Comment on: “Feasibility of electromagnetic methods to detect and image steam-assisted gravity drainage steam chambers” (S. G. R. Devriese and D. W. Oldenburg, Geophysics, 81, no. 4, E227–E244)

Abstract

Devriese and Oldenburg (2016), abbreviated below as DO16, propose a synthetic case study aimed at demonstrating the advantages and limitations in using galvanometric- and induction-based resistivity methods to monitor the growth of steam-assisted gravity drainage steam chambers. They first simulate a simplistic case and then they apply their strategy to a more realistic geometry inspired from a case study operated by Imperial Oil. The geophysical work is based on a set of petrophysical relationships and classical regularization methods for the inverse problem. The petrophysical model used to connect the underlying physics of the problem to the geophysical observable is far from being correct, raising some questions regarding the validity of the input parameters, hence conclusions reached in the paper. The choices for the regularizers and inversion schemes are also problematic as discussed below. I start first with the petrophysical model used in DO16. Despite the claims made in DO16, equation 11 is not Archie’s law for the resistivity formation factor and cannot be found in the seminal paper of Archie (1942). It is a mix of the first and second Archie’s law with a tortuosity added to the equation without any justification. Archie’s law is an equation connecting the intrinsic formation factor to the porosity through a power law relationship. The formation factor can also be defined as the ratio of the tortuosity by the connected porosity (Pride, 1994). In their equation 11, we have both factors used concomitantly, which is not correct (see the discussion in Appendix A of Revil, 2013). Then, DO16 propose to use a Waxman and Smits (1968) equation, see their equations 12–15, to take into account the surface conductivity. Their form of the Waxman and Smits equation suffers four problems: