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

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.

Design of Wideband Absorber Based on Dual-Resistor-Loaded Metallic Strips

A method for designing a dual-polarized wideband absorber with low profile by using dual-resistor-loaded metallic strips is proposed in this paper. Each unit cell consists of a resistive sheet with dual-resistor-loaded metallic strips and an underlying conducting plate. Two-dimensional arrays of two unequal metallic strips are printed on the dielectric substrate, and two resistors are embedded in the metallic strips. By properly designing the resonant frequencies of these metallic strips, a wide absorption band with three resonances is obtained. An equivalent circuit model is introduced, and the current distributions are examined to understand the physical mechanism of the proposed absorber. An example of the absorber is fabricated and measured to verify our designed concept. The measured results show that the wideband absorption performance with a fractional bandwidth of 129% under the normal incidence and the stable angular response are achieved. In addition, the proposed absorber has a low profile with 0.08λL, where λL is the wavelength at the lowest operating frequency.

Design, Preparation and Microwave-Absorbing Properties of Sandwich-Structure Radar-Absorbing Materials Reinforced by Glass and SiC Fibres

In order to broaden the absorbing bandwidth of radar-absorbing materials (RAMs), a type of sandwich-structure RAMs (SSRAMs) derived from a Salisbury absorber and comprising two dielectric layers and one resistive sheet was investigated. In this paper, the impedance characteristics of the SSRAMs were analysed and the mechanisms of broadening microwave-absorbing bandwidth were interpreted using a Smith chart. In order to realise the study’s SSRAMs, plain-woven glass fibre fabric and silicon carbide (SiC) fibre fabric with low electrical resistivity were employed as reinforcements of the dielectric layers and lossy layer, respectively. The microwave-absorbing properties of the SSRAMs were measured and compared with simulated results. The results showed that the experimental and simulated results were in good agreement, that the SSRAMs had better wideband microwave-absorbing properties and that the microwave-absorbing bandwidth at reflectivity below −10 dB can reach 11.6 GHz.