Preventive Corrosion Engineering in Crude Oil Production

ABSTRACT A technique is presented that can be used to determine the produced water level in crude oil production where accelerated corrosion of steel will occur (defined as the Corrosion Rate Break produced water level) and to evaluate the requirement for corrosion inhibitor treatment. The primary focus is preventive corrosion engineering in material selection and chemical treatment initiation to minimize the cost of corrosion control while maximizing its effectiveness. The technique is particularly useful in making recommendations for new crude oil discoveries where no explicit corrosion data exist. Heavy emphasis is placed on the corrosion of steel, since steels are the primary materials of construction for crude oil productions systems. Stress corrosion cracking is not included in the corrosion test procedure, but is addressed in the logic diagram for material selection. Stress corrosion cracking must be considered based on the well fluids analysis using standard tests modified to utilize the well fluids. INTRODUCTION Successful corrosion engineering of a crude oil production facility for maximum safety, reliability and economy includes material selection for the downhole and surface facilities, specification of chemical treatment requirements, and determination of when treatment will be required. To quote from C. Edeleanu (1), "It is simple to recommend that everything in sight be gold plated. ... what is expected from an expert is guidance on how near the precipice it is safe to go without falling over it." The problem is to accurately define the location of the precipice. Accurate predictions about the occurrence of corrosion in crude oil production systems are very difficult to make, The acid gas partial pressures, temperature, produced water composition, crude oil composition, and the level of produced water all combine to influence the corrosion characteristics of the production fluids. The inability to accurately define the production conditions where dangerous levels of corrosion will occur generally necessitates very conservative materials selection due to the consequences of a failure. In crude oil production where steel corrosion rates can increase very rapidly and unexpectedly as the produced water level increases, corrosion may occur in downhole equipment and tubulars before a corrosion monitoring system can reliably detect it. This can result in extensive downhole corrosion damage, requiring well workovers to replace the corroded parts, and has the potential for dangerous failures. The onset of accelerated steel corrosion in crude oil production cannot be reliably predicted by "rule of thumb" produced water levels, the "occurrence of free water" in the produced fluids, or by "water external mixtures and emulsions" (2). The application of experimentally derived equations relating temperature and carbon dioxide partial pressure to the steel corrosion rate are extremely conservative (3-6). These relationships were originally derived for gas systems but attempts have been made to apply the equations to crude oil production. These predictive methods are only approximations at best when applied to crude oil production because of the effect of the crude oil on steel corrosion.