Gyrostab Project: The Missing Link Azimuth And Inclination Mastered With New Principles For Standard Rotary BHAs

Abstract

Abstract Trajectory control in directional wells essentially requires control of only two parameters: well inclination and well azimuth. However, always tighter economic constraints and the increasing complexity of well trajectories keep setting higher challenges that demand purpose-designed and cost-efficient solutions. The standard rotary drilling tools first used for directional drilling gave poor trajectory control. The subsequent addition of Variable Gauge Stabilizers (VGS) addressed only one side of the problem: control of inclination. Both inclination and azimuth were then mastered with Steerable Motor technology, which rapidly gained supremacy. However, there has been discontent with Down Hole Motors (DHM) for a variety of problems ranging from Rate Of Penetration (ROP), sliding mode, friction and lost-in-hole incidents to well tortuosity and hole cleaning. Jumping to rapid conclusions, one might thus think that that motors will soon be replaced by Rotary Steerable Systems (RSS). This evolution of directional drilling techniques has led to the development of more and more sophisticated tools integrating electric and/or hydraulic power systems and loads of "embarked" electronics. The related expenditures have driven service costs to levels that are very often prohibitive for standard drilling programs and/or environments. This paper introduces an innovative technical solution that makes it possible to control both azimuth and inclination using standard rotary Bottom Hole Assemblies (BHAs). It is based on a patented concept permitting command of Bit & BHA walking tendencies through downhole friction management at the level of a "strategic" string stabilizer. On the basis of this concept, a joint industry project led by TotalFinaElf has successfully designed, built and field-tested a first prototype - the "Gyrostab". This missing link in the evolution of directional drilling announces the birth of a new generation of standard rotary drilling tools. Introduction: A short history of directional drilling From the early 1930's into the 1970's, expertise in trajectory control was in large part devised and developed in the "tally book" of directional drillers. Using rough and unreliable directional drilling tools without any continuous measurements, their very exhausting task at the time was to control trajectories single shot after single shot, relying more or less on "feeling" to anticipate what might happen down-hole. Motors for directional drilling were still in their infancy (the first DHM appeared in 1955), and standard rotary drilling was used on a very large scale (Figure 1). However, the challenge for drillers was lightened by the fact that directional well trajectories were simple (2D, J or S shape wells, with moderate inclination values), and long runs were out of the question in any case because of bit life. Towards the end of this period, DHMs and bent subs were employed only to initiate the kick-off. Rotary BHAs were used to complete the build-up section and the following slant and/or drop-off sections, with short motor correction runs to put the well-path back on track when necessary. Left alone the rest of the time, the azimuth would drift by more than 1 degree per 100 feet in some cases, especially when using rock bits.