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Abstract

The presence of borehole rippling, spiraling, and hour-glassing have been known for many years in the drilling industry. Descriptions of these problems have become more precise with improved logging tools, yet the underlying mechanisms have not been mathematically formulated. Many directional models have been presented in the past that attempt to explain the steady state response of the bottom hole assembly, but little has been published on the non-equilibrium or dynamic response of the system.

A theoretical/analytical dynamic model is proposed to explain the conditions required for borehole oscillations to occur. This model is presented along with correlating data from laboratory and full scale field tests for a steerable motor with an 8-1/2" hole size. For different parameters the model correctly predicts the presence and absence of oscillations, the oscillation wavelength, and the effect of bit geometry on borehole oscillations. It also makes testable predictions for the effects of bit speed, penetration rate, and rock strength. From these test cases, general guidelines are presented for selecting the appropriate bit, BHA, and operating parameters for an application.

Background

There are at least three key issues to be understood and optimized when drilling directional wells with motors; tool face control, dogleg severity (DLS) and borehole quality. In particular, this paper will focus on hole problems from the well path, not from sloughing, caving, erosion, etc.

Significant progress has been made in understanding and improving tool face control with PDC drill bits. Specifically, the recent papers on reducing torque fluctuations with PDC bits have had a significant impact on utilization of these bits on steerable motor systems.^1-3

Over the last 30 years several models have been published to predict DLS based on the bit and BHA.^4-23 The experimental and analytical work in this area has significantly improved the state of the art and this continues today.

Finally, the effect of borehole quality on torque and drag, log response, and running casing has been well documented from Lubinski's work in the 1950s to the recent papers by Bellay, Chen et.al.^24-30

Field data and early lab tests showed that short aggressive gauge bits yield poorer quality boreholes in terms of instantaneous or local dogleg severity and caliper measurements than longer full diameter gauge bits.¹ This has been adopted in some areas and resisted by others. However, there is a current trend to look at hole quality more critically and to recognize that there are bit designs and system parameters that affect this.

Recent papers have focused on the oscillating or cyclic nature of some of these persistent borehole problems. In particular, hole spiraling, rippling and hour-glassing have been reported many times and their costly effects are becoming better defined. However the underlying mechanisms for these have not been explained in detail. This will be the focus of the rest of this paper.

The motivation and inspiration for this work is a result of the number of recent reports of hole undulations in the field on a variety of BHAs. These oscillations can significantly affect: 1) torque and drag while drilling, limiting the reach of many wells and causing significant tool failures, 2) log quality, particularly showing up in high resolution image logs causing problems in interpretation, and 3) in the ability to run casing to bottom, the caliper log may not show a problem but the net drift diameter of the hole over a long section can be less than the casing diameter.