An Engineered Approach to Cement-Lined Tubing
Authors: Runyan E.E. 
Published: December 2007

This paper presents previously unavailable date on the physical properties of cement-lined tubing. Using these properties, calculations are made to predict tubing behavior under field conditions. The results of these calculations are verified by laboratory and field tests. The principal conclusions reached are:

cement-lined tubing strings are effective to 9,000 ft, and calculations indicate that satisfactory performance will be obtained at depths to 14,000 ft; hairline cracks do form in deep tubing strings, but they cause no break in corrosion protection; and sour water systems offer no hazard to cement-lined tubing.

Introduction

Lining injection-well tubing with cement is an old and proven approach to protecting tubing strings from internal corrosion. However, little work has been done in determining the actual physical properties of cement lining, and even less work has been done in applying these properties to engineering calculations to describe and predict its behavior. It is the purpose of this paper to contribute to the correction of this situation. The approach taken toward this goal is an examination of basic information on cements, with a cement lining being clearly defined. The properties of the lining are then examined. Engineering calculations are made to predict performance of the lining system. Laboratory and field-test results confirming these calculations are presented. Finally, applications and limitations are discussed along with recommended installation methods. A presentation of case histories concludes the paper.

Background

A wealth of base information on cements and their application is available in the literature. This information is widely known and used by the other engineering sciences. Petroleum engineers, however, have not had ready access and exposure to this information; and, consequently, it has not been effectively applied to many oilfield problems. Some facts pertinent to the cement lining of tubing are:

Properties of cement mortars vary widely, and, therefore, they can be adapted to meet many applications. Cement mortars in themselves are effective corrosion inhibitors. Within limits, cracking of cement lining has no effect on corrosion protection. Cement linings are self-repairing. An excellent bond can be obtained between the walls of the steel tube and the cement sheath.

Properties of cement mortars will vary almost as widely as properties of the constituents used in them. Examples are the new epoxy-cement patching mortars and the new high-temperature mortars. In addition, properties are affected by the method of placing and curing the mortar in the pipe. Obviously then, cement linings can be engineered to many specific applications.

The effectiveness of concrete in protecting steel from corrosion in marine structures has been recognized for many years. This same protective mechanism will protect steel pipe with a lining of cement. Basically, this protective mechanism results from moisture in the cement absorbing calcium hydroxide and maintaining the pH in the steel-contact area above 12. At this pH, corrosion of the steel is prevented by the formation of a protective iron-oxide film on the steel surface. The protective mechanism is effective even in the presence of oxygen as long as the pH is maintained in the inhibitive range. This fact can be easily illustrated by placing common steel nails in two jars of tap water. To one jar add a few pieces of cement mortar. After allowing time for corrosion to occur, observe the nails. The nails in the jar containing fragments of cement will be unaffected while the nails in the second jar will be badly corroded. The protection will be due to the pH rise and consequent steel passivation in the jar containing the cement.

Cement linings always contain some moisture and when in water service are completely saturated. They are occasionally cracked during installation and are usually applied to surfaces which have not been sandblasted. Yet, they give effective protection. Again this protection is due to two properties of cement commonly known to structural engineers but often overlooked by petroleum engineers. The first of these is the passivation of the exposed steel in the crack by the pH rise of the water in the crack as discussed earlier. During installation a tension crack is formed, water immediately fills the crack, it absorbs calcium hydroxide from the freshly exposed cement surfaces, the pH rises and the steel is passivated. Of course there are limits to this process, but authorities agree that cracks of up to 2 mils offer no problem.

JPT

P. 626ˆ