LWD for Imaging, Wellbore Placement, and Formation Evaluation
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 17646, "LWD for Imaging, Wellbore Placement, and Formation Evaluation," by J.C. Rasmus, SPE, C. Esmersoy, SPE, J. Seydoux, and A. Hawthorn, Schlumberger, prepared for the 2005 Offshore Technology Conference, Houston, 2-5 May.
Logging-while-drilling (LWD) measurements are widely available for a variety of applications. Many of the first generation LWD measurements are similar to those obtained traditionally with wireline methods. However, there are measurements available now that are either complementary or are entirely unique compared with traditional wireline measurements.

Introduction
LWD borehole-seismic measurements can enable placing the bit on the seismic map or producing a seismic-impedance map ahead of the bit while drilling the well. These measurements provide accurate depth determination of drilling targets and identification of pore-pressure transgressions.
LWD ultradeep-resistivity (UDR) measurement technology meets the requirements for geosteering at the reservoir level by use of subseismic resolution. Its major applications include delineation of the oil/water contact (OWC) and distance to reservoir caprock and bottom shale.

Well Placement
Since the emergence of LWD in the 1980s, the industry wanted a reliable while-drilling borehole-seismic measurement. However, several technical roadblocks prohibited its realization until recently. These difficulties included the nature of and required accuracy of the measurement, timing issues, sensors, automation, and the lack of an electrical connection between the downhole tools and the surface in a while-drilling configuration.
Technological solutions to these road-blocks have since been found. Seismic-while-drilling data are delivered in real time to make critical decisions while a well is being drilled. They also replace intermediate wireline check-shot surveys, eliminating the need to stop the drilling process and pull drilling equipment out of the hole for the duration of a survey.
Real-time accurate positioning of a horizontal borehole within a reservoir can be performed with standard LWD tools. However, a limitation of these measurements is the relatively small volume of investigation with traditional LWD sensors, which typically probe no deeper than 1 m into the formation. Therefore, unpredictable geological events are, in many cases, detected too late to allow proactive geosteering decisions.
A prototype LWD resistivity tool was developed with a much larger radial response that allows detection of lithology features and fluid contacts that are tens of meters away from the borehole. The tool has a modular design with separate transmitter and receiver subassemblies that can be placed a large distance away, anywhere in the bottomhole assembly (BHA).
The main output is estimated distance to boundaries indicated by resistivity contrasts. These distances are computed through inversion of a formation model in real time. On the basis of tool measurements, determining the optimum formation model and its related parameters allows proactive geosteering decisions and pro-vides significant geological insight into the reservoir structure.