Leaky waveguides in hydrocarbon reservoirs and their implications for oil banks detection

Abstract

The presence of naturally occurring subsurface waveguides for electromagnetic (EM) waves has been previously documented. In particular, the mining industry recognized that a coal seam bounded by layers of conductive rock acts as a leaky waveguide. Consequently, the attenuation constant and phase shift of EM signals propagating through the coal layer are modulated by the thickness of the coal and the EM properties of the three layers forming the leaky waveguide. The radio imaging method was developed based on this discovery to characterize coal deposits. Recent studies have determined that guided waves can provide useful information about the subsurface. Structures with similar dimensions and EM properties are found in oil fields in the form of layers of evaporite (e.g., anhydrite) bounded by hydrocarbon reservoirs. To the best of our knowledge, the feasibility of exploiting such structures to characterize the interwell region has not been investigated extensively. We have conducted a theoretical analysis and 3D numerical simulations in the time and frequency domains to demonstrate that layered structures in oil fields can act as leaky waveguides and efficiently guide EM waves. Our results suggest that such structures substantially enhance the propagation of megahertz EM signals. Among multiple parameters evaluated, the conductivity of the layers has the most significant effect on signal attenuation, and thus its range of propagation. We estimated that EM signals of approximately 10 MHz can propagate several hundreds of meters through a layer of anhydrite in the presence of conductive bounding reservoirs. The received signals are not only affected by the properties of the anhydrite layer, but also by the properties of the bounding reservoirs, conferring sensitivity to changes in reservoir saturation. We conclude that this approach could be further developed to infer fluid saturation and especially to identify the presence of oil banks in water-flooded hydrocarbon reservoirs.