during these experiments.
On a larger scale, work on collected accumulated particulate residues from injection waters is further helping to build up a comprehensive picture of the nature and amounts of fines that might lead to formation damage. The concentration of iron minerals and clays removed are either generally too small for accurate XRD analysis or their signal is masked by high amounts of more abundant minerals such as halite or gypsum. The magnetic techniques, however, are very sensitive and can detect extremely small concentrations of fines, which is a major advantage of this new approach.