Internal corrosion is one of the most common problems within the transportation pipelines of the oil and gas industry. Water wetting is one of the most important issues in the prediction of internal corrosion in mild steel pipelines, and it is affected by water chemistry, flow regime, pipe orientation and water cut. Another possible factor is the nature of the wetted surface itself, such as bare metal surfaces with different degrees of roughness or surfaces covered with iron carbonate film produced by corrosion. The primary objective of this study is to investigate and model the effect of surface state on water wetting and carbon dioxide corrosion in oil-water flow.
Five types of crude oil and one model oil (LVT200) were tested with 1wt% NaCl brine in a 4" I.D. fully inclinable large scale flow loop. Four main techniques were used to determine flow regime: flow pattern visualization, wall conductance probes, wall fluid sampling, and corrosion rate monitoring. Based on the overlapping information from these four techniques, three types of phase wetting regimes (stable water wetting, intermittent wetting and stable oil wetting) were identified. Comprehensive phase wetting maps were constructed based on the results obtained from wall conductance probes, including the transition from intermittent wetting to stable oil wetting.
A goniometer contact angle measurement system was designed and successfully used to investigate the effect of steel surface state on wettability. Contact angle measurements were made for bare steel surfaces with different degrees of roughness, with iron carbonate film, and different pre-wetting scenarios. The results showed that surface roughness and iron carbonate do not affect wettability. However, pre-wetting the steel surface with either water or oil had a great effect on wettability. Pre-wetting the steel surface with crude oil led to a transition of the wettability of the steel surface from hydrophilicity to hydrophobicity. Adding a corrosion inhibitor ("quat") produced a similar effect.
A new mechanistic phase wetting prediction model was proposed. The new model considers the effect of surface wettability to calculate the maximum water droplet size in oil-water flow. The model significantly improves the prediction of the critical oil phase velocity required for full water entrainment compared to the existing water wetting model incorporated in MULTICORP software. The new model was verified with the experimental results for different crude oils and by using different additive chemicals which alter the wettability.