Evaluation of Borehole Casing Sources for Electromagnetic Imaging of Deep Formations
Authors: Marsala Alberto Francesco , Hibbs Andrew D. , Morrison H. Frank 
Published: March 2015

Abstract
Electromagnetic (EM) methods hold considerable promise for differentiating and mapping fluids in hydrocarbon reservoirs. Specifically, the presence of hydrocarbons or carbon dioxide (CO2) produces regions of higher electrical resistivity than regions where saline water is present. Time-lapse EM measurements have demonstrated the capability for gas injection (Wright et al. 2002), water flood (Wilt and Morea 2004) and CO2 injection (Bergmann et al. 2012) reservoir monitoring.
A limitation of EM methods deployed at the surface is their depth of investigation. This is addressed in crosswell EM (Xwell EM) (Wilt et al. 1995) and borehole to surface EM (BSEM) (He et al. 2012) by locating magnetic and electric sources, respectively, at depth within a borehole. However, the deployment of both Xwell EM and BSEM require costly downhole wireline conveyance and are significantly impacted by the presence of a well casing, requiring preferentially the use of at least one open hole well for satisfactory performance.
An innovative approach is to use a borehole casing to provide a way to introduce an electric current into the earth at a considerable depth. One or more remote surface electrodes, located away from the well at a radial distance of approximately the casing depth, are combined with a casing to increase the current flowing in the subsurface at large offsets from the well. In this configuration, a conducting casing is actually an advantage when used in conjunction with an electric source.
In this paper we discuss the effect of the casing conductivity and details of the well completion on the accuracy of the method. We compare variants of the new EM method in which a remote casing is used as one of the surface electrodes. To conclude, we quantify the signal produced by an injected fluid plume.

Introduction
The objective of an electromagnetic (EM) survey is to obtain resistivity and induced polarization (IP), or chargeability, maps of the reservoir, from which it is possible to calculate the saturating fluids distribution. The specificity of borehole to surface EM (BSEM), in respect to crosswell EM (Xwell EM), is that BSEM requires only one surveyed well to obtain an areal map of fluid distribution of a reservoir target layer, kilometers away from the transmitting well (up to 4 km, as demonstrated in Saudi Arabian pilots, Marsala et al. 2011 and 2013a). Xwell EM, on the contrary, allows higher resolution results, but it is limited to cross sections between two or more wells, close enough for EM propagation: about 1 km in open holes, less in cased holes (Marsala et al. 2008).
The BSEM method in the time and frequency domains is the evolution of controlled source EM, a surface-to-surface EM technique. The BSEM technology was first employed in the ex-USSR at the end of the last millennium and has been extensively improved in recent years in China, leading to positive field deployments and resulting in a commercial protocol, subsequently being developed and introduced by BGP (He et al. 2012). Successful pilot studies of BSEM have been reported by Saudi Aramco, producing resistivity and IP images of oil-water contact at reservoir depth (Marsala et al. 2011).