An Experimental and Modelling Study of The Effect of Surface Roughness on Mass Transfer and Corrosion in CO2 Saturated Oilfield Environments

In the oil and gas industry, internal corrosion of carbon steel is commonly encountered during production and transportation of hydrocarbons and water saturated with corrosive gases such as CO2 and H2S, with CO2 corrosion (termed sweet corrosion) the most common one.
This thesis presents an experimental and modelling approach to study the effect of surface roughness on the mass transfer and corrosion of carbon steel.
The influence of surface roughness on mass transfer on a rotating cylinder electrode apparatus is investigated experimentally. Mass transfer from four different samples, with roughness values of 0.5, 6, 20 and 34 μm, is measured using the limiting current technique for a range of rotational speeds in NaCl solutions saturated with N2 at pH=3 and 4. A new correlation for Sherwood number as a function of the Reynolds number, Schmidt number and surface roughness is proposed. Complementary experiments in CO2 environments were used to assess the combined limiting current associated with H+ and H2CO3 reduction. In the CO2 environments considered, surface roughness is found to have no significant influence on the limiting current contribution from H2CO3, which can therefore be determined from Vetter’s correlation.
Novel surface pH measurement methods are also developed to measure surface pH. These were implemented to study the surface pH during CO2 corrosion. Comparisons between mesh capped probes and iridium oxide probes showed that the results in both cases were very similar.
Two mechanistic models were implemented to predict corrosion rates in the CO2 environments. The models were validated against the experimental results from both the literature and the current study. The first, multi-node model is used to predict the concentration of species within the boundary layer. While, the second, two-node model was used to explore the effect of roughness in the CO2 environment. Agreement between experimental and theoretical corrosion rates was good and demonstrated clearly how increased surface roughness accentuates corrosion rates and mass transfer coefficients, and that the latter need to be accounted when implementing theoretical models.