Drag Force
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(•2.51) |
Casing strings are usually reciprocated or rotated during landing and cementing operations, which results in an additional axial load due to the mechanical friction between the pipe and borehole. This force is described as drag force, F^, and is expressed as:
Fd = -fbFn
where:
fb — borehole friction factor.
Fn = absolute value of the normal force.
Thus, the magnitude of the drag force depends on the friction factor and the normal force resulting from the weight of the pipe. Due to the complex geometry of deviated wells, the drag force is a major contributor to the total axial load. It is, however, extremely difficult to predict the borehole friction factor because it depends on a large number of factors, the most important of which include: hole geometry, surface configuration of casing, drilling fluid and filter cake properties, and borehole irregularities.
As a result of field experience and laboratory test results, several methods for calculating friction factor have been proposed. In a recent study, Maidla (1987) proposed the following analytical model for the friction factor:
f Fh — FbUv ± Fvd
h = ТШШ (2л2)
where:
Fh — hook load. lbf.
Fbuv = vertically projected component of buoyant weight, lbf. Fvd = hydrodynamic viscous drag force, lbf.
Wd(l. fb) = unit drag or rate of change of drag, lb/ft.
/ = length of casing, ft I — measured depth, ft.
The above equation was used extensively by Maidla (1987) under field conditions and the values of friction factors reported varied between 0.3 and 0.6. Drag force in a vertical well is relatively low, so methods for estimating friction factor and related drag force are discussed under Casing Design for Special Applications on page 177.