Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Corrosion is an important and costly problem in the petroleumindustry, requiring special considerations in the design of productionequipment. Severe environments involving CO2 or H2S pose particulardifficulties. Four types of corrosion exist: weight loss, stress corrosioncracking (SCC), corrosion fatigue, and galvanic corrosion. Designconsiderations and preventive techniques for each of these corrosion forms arediscussed.
Corrosion costs the petroleum industry hundreds of millions of dollars eachyear. A large number of possibilities exist to reduce or to minimize thesecosts, but they require a continuous and diligent effort. Corrosionconsiderations and appropriate material selections should be an important partof all equipment design, surveillance, and operations actions, particularly forthe high-cost activities of the past few years and anticipated futureactivities.
Deep hot gas wells, CO2 floods, deepwater offshore platforms, and arcticdevelopments are excellent examples of cases that have provided many materialand corrosion problems and are expected to continue to do so. Significanttechnologies have been developed to resolve many of these problems, but muchwork remains.
Although many material and corrosion problems remain to be solved, previousworkers in this field have left a rich legacy of knowledge on which to build. Abrief review of the current knowledge base sets the stage for discussion ofanticipated problems of the future. The forms of corrosion of most importancein oil and gas production are (1) weight loss, (2) SCC, (3) corrosion fatigue, and (4) galvanic corrosion. Methods that have evolved over the years forcombating corrosion include inhibition, material selection, cathodic or anodicprotection, coatings, and control of the environment.
Weight-loss corrosion is the most common problem andoccurs at some finite rate for all materials. The rate varies greatly, depending on the susceptibility of the material under the environmentalconditions to which it is exposed. The corrosion occurs as a result ofelectrochemical reactions between the metal and corrodents in the environment. This reaction is similar to the one that occurs in a battery. In the case ofsteel in acidic environments, iron dissolves at the anode and hydrogen isformed at the cathode. An electrical path (e.g., water) is required. Onepossible chemical reaction is
Fe+2H20 Fe(0H)2 +H2.
In the oil field, water is normally present, often as a brine, and thecorrodents are usually CO2, H2S, oxygen, mineral and organic acids, or otherchemicals that affect the pH or cause oxidizing reactions. Corrosion rates inneutral, low-salinity solutions are normally very low. In contrast, corrosionrates are very high in low-pH solutions that form in the presence of acidicmaterials or high CO2 partial pressures. (Partial pressure is calculated bymultiplying mole fraction of CO2 by the system pressure.) Generally, the systemis considered corrosive if the partial pressure is above 15 psi [103 kPaj;however, significant corrosion may occur below this level. The presence of anoxidant, such as oxygen or free sulfur, causes a significant increase inpitting and crevice corrosion.
JPT
P. 756^
