Abstract
The challenges involved in maintaining straight and truly vertical bores in the vertical section of the well continue to plague the drilling industry. These challenges result in massive losses in drilling efficiency, and subsequently excessive costs associated with complications caused by "corkscrew" wells. Increasingly complex drilling programs are being designed, often involving extended lateral and horizontal sections to be drilled to optimize production through multiple payzones. As the reach of these wells is extended further each year, the importance of maintaining a truly straight and vertical top section becomes more and more critical. The deeper the well and the deviated reach extends, the more difficult it becomes to move the drill string through the well bore and continue drilling ahead to target. The vertical section of the well has a particularly high impact in these wells as the weight that is transferred though this section must be efficiently transferred down the well bore in order to effectively provide weight on the bit to penetrate the formation and continue to drill ahead.
As the drill string drills ahead vertically, natural variations in the earth or tendencies in the formation caused by formation layers that are deposited at an angle to the surface or random deposits of hard rock can push the bit off of its true vertical trajectory. When this occurs, a correction must be made to steer the bit and drill string back on course for true vertical. The more curves or kinks created by correcting the drift of the bit, the more points of contact exist through the vertical section.
The number of deviations and corrections in a well is often referred to as its tortuosity. These points of contact each have a frictional impact on the drill string and ultimately result in increased levels of torque and drag. These forces ultimately hinder the drilling process by interfering with the transfer of weight through the vertical section to the drill string below as well as causing a resistance to the rotation of the drill string.
