3D Azimuthal LWD Caliper
Authors: Labat Chris , Sam Brady , Everett Mark , Ellis Darwin , Doghmi Mohammed , Tomlinson John C. , Shehab Gamal 
Published: December 2007

The need for a caliper measurement for logging-while-drilling (LWD) has prompted developments in several measurement domains. Hole size measurements are mainly derived from ultrasonic and electrical devices. However, both techniques have significant limitations. The ultrasonic measurement requires a good impedance contrast between mud and formation. Its limited range is strongly affected by mud weight, which attenuates the signal. Electrical calipers currently operate in conductive mud systems and offer only an average hole-size measurement.

This paper discusses the development of a new azimuthal density-derived caliper measurement. An algorithm and process were developed to use the 16 sector densities measured by a downhole LWD and measurement-while-drilling (MWD) tool for computations of 16 borehole radii to provide a good representation of the borehole. As with all physical measurements, some limitations exist for accurate measurement in large washouts and in heavy mud systems. However, the measurements are independent of borehole fluid resistivity and can be acquired in oil-, synthetic-, and water-based mud systems. Several comparisons of the derived caliper measurements and wireline multifinger caliper logs confirm the measurement is valid in holes with large washouts. The algorithm was tested in hole sizes from 6 to 13 1/2 in. (bit size from 6 to 10 5/8 in.) and mud weights to 15 lbm/gal. Three-dimensional visualization software allows analysis of borehole shape to be carried out on any personal computer. Timely feedback of this information enables changes in drilling and/or mud parameters to improve borehole shape and drilling efficiency. This responsiveness should lead to better well positioning and better petrophysical data.

Recent applications have demonstrated the benefits of a true three-dimensional (3D) azimuthal caliper measurement. These benefits include borehole shape analysis for stress orientation; detection of problems such as washouts, ovalization, and spiraling; improvement of cement job design and completion strategies; and optimization of drilling parameters. These applications will be discussed and illustrated in this paper.

Introduction

Drilling is an inherent geometric activity, thus efforts to improve drillers' "vision" by providing knowledge of the geometry of the borehole is always of interest. Today, hole size measurements are mainly derived from ultrasonic and electrical devices. However, both techniques have significant limitations. The ultrasonic measurement requires a good impedance contrast between mud and formation. Its limited range is strongly affected by mud weight, which attenuates the signal. Electrical calipers currently operate in conductive mud systems and offer only an average hole-size measurement.

Taking advantage of the compensation technique for dual-detector density devices, we have constructed a density caliper from an alternative interpretation of the information normally collected for compensating the density readings for tool standoff.

As with all physical measurements, some limitations exist for accurate measurement in large washouts and in heavy mud systems. However, the measurements are independent of borehole fluid resistivity and can be acquired in oil-, synthetic-, and water-based mud systems. We begin with an explanation of the borehole size estimation technique and a discussion of the limitations of the method before moving on to some illustrative log examples and applications.