A Deep-Resistivity Logging-While-Drilling Device for Proactive Geosteering
Authors: Seydoux J. , Tabanou J. , Ortenzi L. , Denichou J.M. , De Laet Y. , Omeragic D. , Iversen M. , Fejerskov M. 
Published: October 2008

Abstract
Positioning a horizontal borehole accurately in the reservoir is critical in maximizing the recovery factor. During well planning, the well path is usually optimized based on structural interpretation of seismic surveys, which often lack resolution and accuracy.
More detailed measurements, acquired while drilling, can be used to modify the borehole trajectory to improve its position in the reservoir during drilling. A limitation to geological steering is the relatively small volume of investigation of the traditional logging-while-drilling (LWD) sensors, which can typically probe no farther than a meter into the formation. Unpredictable geological events are therefore, in many cases, detected too late to allow proactive geosteering decisions. Furthermore, traditional drilling and surveying methods do not provide the necessary accuracy to position the wellbore at a fixed distance from fluid contacts over a significant horizontal length.
An LWD resistivity device was developed with a much larger radial response that allows the detection of lithology features and fluid contacts that are in the tens of meters away from the borehole. This paper describes the principle of measurement of the new deepVISION Resistivity** tool, introduces the tool prototype, and proposes a method for computing distance from the tool to boundaries using conductivity contrast. Field test examples from the Grane field (offshore Norway) are also presented.
The new deepVISION Resistivity tool has been tested in several horizontal wells and has demonstrated its capability to accurately determine distance to reservoir boundaries and fluid contacts. The results clearly show the tool's potential for applications such as well landing, geosteering, fault throw estimate, reservoir mapping, and reduction in true vertical depth (TVD) uncertainty.

Introduction
Reading deep into the formation has always been an objective of formation evaluation. Historically, when drilling vertical wells, reading deep was required to image seismic reflectors with intermediate vertical seismic profile (VSP) runs in order to "look ahead" and to overcome invasion or borehole damage with deep-resistivity devices.
With horizontal well drilling, measurements are now used to identify nearby bed boundaries for geosteering purposes1,2,4. This technique requires a detailed description of the geometry and petrophysical characteristics of the reservoir. Placing the well in the right place inside the reservoir requires the ability to steer the well path according to the reservoir boundaries. However, reservoir boundaries are often beyond the radial response of LWD devices and only specialized wireline services such as the Borehole Accoustic Reflection Survey tool (BARS) have been able to detect such boundaries from within a borehole. Indirect techniques that use resistivity height functions3 derived from offset wells to locate fluid contacts have also been used with varying degrees of success.
The new deepVISION Resistivity LWD device was developed for proactive well placement. The tool has a very large radial response that allows accurate determination of the distance to reservoir boundaries and fluid contacts tens of meters away from the borehole. The tool has a modular design with separate transmitter and receiver subs that can be placed anywhere in the bottomhole assembly.