Friction Analysis for Long-Reach Wells

Abstract

Presently wells are drilled in the North Sea approaching a horizontal reach of 8 km. Plans for the near future is to extend these towards and beyond 12 km. Well friction is one of the most important limiting factors in this process. Torque and drag prognosis are today developed on in-house simulators. Although a good tool for planning, improvements are made on an trial and error basis, and, these simulators have limited availability. To provide more insight into the frictional aspect, a larger study was undertaken. Explicit analytical equations are derived to model drill string tension for hoisting or lowering of the drill string. The equations are developed for straight sections, build-up sections, drop-off sections and side bends. Both constant curvature models and a new modified catenary model are derived. The new catenary model is developed for arbitrary entry and exit inclinations. Equations to determine well friction in fully 3-dimensional well profiles are also given. Furthermore, based on the tension equations, expressions for torque and drag are developed. Equations for combined motion and drilling with motor are also given. Using these equations, the total friction in a well is given by the sum of the contributions from each hole section. A field study offshore Norway is included in the paper. Using the equations derived in the paper, the well path is chosen to minimize the torque on the rig, which is the limiting factor. The paper also summarize a number of guidelines for extended-reach well design, and shows the design of an ultra- reach well of more than 12 km reach. P. 819

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