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Broadening the applicability of OMMICA in oil samples.

History


The OMMICA assays were originally designed as an answer to a request that a collaborator came to us with: to create an accurate and portable analysis method for methanol contamination in crude oil. The particular focus was on the level of methanol in oil which can incur fines (which is typically below 50 ppm), so this was the concentration that the kits were developed to analyse.


When the OMMICA testing kit range was broadened to include MEG (ethylene glycol) in oil, the same working range was used.  


Fairly early in the commercial life of the OMMICA products, their broader potential was recognised, for example in monitoring the methanol levels at initial start-up of a well (when they can be as high as 1% or more), and in mapping the way methanol can partition between the oil, water, and gas phases (rather than just assuming the theoretical calculations are correct).


Method


Unlike when analysing methanol or MEG in an aqueous solution, you can’t just dilute the test sample with water, as oil and water are not miscible, so the resulting mixture won’t give an accurate result.


For samples which are up to 1000 ppm, the volume of oil sample can simply be reduced (good pipetting skills are helpful here), and the assay run as normal. The supplied calculation spreadsheet includes the requirement to enter the oil sample volume, and will then determine the concentration of the sample without any additional steps required by the operator.


For samples which are more concentrated than this, the recommended approach is to dilute them using a mutual solvent (i.e. one which can dissolve both the oil, and the test sample of interest, either methanol or MEG). The suggested solvents are xylene (for methanol samples) and acetone (for MEG samples).


Both methods are described in the instruction booklets supplied with the kits.


This video demonstrates the approach using a MEG sample in hydraulic oil.  



 A 1:10 dilution is recommended; e.g. 1 ml test sample plus 9 ml dilution solvent. If a single 10-fold dilution still gives results which are off scale (with an absorbance of greater than 2.5 AU), then a second 10-fold dilution should be carried out on the sample, for an overall dilution of 1:100. Alternatively, both a 1:10 and 1:100 dilution can be prepared initially, and analysed in parallel, if you are unsure which would be more suitable.


Positive displacement pipettes, such as the Eppendorf E3 shown in the video, are suggested to give the most accurate results.


Validation


This dilution method was used as part of a project by an external company (a global analysis house) to compare GC (their standard in-house method) with three alternative commercially available ways of analysing methanol concentration in oil (including OMMICA). OMMICA gave very good agreement with GC (on samples they had prepared in-house), both when the samples were on scale as supplied, and when they had to be diluted before analysis.  

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