Continuous Borehole Curvature Estimates While Drilling Based on Downhole Bending Moment Measurements

Abstract

The paper shows that a downhole bending moment measurement combined with a simple mathematical model can provide a good estimate of the wellbore curvature, matching the results obtained from survey data analysis quite well. In addition, the bending moment measurement delivers wellbore curvature information at a much higher resolution than the standard Minimum Curvature method, which assumes a constant curvature between the survey stations. The paper presents numerous examples of downhole bending moment data measured in deployments with both rotary steerable and steerable motor BHAs and compares the dogleg severity estimates from the bending moment data with the dogleg severity derived from the survey measurements. Accuracy and limitations of the measurement and the model predictions are discussed. The paper also introduces a new method of analyzing cyclic hole effects applying FFT techniques to the depth-based bending data. Introduction Any drilling bottom hole assembly (BHA) in a directional wellbore is subjected to bending moments due to side forces acting on the BHA. These side forces may be caused by gravity, by dynamic effects, or by wall contacts of the BHA in curved wellbore sections. In the past, mathematical models have been developed5 to predict the side forces, bending moments and stresses that are introduced into a BHA in a given borehole curvature profile, usually derived from measured survey data. This paper discusses the application of mathematical models in the opposite direction: to predict the borehole curvature from measured bending moment data. The bending moment data have been acquired with a recently introduced drilling dynamics tool3 in a variety of different applications and tool configurations including rotary steerable systems (RSS) and steerable motor BHAs. The bending moment data can be both transmitted to the surface while drilling and recorded in on-board memory for post-run processing and detailed analysis. Bending moment data recorded during directional drilling operations have been published in 1989 by Cook et al.6; however, their analysis was limited to some qualitative statements. Hood et al.7 demonstrated recently how real-time bending information can successfully reduce the risk when drilling hard interbedded formations with an increased tendency to develop high local doglegs at the formation interfaces. Dogleg Severity The drilling industry uses the term dogleg severity (DLS) to describe the total curvature of a directional wellbore. The dogleg severity is derived from the directional survey data as explained in detail in Appendix A. All DLS calculations from survey data in this paper are based on the Minimum Curvature method. Here, the well path between two survey stations is assumed to be a circular arc with constant curvature, i.e. constant dogleg severity. Appendix B shows that the bending moment data constitute a measurement of the curvature of the BHA at the point of the bending moment sensors. With the assumption that the curvature of the BHA does not differ significantly from the curvature of the wellbore the BHA is in, an estimate of the wellbore curvature is obtained. The goal of this paper is to demonstrate the validity of this assumption for a variety of cases and to discuss the influencing parameters. The examples in the next section compare the DLS calculated from the survey data with the DLS estimated from the bending moment data. For better readability, the terms survey DLS and bending DLS are used from hereon. A sensitivity analysis in the section after next illustrates the influence of various factors on the accuracy of the bending DLS estimate.

Visualization of BHA Dynamics Improves Understanding of Downhole Drilling Conditions, Speeds Up Learning Curve

“…The model has been successfully validated using downhole bending moment measurements as documented in earlier publications 6,7 . Again, graphical visualization tools are a great help to get an overview of the model results and to understand the static response of BHAs in directional wellbores.…”

High-Speed Wired Drillstring Telemetry Network Delivers Increased Safety, Efficiency, Reliability and Productivity to the Drilling Industry

“…This measurement can be used to accurately estimate borehole curvature 28 and has also proven to be valuable when drilling through highly interbedded formation sequences where formation strength varies greatly across interfaces. This measurement can be used to accurately estimate borehole curvature 28 and has also proven to be valuable when drilling through highly interbedded formation sequences where formation strength varies greatly across interfaces.…”