Abstract

Present methods of corrosion mitigation, although successful and Present methods of corrosion mitigation, although successful and advantageous over past methods, appear to be highly overdesigned and thus prove to be expensive. prove to be expensive. In order to reduce the cost of corrosion control, the authors have proposed a series of calculation steps that limit the corrosion control to specific intervals of tubing rather than the whole string of tubing. Further cost saving measures can be realized by completing the corrosive wells with a system that monitors corrosion and regulates the mass flow rate of inhibitor into the tubing automatically.

Introduction

In order to review the fundamentals of subsurface system design, our approach is first to review the steel structure, basic causes of corrosion, failure analysis and present technology as employed in the oilfield to mitigate corrosion and then to present our design methodology, which is suitable for the corrosive environment.

Steel Material

In discussing the cause and effect of corrosion on tubular goods used in oil wells, one should first review the basic micro-structure of steel, and then address the causes of corrosion. Once a systematic approach is taken, the design of production tubing for a corrosive environment can proceed toward fruition.

Basically, steel is defined as iron containing carbon, 0.05 percent or greater by weight. Pure iron is called "ferrite". It is a soft ductile material of a tensile strength of 40,000 psi. Obviously, if high quantities of ferrite are found in a tubing under high tensile stresses, it is likely that the tubing would fail. What keeps the iron and the carbon together is a material called "cementite", with the chemical formula Fe3C. Cementite is a very hard, non-ductile material of high tensile strength. When cementite and ferrite are mixed together, they form a laminar structure called "pearlite". As the temperature rises, the lower critical or upper critical temperature levels are reached, depending on the percentage of carbon present in the steel. At the lower critical percentage of carbon present in the steel. At the lower critical temperature, "austenite" is formed. Austenite forms from pearlite by the solution of carbon from cementite into ferrite. The process continues until the upper critical temperature is reached. At this stage, austenite dissolves all of ferrite and the material becomes all austenite. The formation of microstructures are shown in Figure (1-a). At some point, where all the constituents of steel meet and the carbon content is 0.83 percent by weight, the material is all pearlite and the result is a percent by weight, the material is all pearlite and the result is a "eutectoid" structure. Figure (1-b) shows the details of regions one through five and ranges of temperature for annealing and hardening.

Except for gold and platinum, all metals are subject to corrosion. In particular, the above mentioned steel structures are subject to corrosion particular, the above mentioned steel structures are subject to corrosion because iron is the basic substrate where all corrodents come in contact and interact or react with it. For example, kansite (Fe9S8), troilite (FeS), or pyrite (FeS2) can form by the reaction of hydrogen sulfide with steel.

Basic Causes of Corrosion
Carbon Dioxide

There are at least three sources of carbon dioxide in a given oil and gas bearing formation. The first source is attributed to thermocatalytic decomposition of oxygen containing groups in organic matter. Usually, decarboxylation of "COOH" groups in organic structures, which occurs in the diagenetic stage of geologic evolution, leads to a by-product of carbon dioxide. The gas in this form may be utilized by microbes, or dissolve as (HCO), (CO), (-OCH). The second source is attributed to a reaction of methane and water at high temperatures to give carbon dioxide. The third source is found in thermal decomposition of carbonates near the igneous sources at temperatures from 200 degrees F and above.

Carbon dioxide is a colorless gas with a slight acid odor and taste. It is the most stable oxide of carbon at room temperature. Carbon dioxide dissolves in water easily at standard pressure and temperature to give 0.04 percent molar solution in which the carbonic acid is slightly ionized. percent molar solution in which the carbonic acid is slightly ionized. Carbon dioxide itself is non-corrosive.

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