Evaluation of Borehole Casing Sources for Electromagnetic Imaging of Deep Formations

摘要

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.