Iron and manganese counts have been used as Doe method for monitoring corrosion. The interpretation of iron and manganese counts has been, however, a source of controversy and debate.
This paper will explore the relationship between iron and manganese counts for three heavy oil fields in West Central Sask. Correlation coefficients for the iron and manganese counts are discussed. The potential usefulness of these correlation coefficients is also presented.
It has been a long standing practice that iron counts from produced waters are used as a measure of corrosion. It has been assumed that iron present in produced water is primarily a byproduct of the corrosion process.
It has become apparent. however, that iron counts can be unreliable predictors of corrosion. Practical field experience has demonstrated, particularly in sour systems, that iron counts show extreme variation. The exact cause of this variation is not clear, but the result has been a reduction in the importance of iron counts as a corrosion monitoring tool.
In an effort to establish an alternative to iron, manganese counts have been used as a means of monitoring corrosion. It is assumed that the primary source of manganese in produced waters is from the corrosion process. Thus, manganese counts are becoming a widely accepted method of corrosion monitoring.
Although manganese counts are currently a popular method of monitoring corrosion, there are some difficulties associated with their use as well.
First of all, it is possible to obtain very low observed values of manganese in oilfield waters. Ranges from 2 ppm manganese down to 0.002 ppm manganese for produced waters are entirely possible. The interpretation of these values, particularly lower values. may be difficult.
Interpretation of manganese counts from produced waters can be further obfuscated by iron counts. It is quite possible to have high iron counts with low manganese counts or conversely high manganese counts and low iron count. How, then, does one interpret mangane counts? Do you refer to the often used 0.2-0.3 ppm treating ranges or, as is often the case, do you simply ignore iron and manganese counts?
The viewpoint expressed in this paper is: that iron and manganese counts from produced water should be an integral part of any corrosion monitoring programme. It is not the counts themselves that are problematical but rather how they are interpreted.
In an attempt to find a method of interpreting iron and manganese values from produced waters, correlation coefficients will be used. Values for Pearson's r and Spearman's r3 will be presented for three heavy oil fields in West Central Saskatchewan. From this data, it will be demonstrated that Spearman's rank correlation (r3) has the potential to be a very useful tool in interpreting iron and manganese counts from produced waters.
Iron and manganese are both found as relatively cornmon elements in the earth's crust. From table 11, it is obvious that iron is much more common than manganese.