Abstract

The optimum placement of centralizers does not only incorporate the achievable standoff. In highly inclined and horizontal well situations the consideration of friction forces is relevant to assure, that a string can be run safely to bottom.

The set of equations described in SPE 21282 and the API Specification 10D of January 1, 1995 were revised and corrected. The enhanced equations and a newly developed algorithms to calculate torque and drag were described in IADC/SPE 36382.

A new software based on these mathematical models was developed and installed strategically in over 100 locations worldwide. The software was programmed in "C" and runs under Windows TM. It allows the interactive simulation of different centralizer pattern and its effect on the expected hook load and torque. Simulated data compared with real field situations showed excellent compliance.

Introduction

Careful planning and engineering are critical for the success of the cementing job with proper centralization necessary for good mud removal (Fig. 1). The optimum placement of centralizers does not only incorporate the achievable standoff. In highly inclined and horizontal well situations the consideration of friction forces is relevant to assure, that a string can be run safely to bottom.

A new mathematical simulation model based on revised API-10D formulas is used to calculate the optimum spacing of centralizers to obtain the best standoff at a given borehole location. The revised mathematical model was described in [3]. It is associated with a torque and drag analysis, taking into account the known running forces of the centralizers as well as the friction factors depending on the mud type. This analysis is important in order to evaluate whether the desired centralizer spacing can be run or rotated without delays, problems due to high drag forces, or without damaging the pipe connections.

A powerful computer program (CentraPro PlusTM) has been developed which runs under a Windows operating system. It makes use of the standard conventions defined for these systems. Users familiar with a Windows environment are able to run the software intuitively based on those conventions.

2 Technical and Mathematical Background

Centralizers are used to optimize the hydraulic flow during the cementation process and to minimize drag forces caused for example by differential sticking. Two types of centralizers are known: bow-type centralizers and rigid centralizers. The bow-type centralizers are made of two collars and a number of metal springs attached to them. The characteristics of the metal springs is known, so that the deflection of a centralizer in a given load situation can be determined. Rigid centralizers have a fixed outer diameter which is basically independent of lateral forces. The decision which type of centralizer to use depends on several factors such as the borehole condition (see table 1).

During the cementation process, the mud used to drill the well has to be replaced by cement.

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