Integrated CO2 Corrosion - Multiphase Flow Model

Abstract

Abstract An integrated CO2 corrosion - multiphase flow model was built which takes into account the effect of most important variables. The model is mechanistic in nature and resides on clear theoretical foundations. All the assumptions in the model are explicitly stated and are open to future adjustments and improvements. The overall model was extensively verified with a large experimental database and was able to perform reasonably well in all cases. The multiphase flow model was also benchmarked against a well-established commercial package. Introduction A variety of prediction models for CO2 corrosion of carbon steel exist. 1–19 Most of these models are semi-empirical, or even fully empirical with only a handful of the more recent models being based on mechanistic descriptions of the processes underlying CO2 corrosion 14–19. A recent paper by Nyborg20 reviews the performance of a representative group of models concluding that most of the models predict well the "worst case" CO2 corrosion rate but vary widely when more complex effects (e.g. protective films, water entrainment/wetting, H2S, etc) are included. The main reason for this spread lies in the arbitrary nature of the empirical correction factors that are employed to account for the various complicating effects. The present paper describes a newly developed CO2 corrosion multiphase flow model that takes into account the effect of most important variables. The majority of the model is mechanistic in nature, fully transparent and resides on solid theoretical foundations. Some aspects of the model that cover areas where insufficient knowledge exists (e.g. H2S effect) employ a semi-empirical approach. It should be noted that the current model is a decade long project building on previous developments published in the open literature21–23. Integrated Model The integrated model consists of two main models: the corrosion model and the multiphase flow model. An interface module enables the transfer of the data between the two. Corrosion model. The transient mechanistic CO2 corrosion model of Nesic et al.21–23 is used as the backbone of the corrosion model of the integrated package. The corrosion model comprises of the following sub-models covering:Kinetics of electrochemical reactions at the steel surface including:H+ reductionH2CO3 reductionHAc reductionFe oxidationTransient one-dimensional transport of species between the bulk solution and the steel surface, through the turbulent boundary layer and through a porous surface film.Kinetics of chemical reactions including precipitation.Growth of iron carbonate films.Effect of traces of H2S.Effect of steel type.Effect of inhibition by crude oil and/or corrosion inhibitors.Possibility and morphology of localized attack. The physical, mathematical and numerical aspects of the electrochemical, transport and chemical models have been published previously, and only a very brief outline is given below to facilitate the understanding of the text to follow.