Prediction of the Risks Of CO2 Corrosion in Oil and Gas Wells

Abstract

Summary This paper discusses the various effects of CO2 on corrosion in general, apolicy for fighting corrosion in offshore wells, the mechanisms of local CO2 attack, and a method of predicting the risks of CO2 corrosion in wells. Introduction An anticorrosion policy, just one part of the production of oil and gas, issensitive to the overall industrial environment. The old on-shore and the newoffshore anticorrosion policies are often profoundly different. CO2 corrosionis an old problem in the oil and gas industry. Despite several publishedstudies, the problem is still not completely solved. Through published studies, the problem is still not completely solved. Through hindsight, we can see that CO2 has several effects on corrosion and can cause a variety of problems, depending on the mode of production, its industrial context, and the company'sproduction policy. Thus, a comparison of the literature to our fieldconstraints made the segmented nature of the problem and the specificity of thesegments obvious at an early stage. The first study on the problem waspublished in 1986. Progress since then has led to a new level of understandingof CO2 Progress since then has led to a new level of understanding of CO2 corrosion. The goal of this paper is to review current knowledge of CO2 corrosion, starting with descriptions of the various effects of CO2 oncorrosion in general and the diversity of the definitions of corrosivity. Methods for predicting the risks of CO2 corrosion are then discussed. Effects of CO2 on Corrosion and Resulting Problems Importance and Extension of CO2 Partial Pressure. Measuring the CO2 contentin water under pressure is never easy. From the reservoir to the pipelines, however, the water, oil, and gas phases are always close to thermodynamicequilibrium. Therefore, it is much easier to characterize the presence of CO2 by its chemical potential-i.e., by its partial pressure in the gas phase. Byextrapolation, we also use the CO2 partial pressure in systems where the gasphase is totally absent. This "virtual partial pressure" is de-fined asthe value of the chemical potential of CO2 pressure" is de-fined as thevalue of the chemical potential of CO2 expressed in bars (or psi). Experimentally, it corresponds to the real partial pressure of CO2 that wouldappear in the nascent gas phase if the partial pressure of CO2 that wouldappear in the nascent gas phase if the system were depressurized to itsbubblepoint. Thus, it is possible to define a CO2 partial pressure in anyoilfield effluent or in any laboratory medium, even in the absence of gas, andto express the activity of CO2 solely in terms of this partial pressure. Effect of CO2 on Produced Water. The presence of a certain CO2 partialpressure in an oilwell effluent leads to a proportional dissolution in theproduced water. This dissolution acidifies the water to an extent that dependson its composition and the resultant buffering effects (Fig. 1). It introducesspecific physical/chemical equilibria, particularly for the solubility of CaCO3. It also introduces an additional particularly for the solubility of CaCO3. It also introduces an additional oxidizing species, CO2/H2O or H2CO3. The CO2-induced acidification can also cause partial reassociation of anions, such as acetates and propionates, to form organic acids. This further increasesthe oxidizing power of H+ by raising the limiting diffusion current forcathodic reduction. However, it also supplies very soluble corrosion products, such as iron acetate, which might reduce the protectiveness of the corrosionlayers. In the same way, the presence of protectiveness of the corrosionlayers. In the same way, the presence of CO2 increases the solubility of theferrous oxides and hydroxides (by lowering the pH) and leads to siderite(FeCO3) deposition. Because CO2 always acts at the heart of the corrosionmechanisms, the presence of CO2 can modify several aspects of the corrosionproblem; each presence of CO2 can modify several aspects of the corrosionproblem; each modification can be called CO2 corrosion. The four principaleffects are the enhancement of uniform corrosion, the modification ofinhibition conditions, the reduction of erosion-corrosion velocity thresholds, and the creation of a specific form of local attack known as mesa-typecorrosion. Effect of CO2 on Uniform Corrosion. The acidification and additionaloxidizing power from the CO2 can lead to a considerable increase in the rate ofuniform corrosion of carbon steels. This is observed systematically as long asthe metal remains bare. Furthermore, by increasing the solubility of thecorrosion products, the CO2-induced acidification enables the buildup of higherconcentration gradients in the liquid phase in contact with the metallicsurface, resulting in more efficient transport processes by diffusion. Thisfacilitates the dispersion of corrosion products into the bulk solution andincreases the thresholds for the precipitation of protective solid corrosiondeposits. For example, in a laboratory test, the physical chemistry of the testmedium must be strictly controlled to avoid an artificial drift in pH and inthe degree of saturation in FeCO3. However, even when the usual experimentalartifacts associated with long-time autoclave tests are avoided, a certainlength of time is still necessary before deposits will form. This periodincreases with the solubility of the corroded iron. This deposit precipitationthreshold depends not only on the CO2 but also on the initial corrosion rate ofbare metal, the level of saturation in CaCO3 (owing to competitive saturationbetween FeCO3 and CaCO3), and, of course, the degree of turbulence. Thus, it isextremely difficult to predict the uniform corrosion rate of a steel withcertainty. predict the uniform corrosion rate of a steel with certainty. It wasexpected, however, that as long as the corrosion remained uniform, it could notoccur more rapidly than the initial rate on the bare metal. The initialcorrosion rate represents a "potential corrosivity" that should not beexceeded. It also represents the maximum penetration rate of any localizedcorrosion that does not significantly penetration rate of any localizedcorrosion that does not significantly modify the system's overall corrosionpotential. This theory of the worst case recently was proved incorrect whencorrosion rates much higher than the initial rate were observed beneath some FeCO3 scales. On the other hand, depending on the protective nature of thecorrosion deposits, the real long-time corrosivity also can be much lower thanthe potential value. Thus, two-thirds of our CO2 fields are, in fact, notcorrosive. Uniform corrosion rates as high as 10 times the potentialcorrosivity and lower than 1/100th of the potential corrosivity are currentlybeing observed in the laboratory. Therefore, potential corrosivity is in no waya prediction of any long-term corrosion rate; It is simply an electrochemicalcharacterization of a given metal/electrolyte couple. This value can be modeledfrom the dependence of the anodic and cathodic polarization curves on theactual water chemistry. polarization curves on the actual water chemistry. Inaddition, if localization of the attack markedly increases the mean corrosionpotential and thus increases the anodic polarization of the local anodes, thenthe "real corrosivity," expressed in terms of the local penetrationrate, could also exceed the potential corrosivity. penetration rate, could alsoexceed the potential corrosivity. The predictability of the real corrosivity ofa CO2 field in terms of uniform corrosion remains imperfect. The only possiblemethod often is to estimate experimentally a potential corrosivity in the fieldand to decide later which treatments to apply. November 1991 P. 449