Prediction of the Risks of CO2 Corrosion in Oil and Gas Wells
Authors: Crolet J-L. , Bonis M.R. 
Published: October 2008

ABSTRACT
This article discusses the various effects of CO2 on corrosion in general, ELF AQUITAINE's policy for fighting corrosion in offshore wells, the mechanisms of local CO2 attack, and finally, the method of predicting the risks of CO2 corrosion in wells.

INTRODUCTION
An anti-corrosion policy is just one element of the general approach to oil and gas production, and is therefore sensitive to the overall industrial environment. From this standpoint, the offshore and onshore contexts are often profoundly different. While this is perhaps not true for sour fie1ds, because of the safety requirements around the wells, in the case of sweet fields the difference can be fundamental.
CO2 corrosion is in fact an old problem in the oil and gas industry, having been encountered in the United States as early as in the 1940's [1]. Since then, it has been the subject of numerous studies throughout the world [2,3], with peak activities in the 1940's, then in 1980. In spite of this, the problem is still not considered to be completely solved.
With hindsight, it can now be seen that there are actually several effects of CO2 on corrosion, and also several problems, depending particularly on the mode of production and its industrial context, and in general, on the production policy of each company.
It so happens that the context in which ELF AQUITAINE operates is quite different from the average for the profession, particularly in North America, and that the company's field observations and constraints are also different. The segmented nature of the problem and the specificity of the segments became evident at an early stage. An initial study was published in 1985 [4], and continued progress since then has now led to a new level of understanding.
The aim of the present article is therefore to review current knowledge of CO2 corrosion, starting with a description of the variety of the effects of CO2 on corrosion in general, and the resulting diversity of the definitions of corrosivity. The methods employed for predicting the risks of CO2 corrosion are then discussed, based on the specific problems encountered by ELF AQUITAINE, and with the corresponding definition of well corrosivity.

THE VARIETY OF THE EFFECTS OF CO2 ON CORROSION AND ON THE RESULTING PROBLEMS

Importance and extension of the notion of CO2 partial pressure
For millions of years, the CO2 in hydrocarbon deposits has been in equilibrium with the three water, oil and gas phases. The quantity of CO2 present in each phase is therefore proportional to their relative abundance. The variations in concentration result simply from the differences in the solubilities and the activity coefficients in the three phases. Thus, in three phase effluents, which are generally highly turbulent, even though the CO2 equilibrium is never perfect, any deviations remain relatively insignificant. Furthermore, sampling and analysis of CO2 in the high pressure condensed phases is difficult and cannot be transposed from one phase to the other (nor even from one oil to another). For all these reasons, whatever the phase considered and the number of phases present, it is more reasonab1e, and above all much easier, to express the amount of CO2, not in terms of its concentration in each phase, but as its chemical potential.