Evolution Of An Lwd Toolstring With Applications For Petrophysical Logging And Drilling Control
Authors: Dowell I.A. , York P.L. , Jackson C.E. 
Published: October 2008

The first Measurement While Drilling (MWD) devices were used in the late 1970s and gave only directional information during drilling. Since then, MWD technology has advanced rapidly, with recent developments making reliable Logging While Drilling (LWD) and Logging After Drilling (LAD) for formation evaluation a reality. This paper discusses the development of a new LWD toolstring that accurately measures numerous petrophysical parameters and gives useful drilling information in a timely manner.

The new LWD toolstring furnishes a Triple Combination Suite of logs"namely, resistivity, density, and neutron with supporting gamma ray, caliper, and directional measurements. Tool development was based on studies that used computer simulation, test formation data, scale models, and information obtained from years of experience in MWD and wireline operations.

The toolstring combines features from both MWD and wireline disciplines to optimize the measurements. Important features include:Borehole-compensated resistivity, density and neutron measurementsResolution-matched symmetrical resistivity MeasurementsPlacement of the density sensors behind a mud-excluder device to minimize borehole effectsUltrasonic standoff and caliper measurementsA Pe measurement for Iithology identificationA downhole shock indicator

The caliper is a three-transducer ultrasonic device that provides both standoff and borehole geometry information used in correcting the other tool measurements for environmental effects. In addition, the caliper allows borehole volume to be calculated and helps identify and characterize intervals that cause drill pipe to stick.

Several features of the MWDILWD toolstring help ensure smooth drilling operations. The downhole shock indicator gives real-time warnings of critical bottomhole assembly (BHA) vibration. The tools have been designed to easily traverse doglegs and horizontal holes. Numerous safety features ensure the reliable containment of all nuclear sources when used in the LWD tools.

The new MWD/LWD toolstring provides for integrating its data with drilling, geological, and wireline data as part of a system of continuous logging services. This enables comprehensive evaluation of downhole conditions from spud to well abandonment.

Introduction

In the mid-eighties Halliburton introduced the Dual Resistivity MWD tool1. This device provides two qualitative resistivity measurements. The first measurement is made with a well-focussed lateral-type instrument with a depth of investigation similar to a wireline shallow Iaterolog device (see Figure 1). This lateral measurement is very successful in high-salinity mud. The second measurement provided MWD's first genuine "at the bit" reading. This bit resistivity measurement is most effective when the dual resistivity tool is located immediately above the bit with no near-bit stabilizer. Although it has proven very difficult to quantify the bit resistivity due to the wide variety of bits in common use, the measurement has an established niche where early and accurate picking of coring and casing points are essential. This is particularly the case in oil based mud where a high-definition bit-sensitivity log can be produced, giving excellent thin-bed delineation. There are occasions, however, when the environment severely challenges the Dual Resistivity device to produce logs comparable to wireline logs. In oil-based mud, and in freshwater mud in a Iow-resistivity formation (e.g., the U.S. Gulf Coast), this is especially true.