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Oil, water and corrosion

Oilfield Technology,

Shale oil and gas activity in the US continues to grow as a number of other countries around the world begin to explore their own shale prospects. There are many well documented challenges relating to the exploration and production stages of this burgeoning industry, however it is becoming clear that there is work to be done to fully understand the difficulties further down the line. One such area of concern relates to the safe delivery, handling and storage of shale products, in particular crude oil, in preparation for refining. This article will offer an insight into the issue of corrosion in the shale oil distribution and storage network and look at the role protective coatings can have in mitigating the risks associated with the handling of shale crude.

Transportation and storage

The crude oil supply network in the US is not currently configured to cope with the boom in shale oil production from the country’s largest shale plays, particularly those situated in interior locations between North Dakota and South Texas. This has led to a scramble to quickly connect these new crude supplies to existing processing facilities in a safe and profitable manner. The requirement for speed, profitability and safety has led oil and gas producers to consider a range of options, with different solutions favouring different locations.

Corrosion concerns

Historically, pipelines have been the primary mode of onshore transportation for crude oil and petroleum products, with approximately 70% of product transported in this way.1 However, during the early stages of the shale movement the remote location of shale activities has vastly limited access to crude pipeline facilities. A number of expansions of the pipeline network have been proposed to help service the changing energy landscape of the US and provide a cost effective method for distributing shale crudes to refining facilities around the country. However, environmental concerns, difficulties in financing construction and the availability of alternative methods of transport have delayed progress.

An alternative to pipeline transportation which is increasing in favour due to its flexibility and speed of deployment, is rail. In comparison to the use of pipeline, transportation by rail is not cheap. In fact it can be more than double the cost. However, its ability to rapidly respond to the needs of the market and quickly open up access to remote supplies is proving an attractive prospect for the fast moving industry. According to the Association of American Railroads, a record 97 135 carloads of crude oil were transported by rail in the first quarter of 2013. That is 166% more than during the first quarter of 2012 and 922% more than the whole of 2008.6

Regardless of the method of transportation used, there will be a requirement, either at the well site, midstream or at the product’s final destination, for static storage. Exposure to shale crudes has led to numerous cases of storage assets falling short of their expected service life. The risk of containment loss is leading to costly maintenance and replacement and for these reasons corrosion will continue to be a concern for the industry.

Experience in the field

Theoretically, in the absence of water, crude oil is non-corrosive and in immersion, steel does not require a significant corrosion protection system. However in the field, different crude oils contain varying levels of water and sediment. It is the presence of this water and sediment that creates the conditions required for corrosion to occur.

To determine the corrosivity of different types of crude oil, including shale crude, it is important to understand a number of different factors. These include:

  • The temperature of the cargo.
  • The amount of water present.
  • How the water interacts with the oil.
  • The composition of the water phase.
  • The amount and type of sediment present.
  • Hydrogen sulfide content.

Different types of crude oil contain different quantities of water but also the method of extraction and separation can alter the final amount of water contained within the crude mixture. The greater the proportion of water present, the higher the risk of corrosion to the storage vessel. Due to the nature of the fracturing process and the large quantities of water used, it is common for water to be present in shale crudes.

Solutions for minimising the impact

The potential risks from corrosion in the transportation and storage of crude shale are significant and although there are some similarities with the issues experienced during the handling of conventional crude oil, there are also notable differences which require a more robust approach to lining selection.

As with any coating application, it is important to develop a solid specification for the requirements of the coating system based on the environment it is likely to encounter during the life of the asset. It is beneficial at this stage to involve coating manufacturers in the discussion as they will be able to provide input based on previous experience in similar applications which will improve the overall quality of the specification. Some key things to consider include:

  • An accurate classification of the cargo which is to be stored, including the water and H2S content and any contaminants which may be present.
  • The temperature at which the cargo will enter the tank as well as the temperature it will reach during transit to ensure the coating is able to withstand the highest value.
  • Likelihood of the water present to be held in emulsion or separated from the crude.
  • The microbial content, amount of sediment present in the material and likely interval between cleaning.
  • Life expectancy of the asset and coating system.

The greater the understanding of these factors, the greater the chance that the coating system selected will protect the asset for the required period of time.

Written by Edward Shanks, International Paint Ltd., and adapted for the web by Claira Lloyd.

The full article can be found in the March issue of Hydrocarbon Engineering.

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