This is a repository copy of An experimental and numerical investigation of CO2 corrosion in a rapid expansion pipe geometry.

Drill-pipe corrosion is a critical issue for any drilling operation, particularly under high-pressure, high-temperature (HPHT) downhole conditions. However, most laboratory studies have been conducted under ambient and static conditions, with only a few downhole studies based on flow loop showing inconsistent results. In this study, we proposed a novel simple method to simulate pipe corrosion/erosion in a reservoir-like environment under both the static and dynamic conditions and investigated the influences of wellbore conditions, including temperature, pressure and salinity of water-based drilling fluids, on the corrosion behaviour of the drill pipe. The results showed that the erosion effect of the drilling fluid (without drilled cuttings) was negligible. Furthermore, we found that the corrosion rate increased with an increase in the temperature, pressure and rotational speed; however, it decreased with an increase in the salinity. In addition, the proposed method can be used to simulate other complicated conditions.

The influence of surface roughness on mass transfer on a rotating cylinder electrode apparatus is investigated experimentally for a roughness pattern consisting of grooves parallel to the direction of fluid flow. Mass transfer from four different samples, with roughness values of 0.5 μm, 6 μm, 20 μm, 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. Comparison with available correlations for the Sherwood number in literature (which are independent of surface roughness and are either for specific or arbitrary roughness patterns) shows that H+ mass transfer only correlates well for particular levels of roughness and that their accuracy can be increased if a correlation is utilized which is a function of surface roughening. A new correlation for Sherwood number as a function of the Reynolds number, Schmidt number, and surface roughness is proposed which agrees well with the mass transfer observed from all of the rough surface cases considered for this particular roughness pattern. Complementary experiments in CO2 environments were used to assess the combined limiting current associated with H+ and H2CO3 reduction (with the latter occurring via the buffering effect and being associated with the slow CO2 hydration step). Although the increase in sample roughness clearly leads to an increase in the rate of H+ mass transfer, 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 equation across this range of operating conditions.

<i>This paper presents an experimental and theoretical investigation into water condensation and corrosion under non-corrosion product forming conditions at the top of line in a static, CO<sub>2 </sub>environment. An experimental test cell is developed to measure droplet lifetimes, condensation rates, as well as in situ and integrated corrosion rates (using miniature electrodes and mass loss specimens, respectively), as a function of the surface and gas temperatures, when the gas flow is dominated by natural convection. Experimental results show clearly that that water condensation rate (WCR) is not very influential on corrosion rate at low surface temperatures (T<sub>s</sub>) (particularly below 25<sup>o</sup>C) but becomes much more important at higher surface temperatures (>40<sup>o</sup>C). These findings are summarised in a new empirical correlation for TLC rate as a function of the condensation rate and surface temperature. A model for condensation at the top of the line for static, buoyancy-driven conditions is also presented and is shown to predict dropwise condensation rates accurately for a range of experimental conditions. The developed miniature electrodes for in situ electrochemical measurement are shown to provide an accurate interpretation of the transient response in general corrosion behaviour by giving real-time corrosion rates to complement the mass loss measurement.</i>