Directional Drilling - A Comparison of Measured and Predicted Changes in Hole Angle

This paper compares measured hole angle changes and predicted angles between bit force and hole direction. The predictions were made using an analytical program that assumes the drillstring is a static beam with varying properties constrained by borehole geometry. Data were taken from a field study conducted in Dubai. Agreement was good for 8 of 12 cases and improved with use of an interpretive length. Introduction Directional drilling continues to play an important part in offshore field development. A more thorough and firmly based understanding of the factors affecting hole angle and direction are needed so that drilling can be done as economically as possible and so that overall drilling technology can advance at a uniform rate.Probably the most concentrated recent effort toward a logical and thorough understanding of the factors involved in directional drilling is that of Millheim. Among his works are a series of articles dealing with proper use of directional tools, bottomhole assembly (BHA) mechanics, and behavior of single- and multiple-stabilizer BHA's. In the first of these articles, he lists 46 references but states that there is still much to learn. Later in the series, he presents material derived from postanalysis of more than 70 wells in the U.S. and overseas, field tests, computer analyses, literature, and research.In other articles, Millheim et al. describe a finite-element method of analyzing BHA's and Millheim discusses hole curvature as it relates to hole trajectory. Among the conclusions reached are that reaction forces at the bit can be calculated, the dropping or building tendencies of various assemblies can be assessed, and hole curvature can play a significant role in deviation and deviation control. Walker and Friedman present a three-dimensional force and deflection analysis of a drillstring and state that the use of the model can improve the understanding of how various assemblies compare and can assist in determining the direction the bit will drill. Fischer describes an analysis of a drillstring in a curved borehole and concludes that valuable insight into the mechanical performance of drillstrings can be gained using such an analysis. In discussing the factors that affect the control of borehole angle in straight and directional wells, Bradley gives information relating to the location of drillstring tangency, drill collar stiffness and weight, placement of stabilizers, and downhole motor case flexibility. He emphasizes the need for additional research into the bit/rock interaction as it affects the deviation process.Analytical programs like those in Refs. 1 through 6 calculate the force acting at the bit. Although it generally is agreed that the bit tends to drill in the direction of this force, Millheim and Warren indicate that no method is available to predict rates of build, drop, or turn for a particular set of conditions. JPT P. 2090＾