Anisotropy and True Formation Resistivity Measurements with a New LWD Resistivity Sensor

Abstract

Abstract The determination of true formation resistivity is always a challenging component of oil field exploration. The electromagnetic (EM) resistivity tool, one of the most well-known logging tools, provides geological information by detecting electrical properties of the formation downhole. Accurate geophysical knowledge assists petrophysicists to develop a better reservoir and to enhance oil production. However, it is often critical to know the hydrocarbon saturation, which requires reliable horizontal and vertical formation resistivities. During the last decade, wireline triaxial induction logging has successfully provided resistivity anisotropy in vertical wells, whereas logging-while-drilling (LWD) azimuthal EM tools have demonstrated similar field results in deviated wells with only high relative dip angles. Low dipping drilling conditions have posed difficulties regarding conventional LWD measurements for resistivity anisotropy determination because of simple antenna structures. This paper describes the development of a new LWD EM resistivity sensor that uses an antenna configuration with tilted transmitter and receiver antenna orientations rotated relative to the borehole by rotary steerable drilling. Measurements made by this antenna configuration are obtained by unique processing schemes that enable the determination of the dip angle, strike angle, and vertical and horizontal resistivity of the formation at any relative dip angle while drilling. The paper discusses the design concept of the sensor and the environmental parameters that influence tool responses. Several test results, with data processing and the inversion schemes, are shown and compared to various resistivity logging tools. These results indicate that the new LWD EM resistivity tool provides reliable conventional resistivity readings and determines resistivity anisotropy and dip at any relative dip angle. The paper also highlights additional discussions about the calculation of anisotropy and dip angle and about related applications using the sensor.