Such deposition has posed a serious threat to the continuity of oil production on the Statoil-operated Heidrun field in the Norwegian Sea ever since formation water began to be produced with its oil in 1996.

Intensive efforts have been made by Statoil researchers and operational staff together with chemical suppliers to find ways of preventing and treating the problem.

For many years, the accepted view has been that calcium naphthenate deposition resulted from a reaction between natural naphthenic acids in the oil and calcium ions in the formation water.

This still holds true. But nobody was able to identify which naphthenic acids are active in the process, quantify them, and unravel their elusive molecular structure. 

In the summer of 2002, however, ConocoPhillips and Statoil made a decisive breakthrough with a study of advanced analytical data. Both partners have now agreed to publish their findings, which were initially confidential. 

The crucial revelation hinges on the identification of a new family of naphthenic acids with molecular weights three times higher (1,227-1,235 g/mol) than the average molecular weight of naphthenic acids found in crude oil (430 g/mol).



Called the ARN acid family, it is also unique in the sense that the acids are four-protonic. The other acids encountered in Heidrun crudes, which constitute over 99.99% of the total, are mono-protonic.  

Another finding is that the molecular structure is not peculiar to Heidrun, but occurs in naphthenate deposits from China, the UK, west Africa and elsewhere in Norway which have been studied.

This strongly suggests that the structure is a worldwide phenomenon. If so, the commercial impact of this discovery could be far greater than previously imagined. 

Statoil is now focussing on the development of more effective and environment-friendly naphthenate inhibitors, and on methods for evaluating the relative susceptibility of fields and wells to naphthenate deposition.