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 isátantamount 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 an enervating factor. (Considine, 1983, 793).
Electrochemical corrosion occurs under the following conditions: 1) the presence of a driving force or electrical potential, 2) the presence of an electrolyte, 3) the presence of both an anodic and cathodic sites, and 4) the presence of an external conductor. In most cases this last role is filled by water (Bertness and Chilingarian, 1983, 560).
The basic electrochemical reactions that cause corrosion are as follows: Hydrogen, or the acid ion(+), t...