Corrosion in Drilling Operations

Corrosion, in terms of drilling operations, was responsible for at least 70 billion dollars in losses in 1975, or 4% of the gross national product. Reportedly, 10 to 45% of that loss was preventable (Bertness and Chilingarian, 1983, 559). Understanding the causes of corrosion in terms of drilling operations is tantamount to saving time and money. The trends towards drilling of deeper wells, the use of higher strength steels, the presence of higher stress levels and the use of lower pH drilling fluids all contribute toward increased corrosion susceptibility.

The destruction of metals by corrosion is caused by direct chemical attack at elevated temperatures in a dry environment or by electrochemical processes at lower temperatures in a moist environment. In drilling operations steel and other metals regularly come into contact with oxygen, carbon dioxide, hydrogen sulfide, salts and organic acids. Various physical and chemical factors act in concert to create corrosion damage (Bertness and Chilingarian, 1983, 559).

The conditions that promote corrosion include, but are not limited to, differences in the electromotive scale on metal surfaces. Metal surfaces are exposed to salts, corrosive solutions, electrolytic solutions, deposits at depth and temperature gradients. Because metal surfaces contain imperfections and are not impervious to change, chemical reactions are bound to occur. In all cases, the presence of water is

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s. Galvanic or twometal corrosion often occurs when two different metals are in contact in a corrosive environment (Bertness and Chilingarian, 1983, 566). Intergranular corrosion has several root causes. Improper heat treatment of metal alloys may cause nonhomogeneity of metal structure at the grain boundaries, which allows a preferential attack. Weld decay is related to intergranular corrosion. The narrow band along either side if a weld may experience changes in grain structure. Ring worm corrosion is in the same category as Weld decay. Ring worm corrosion forms around a pipe near a box where selective welding has occurred. Appropriate heat treating or annealing can prevent both of these effects (Bertness and Chilingarian, 1983, 567). In Selective leaching, an alloy such as brass is selectively corroded. In Erosioncorrosion, the combination of erosion and corrosion results in sever localized attacks on metal. Pitting initiates the processes, and continued erosion completes the destruction. Cavitation corrosion is not completely understood. Apparently, changing internal pressures create bubbles at a pipe or pump surface, allowing a spongelike erosion of pipe surfaces. By increasing the suction head on pumping equi

Category: Science - C