Do metal infrastructure effects cancel out in time-lapse electromagnetic measurements?

Abstract

Controlled-source electromagnetic (CSEM) methods have the potential to be used in reservoir monitoring problems, due to their sensitivity to subsurface resistivity distribution. For example, time-lapse electromagnetic (EM) measurements can help to determine reservoir changes during enhanced oil recovery (EOR) processes such as water/steam injection or CO2 sequestration. Although metal infrastructure such as pipelines and casings can strongly influence EM data and mask the underlying geological response, one may presume that these effects cancel out during time-lapse surveys. In this paper, we analyze the effects of well casings on time-lapse surface-to-surface EM measurements. First, using a synthetic example of an onshore 1D hydrocarbon reservoir we quantify the effect of single and multiple casings at several source and receiver locations. We show that time-lapse responses are distorted significantly when a source or receiver is located near a casing. Next, we study a more realistic scenario where we approximate the hydrocarbon reservoir as a thin bounded resistive sheet. We present a Method of Moments (MoM) algorithm to calculate the secondary currents and charges on a well casing and resistive sheet combination and validate the electric fields these secondary sources generate against finite element modeling. Finally, we calculate and explicitly demonstrate time-lapse amplitude changes in the well casing-thin sheet interaction matrix, secondary currents, charges, and surface electric fields. Our 3D modeling results show that the conductive casing reduces the ability of the resistive sheet to impede the current flow and distorts time-lapse responses. Therefore, one cannot fully eliminate casing effects by subtraction of time-lapse data and must fully incorporate such infrastructure into forward models for time-lapse EM inversion.