Magnetic susceptibility as a rapid, nondestructive technique for improved petrophysical parameter prediction

Magnetic measurements provide a rapid, cheap and nondestructive means of characterizing high-resolution mineralogical variations between core samples. Until now they have been a relatively unexploited tool in petrophysical core analysis. One of the most useful measurements is low-field (initial) magnetic susceptibility. This paper will demonstrate how such measurements on core plugs, both as raw values and after novel processing of the results to mineral contents, can provide rapid predictions of key petrophysical parameters. The measurements allow "quick look" petrophysical appraisals to be made long before the usual routine or special core analysis data becomes available, since all the core plugs from a well can potentially be magnetically screened in one day. In the present study the negative or positive sign of the raw magnetic susceptibility signal showed a strong correspondence with the main lithological and permeability zones. Moreover, the processed results exhibited strong correlations between magnetically derived illite content and permeability in an oilfield where the porosity-permeability relationship is very poor. This provided a new permeability predictor where prediction had previously been problematic. Furthermore, the magnetic measurements correlated with the flow zone indicator (FZI), the cation exchange capacity per unit pore volume (Q _v), and the wireline gamma ray signal. The relationship between magnetics and wireline gamma ray provides a quantitative means of distinguishing clean sand from muddy sand, even in the presence of a high-gamma ray-emitting (but low magnetic susceptibility) drilling mud. The results also allowed the wireline gamma ray data to be related to illite content. © 2007 Society of Petrophysicists and Well Log Analysts. All rights reserved.