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Abstract

This paper presents a method to evaluate an overall friction coefficient between borehole and casing. The computation is done by matching hook load data, recorded in the field, with the calculated hook load obtained by assuming a friction factor coefficient. The equations for predicting surface hook governing differential equations. The iterative nature of the calculations are handled by a computer program. One case history from a rig offshore Louisiana State, confirmed the possibility of evaluating this friction coefficient, while also describing the equipment and necessary care used to record the data.

Introduction

A new method for casing design in directional wells was presented in Nov/1985. It introduced a new constraint to the casing design process by imposing the need of upward pipe movement, whether for reciprocation pipe movement, whether for reciprocation while cementing, whether for pulling out part of the (or the entire) casing string from the hole, due to any technical problem while running it in. This de sign method requires, among other data, the knowledge of an overall friction coefficient.

Other design methods have simply ignored the effect of wall support on casing, and simple don't design for the possibility of any upward pipe movement. The method of the vertical projected depth is an example of this. In this situation, when reciprocation for cementing purposes is required, a reciprocation trial takes place when the casing shoe reaches the bottom of the hole that isn't always successful.

In this research work, friction between the borehole and casing is looked at in a macroscopic scenario, where its overall average value is relatively constant along a borehole. This approach is similar to that undertaken in previous research on the drillstring borehole friction. The overall friction coefficient is a simplification of what is believed to be a complex mechanism of mechanical interaction between a tubular string and a borehole. Though it basically ignores the effects of lithology stratification the compressive and shear strength of the rocks, rock and casing hardnesss, and the everchanging borehole diameter. It provides an attractive simplification useful in directional casing string design for a given location.

The overall friction coefficient equation can be summarized as:

FF = HL-VPBW (1)NF

Unfortunately equation (1) is of no practical importance for this situation due practical importance for this situation due to the dependence of the normal force (while on a curved path), on the axial load immediately below. Thus a computer is coefficient having a recursive procedure and an iterative search procedure built in it.

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