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Drilling services and oil companies have long been interested in acquiring the capability of landing a well accurately in a hydrocarbon reservoir and remaining in it for optimal drainage. Although traditional logging-while-drilling (LWD) propagation resistivity tools can help to achieve this goal, their overall effectiveness is not satisfactory because they lack azimuthal sensitivity. Ideally, geosteering and advanced formation evaluations, such as anisotropy calculations, require azimuthally sensitive measurements. This paper discusses a newly developed propagation resistivity tool that is designed to be azimuthally sensitive for use in geosteering and formation evaluation while drilling. It uses the tilted antenna concept to produce directionally sensitive measurements that are lacking in traditional LWD propagation tools. This paper also discusses the theory and the development of this tool, as well as the experimentation and numerical modeling data used to characterize its azimuthal capability. Advanced application algorithms used to calculate the horizontal and the vertical resistivity (anisotropy calculation), as well as dipping angle will be explained in detail. Finally, the paper presents and discusses field examples to demonstrate that this newly developed tool is a two-in-one service: geosteering and advanced formation evaluation. The azimuthal deep-reading resistivity is shown to bear the promise for use in optimization of well trajectory and well placement, as well as in advanced formation evaluation while drilling. This newly developed tool is superior to traditional propagation tools in locating bed boundaries and in keeping the well in the desired pay zone. In addition to providing traditional multi-depth of investigation resistivity measurements, this new tool provides multi-depth of investigation azimuthal resistivity measurements. Introduction Since the introduction of LWD electromagnetic wave propagation resistivity service in the 1980's there has been steady advancement in tool features and capability. The first LWD resistivity tool was introduced in 1983 with single frequency, single spacing, and one depth of investigation1. Multiple depth of investigation capability from single spacing, single frequency, via measurement of phase shift and attenuation was introduced in 19882. In 1991 multiple depth of investigation tools were introduced. Multi-spacing, multil-frequency3, 4, compensated LWD resistivity tools were introduced in 19945. However, most wave propagation tools have followed the traditional design of coaxial transmitters and receivers. Progress in the traditional wave propagation tool design includes adding more transmitter-receiver spacings and frequencies to provide multiple depths of investigation and desirable responses under challenging LWD environments. Today, with the advancement of directional drilling technology and the use of rotary steerable systems, more complex and hard to reach reservoirs are being drilled and evaluated. Traditional wave propagation technology is not adequate because it lacks the azimuthal sensitivity that provides the directionality information and data necessary to evaluate complex anisotropic reservoirs. In 2000, the concept of directional LWD was introduced and that gave way to a new generation of directional tool and services6, 7. This allows accurate geosteering and optimized well placement for maximum oil recovery. Also, it provides the ability to accurately measure the formation anisotropy, dip angle and azimuth for better fluid saturation estimates.

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In the development of many of today's reservoirs, the oil and gas industry is challenged to drill more efficiently and is asked constantly to maximize recovery and production. However, drilling through these reservoirs is challenging because geologic models often are limited to the resolution of seismic data, and offset wells often have significant variations. Geosteering is the process of adjusting the borehole trajectory in real time to correct for unanticipated variations in geology and structure to avoid exiting the target zone. It is a technique currently used on many horizontal and deviated wells for better well placement and for efficiently draining a reservoir. Recent improvements in well-placement and formation-evaluation technologies have helped in gaining access to bypassed reserves that originally were not thought to be practical targets. Examples highlight the economic benefit of geosteering and well-placement technology. Maximum reservoir contact in the sweet spot leads to increased production, and early warning of approaching faults and bed boundaries results in reduction of sidetracks. Furthermore, keeping the well trajectory away from the oil-water contact optimizes production by producing less water. Finally, maximizing production by placing the wellbore entirely within the best reservoir zone boosts productivity so that wells that previously appeared difficult or uneconomic are now becoming viable.

Summary Optimal field development often entails placing the wells in prescribed locations within the reservoir. An error of a few meters in height above the oil/water contact or with respect to the roof may result in leaving behind a significant portion of the producible reserves. Driven by this key requirement, new technologies continue to emerge to help geologists, drillers, and reservoir engineers geosteer the wells. In recent years, two types of logging-whiledrilling (LWD) information have been used. On one hand, wellbore imaging can determine when a well path has left the reservoir and the angle of exit. On the other hand, traditional axisymmetrical resistivity logs help to quantify the distance to an approaching boundary through inversion, but fail to tell its azimuth. A newly deployed azimuthal deep resistivity instrument recognizes an approaching geological event before it intersects the well while continually imaging it at multiple depths of investigation. Of particular interest is the azimuth of approach with respect to the well path, advising in real time the most favorable change of direction. In addition, a series of transverse electromagnetic measurements specific to azimuthal resistivity, called geosignals, are presented. Geosignals help to quantify the distance and the rate of approach with great accuracy, before the actual intersection could occur. With this real-time information, geosteering engineers can remain at prescribed distances from important boundaries, including oil/water contacts and overlying shale roofs. Modeling and actual logs demonstrate that the new LWD instrument performs at its best when the reservoir is overlaid by shale. Modeling suggests, however, that suboptimal performance occurs in reservoirs with very resistive caprock, such as anhydrite.