CASING DESIGN IN DEVIATED AND HORIZONTAL WELLS
Calculation of the axial loads is the most challenging part of directional-well casing design. Using the maximum load principle, the concept of the maximum pulling load is applied. This concept is derived from the observation that the greatest value of tensile stress in directional-well casing occurs during the casing running operation. Working the string up generates the highest tensile loading because the friction (drag) generated by the normal force between the rasing and the borehole and the friction factor (this last parameter is discussed later in this
chapter) oppose the direction of motion of the string.
The magnitude of the drag force depends on the friction factor and the normal force exerted by the casing. Maximal drag force is usually experienced either when the casing is pulled on after it is stuck in a tight spot or on the upstroke when reciprocating the pipe during cementing operations. In order to determine the drag-associated axial force, one requires an accurate knowledge of well profile, hole geometry and borehole friction factor.