Summary In petroleum exploration, reservoir navigation is used for reaching a productive reservoir and placing the borehole optimally inside the reservoir to maximize production. For proper well placement, it is necessary to calculate in real-time the parameters of the formation we are drilling in and the parameters of formations we are approaching. On the basis of these results, a decision to change the direction of drilling could be made. Modern logging-while-drilling (LWD) extra-deep and azimuthal resistivity tools acquire multicomponent, multispacing, and multifrequency data that provide sufficient information for resolving the surrounding formation parameters. These tools are generally used for reservoir navigation and real-time formation evaluation. However, real-time interpretation software is very often based on simplified resistivity models that can be inadequate and lead to incorrect geosteering decisions. The core of the newly developed software is an inversion algorithm based on transversely isotropic layered Earth with an arbitrary number of layers. The following model parameters are determined in real time: horizontal and vertical resistivities and thickness of each layer, formation dip, and azimuth. The inversion algorithm is based on the method of the most-probable parameter combination. The algorithm has good performance and excellent convergence because of its enhanced capability of avoiding local minima. This capability enables interpretation of real-time resistivity data, including azimuthal and extra-deep measurements. A graphical user interface (GUI) was developed to provide an interactive environment for each stage of the resistivity data interpretation process: preview of input resistivity logs, initial preprocessing and filtering of raw data, creation of initial guess, running inversion and viewing inversion results, and quality-control indicators. Applications of the developed software will be shown on a series of synthetic examples and field data from the North Sea and Gulf of Mexico (GOM). This newly developed software is currently in use for real-time reservoir navigation and post-well analysis.

The relatively recent development of azimuthal resistivity measurements enables proactive geosteering within complex reservoirs. These successful tools are the major contributor to the substantial expansion of horizontal drilling. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low resistivity layer, the depth of detection of an adjacent high resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering. An extra-deep resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies, has large transmitter-receiver spacings and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional resistivity instrument. The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled. Examples are presented from one well where the extra-deep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extra-deep measurements from the tool also provide valuable reservoir understanding and knowledge for future well planning purposes.

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The relatively recent development of azimuthal-resistivity measurements enables proactive geosteering within complex reservoirs. The tools enable determining the distance (up to 5 m in ideal conditions) and the azimuthal direction to a resistivity boundary. In ideal conditions, the well is inside a high-resistivity layer and the shoulder bed is low resistivity, giving geologists warning of approaching adjacent conductive beds. When the tool is in a low-resistivity layer, the depth of detection of an adjacent high-resistivity layer is much smaller. In these situations, it is often not possible to use the tool for effective geosteering.An extradeep-resistivity tool has been used for several years in Norway and has been introduced in the Peregrino Field in Brazil. It operates at lower frequencies than the shallower reading tools, has large transmitter/receiver spacings, and a depth of detection up to 25 m. This tool was deployed in addition to the conventional directional-resistivity instrument.The new application in Brazil was supported by inversion software (still in development) to enable possible interpretation of the geology within the tool range. The inversion results provide information that can help identify adjacent reservoir layers while in the target zone and measure the thickness of the reservoir layer being drilled.Examples are presented from one well where the extradeep resistivity provided early warnings and additional information that helped to steer the well successfully and maximize reservoir coverage. The extradeep measurements from the tool also provide valuable reservoir understanding and knowledge for future well-planning purposes.Extradeep-Resistivity Measurements. To illustrate how the extradeep-resistivity measurements complicate visual interpretation, consider a simple model in Fig. 1.

The presented study demonstrates the effect of different technological activities of drilling through a reservoir interval on induction logging while drilling and near wellbore resistivity. The drilling operations include drilling with different penetration rates, shutdowns for drill-pipe addition, back reaming and so on. To estimate this effect, numerical modeling has been applied for simulation of mud filtrate invasion followed by electromagnetic modeling of induction tool signals. When modeling the invaded zone, the developed algorithm takes into account the drilling mode and mudcake formation. The computed near wellbore distributions of invasion zone parameters such as water saturation and salinity are then used to determine the resistivity distribution in the near wellbore area. The obtained geoelectric model of the well and the invaded zone makes it possible to compute the induction signals measured while drilling. It has been shown that neglecting features of the drilling operation may lead to improper interpretation of logging measurements.