This study describes a novel approach to propagate high-frequency (MHz) electromagnetic (EM) signals through the subsurface. Our technique relies on the presence of highly-resistive evaporite seals bounded by hydrocarbon reservoirs acting as planar transmission lines to achieve greatly increased EM propagation. As the signal propagates through the seal, variations in the fluid saturation of the bounding reservoirs modulates the signal amplitude and velocity. This technique will enable improved EM surveys for reservoir characterization.
The concept, known as Proximity Sensing, has been previously demonstrated via numerical simulations and laboratory experiments. In this work, we present quantitative 3D numerical simulations in the frequency domain to estimate the maximum depth of investigation (DOI) under multiple reservoir saturation scenarios. The models consist of: three layers representing an evaporite seal bounded by hydrocarbon reservoirs, and two fluid-filled cased wellbores with dipole antennas representing the transmitter and receiver. Multiple parameters were evaluated to quantify their impact on maximum DOI, including: antenna location, wellbore fluid properties, reservoir saturation and seal conductivity.
The results show that EM signals are detectable beyond half a kilometer separation between transmitter and receiver wells if the evaporite seal is highly resistive. This method can be used to map reservoir saturation based on the relationship between bounding reservoir conductivity and signal propagation. Formation conductivities and antenna depth were found to have the most significant effect on the maximum DOI. Wellbore fluid conductivity has a negligible effect on the DOI. Overall, the more conductive the bounding reservoirs are, the more efficient they are at containing the signal within the channel. However, more conductive reservoirs translate in higher signal attenuation before it enters the channel. Therefore, there is a tradeoff between reservoir conductivity and maximum DOI.
The results presented confirm the viability of Proximity Sensing to enhance propagation of high-frequency EM signals and provide valuable reservoir saturation information. This study provides additional insight as to the viability and ideal configuration for field testing and paves the way for a new research direction in EM surveys.