Borehole interface converted electromagnetic wave of double cylindrical formation
Authors: Wang Wei-Hao , Cui Zhi-Wen ,   College of Physics, Jilin University, Changchun 130012, China
Published: December 2020

In recent years, electrokinetic effect has received wide attention. The electrokinetic effect refers to the coupling and conversion of acoustic waves and electromagnetic waves in a porous medium. Due to the electric double layer structure of porous medium, electromagnetic waves are generated during acoustic waves propagation in the liquid-containing porous medium. Based on this phenomenon, one proposes a method for electrokinetic effect logging.The electrokinetic effect has great application prospects in earthquake prediction and engineering exploration. The generation mechanism and propagation law of acoustic-electric coupling wave in electrokinetic effect logging of double-cylindrical-layer porous medium are explored in this paper. By calculating the vertical secant integral of the electric field branch point, the total response of the interface converted electromagnetic wave can be obtained. Previously, in the cylindrical layered medium, the secant integral corresponding to the borehole fluid and the inner layer medium branch point is zero. So the examples in this paper are calculated from the vertical secant integral of branch point <i>k</i><sub>em2</sub>, which corresponds to the outermost infinite medium. It is found that when each time the longitudinal and transverse waves of the formation propagate to the interface, the interface converted electromagnetic waves are generated. These converted electromagnetic waves constitute the total response of the interface converted electromagnetic wave. At the same time, the influence of porous medium fluid salinity on interface converted electromagnetic wave is investigated. It is found that when only the inner layer porous medium fluid salinity changes, the amplitude of the interface converted electromagnetic wave response of the two interfaces is affected. When only the salinity of the outermost porous medium fluid changes, only the amplitude of the interface converted electromagnetic wave generated at the second interface is affected, but the interface electromagnetic waves generated at the internal interface are not influenced. When the difference in pore fluid salinity between both sides of the medium interface is larger, the amplitude of the interface converted electromagnetic wave is greater and the electromagnetic waves generated from the two interfaces are independent of each other.The case where fluid intrusion zone in porous medium is investigated, and the generation mechanism of interface converted electromagnetic wave is also studied when only a mineralization interface exists. It is found that the converted electric field is also generated when the acoustic waves propagates to the salinity interface. Since there exists neither multiple refraction of acoustic wave nor reflection of acoustic wave in the porous medium, the interface electromagnetic wave has only two wave packets, which are interface converted electromagnetic waves generated respectively at the borehole wall and at the electrochemical interface.The interface converted electromagnetic wave can be used to detect the location of the medium interface and the salinity interface. It can also clearly reflect the refraction and reflection law of wave propagating in stratified porous medium, which is of great significance for understanding the interface response mechanism of double-cylindrical-layer porous medium in electrokinetic effect logging.