Research On The Electromagnetic Response Of Azimuthal Resistivity Logging While Drilling

Because of its important value in geosteering and boundary detection, electromagnetic measurement while drilling has been widely developed and applied in the field of oil drilling, especially offshore drilling. However, most measurement while drilling tools have followed the traditional design of coaxial transmitters and receivers, although the location of oil and gas reservoir can be detected, the directionality information and data necessary to evaluate complex anisotropic reservoirs can’t be provided because the lack of azimuthal sensitivity. In 2000, the concept of directional logging while drilling (LWD) was introduced and that gave way to a new generation of directional tools and services. These tools contain tilted and transverse antenna except coaxial antenna. They provide the ability to accurately measure the formation anisotropy, dip angle, and azimuth for better fluid-saturation estimate. Also, this allows accurate forecasting formation boundaries and geosteering. By studying the reflection and transmission of electromagnetic wave in each layer, the analytical expression of electromagnetic wave is derived, the amplitude attenuation and phase shift of electromagnetic resistivity measurement while drilling are computed, and the sensitivity to formation boundaries of directional electromagnetic measurement is analyzed. Meanwhile, the electromagnetic field of different azimuth is calculated by analytical method, and the rule of azimuthal resistivity image is analyzed.First of all, based on the Maxwell’s equation, the transmitters of logging tools are reduced to magnetic dipoles, the electromagnetic wave in each layer is resolved to TE and TM wave. By studying the reflection and transmission of TE and TM wave in each layer, an analytic expression of magnetic dipole in horizontal layered medium, which satisfies boundary condition of each layer, is obtained. On the basis, the amplitude attenuation and phase shift of electromagnetic measurement while drilling are computed.Secondly, the amplitude attenuation changes in the relationship between azimuth, the resolution of the azimuthal resistivity image is analyzed using the models of three-layer and multiple thin-layer with varying thicknesses of the target layer. The results show that formation boundaries can be detected when the target layer is thicker. However, a misinterpretation may happen when the target layer is thinner, so the logging decisions will be influenced.Finally, based on the amplitude attenuation and phase shift of electromagnetic measurement while drilling, combined the azimuthal resistivity image, The sensitivity to formation boundaries of directional electromagnetic measurements is analyzed, and the influence of formation dip angle and receiver antenna’s tilting angle on directional measurements is studied. The results show that the directional detection range will increase with decreasing frequency, increasing coil spacing, thickness of target bed, resistivity contrast between target layer and shoulder layer. The change of directional amplitude-attenuation became more obvious with increasing formation dip angle and receiver antenna’s tilting angle when the tool approached formation boundary, and thus the sensitivity to formation boundaries increased. Meanwhile, the azimuthal resolution property of directional electromagnetic response is analyzed, and thus the formation boundaries can be accurately forecasted.All above results will provide practical significance for azimuthal resistivity logging and the theoretical foundation for the analysis of directional electromagnetic response and azimuthal resistivity image.