Abstract

Under a corrosive downhole environment, the completion of a well might be damaged if no precautions are taken. Pipe made of corrosion resistant alloys (C.R.A.) is used to minimize corrosion. An analysis of various corrosion mechanisms shows that the surface integrity is an important factor for corrosion prevention.

Conventional spider and elevator inserts cause die marks that support corrosion. A new developed gripping system distributes the required load equally onto a large number of small peaks, minimizing the indentation of each single peak.

Electrochemical investigations, the measurement of the surface indentations and microscopic analysis compare the effect of conventional dies and the new gripping system on C.R.A.-pipe.

Using the new gripping system, the surface integrity is not damaged and the corrosion potential is virtually eliminated.

Introduction

Depending on the conditions within a borehole, a completion may be subjected to corrosive agents. Corrosion resistant alloys (C.R.A.) are used in order to reduce corrosion damage to casing- and tubing strings and thus to prolong the life span of the completion.

The degree of resistance to corrosive agents such as acids, acid gases or salts is mainly determined by the surface integrity of the pipe. Surface damages caused by inappropriate handling or by conventional slip markings contribute directly to the development and the propagation of corrosion.

In order to eliminate these undesired effects, a new spider gripping system has been developed. It practically eliminates pipe surface damage during handling and running. Any corrosion potential is virtually eliminated.

The paper describes a series of laboratory tests performed at the Institute of Metallurgy of the University of Hannover in Germany. These tests show, that the corrosion potential of the surface handled by the new spider and elevator gripping system is virtually eliminated.

2 Corrosion Mechanisms and Corrosion Protection

Corrosion is defined as a change of metallic materials caused by an interaction with its environment, especially when it is undesired and leads to destruction. The basic reason for corrosion is the thermodynamic instability of most metal alloys which tend to reach a stable condition under given circumstances. In general, corrosion is an interface reaction between metal and an electrically conductive water solution.

The different kinds of electrochemical corrosion can be categorized as follows (figure 1):

  • constant surface corrosion with hydrogen development and/or oxygen consumption (reduction)

  • contact corrosion

  • selective corrosion

  • crevice corrosion

  • pitting corrosion

  • intercrystalline corrosion

  • stress corrosion cracking.

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