Abstract

The objective of this paper is to show the application of a computer program developed at CENPES (PETROBRAS Research Center), which performs analysis of a drillstring. In the case of a horizontal well it is possible to simulate several designs, with the goal of extending the length of its horizontal span and thus increase of its productivity. The working and the basic characteristics of the program are described, as well as the analysis of the factors that influence and limit the maximum examples are shown to validate the program and to calculate the lateral forces, drag, torque and buckling load, in order to verify the design.

Introduction

A design optimization of a horizontal well requires, necessarily, the strength analysis of a drillstring and the computations of the maximum torque and drag imposed on the rotary and hoisting systems of a drilling rig. For vertical or slightly deviated wells, the only lateral force reaction of importance is the one on the bit. All other are negligible. In this cases, it is a common practice to simulate only the bottom hole assembly. However, for highly deviated wells, specially for horizontal ones, large lateral reactions arise, due to the contact between formation/ casing and drillstring. Under these circumstances, it is imperative to simulate the structural behavior of the whole column. In doing so, it is possible to compute the lateral reactions, from which the torque and drag are then obtained with necessary reliability. Also, the buckling load depends on this analysis. In order to consider the horizontal length as resting on the bottom wall of the well. A computer simulation program that uses a linear theory is not capable of computing such calculations. A nonlinear method is therefore required in order to adequately describe two important phases (1) the large deviation of the straight drillstring, to comply with well trajectory and (2) the lateral displacement constrained to the gap between column and well. In recent years, a powerful tool for structural analysis has been developed at Petrobras Research Center CENPES), by the Petrobras Research Center CENPES), by the Drilling Sector, for use in microcomputer environments. Among several other versions, there is a static analysis program which considers the limited lateral displacement (gap), as in the case of a drilling column confined in an oil well. A nonlinear structural analysis theory is used, combined with the finite element method. Their combination yields the necessary accuracy to obtain satisfactory results for slender structures subject to axial and transverse loads. In order to reach the desired values, a beam column finite element resting on elastic foundation has been implemented. Then, it is possible to control, for each point of the lumped structure, the lateral displacement of the column and the reactions at points of contact with the casing or rock formation. The basic equations are presented below. The output yielded by a drilling column simulation includes the following data: the displaced column, reactions forces on the points of contact, bending moments, and axial forces. With this basic information provided by the structural calculations, an provided by the structural calculations, an analysis of the mechanical behavior of the drillstring can be performed. Horizontal wells have been demonstrating their validity in production increase. Extension of the horizontal span to override the limitations imposed by the drilling operation. With this objective in mind, the following aspects are verified: limitations imposed by the maximum admissible torque of the column or the rotary table; limitations due to the maximum admissible drag; limitations arising from the buckling load in the horizontal phase. All these aspects are used to verify phase. All these aspects are used to verify the desired replacement of drill collars or heavy-weight pipes by drill pipes over the horizontal length.

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