Optimization and validation of low field nuclear magnetic resonance sequences to determine low water contents and water profiles in W/O emulsions

Abstract

In earlier studies, several NMR procedures have been developed to follow the separation of water and oil. In particular, one of these procedures allows for determining the water content profile of the samples, i.e. the concentration of water as a function of the vertical position in the sample. This method is capable of following the separation but has a detection limit close to 5v/v% water content with the used settings. Since an oil is required to contain less than 0.5v/v% water to be qualified as export quality, this detection limit of 5v/v% is not sufficient for assessing whether an oil is dry or not. For this reason, the above-mentioned water content profile determination method and the CPMG sequence which only measures the average of the water content of the entire sample have been optimized specifically for determining low water contents. These methods have been tested using both model emulsions and petroleum crude oil emulsions in samples with or without free water.It is shown that the optimized NMR sequences are capable of measuring right low water contents. In one case, however, some discrepancy is found between results from NMR and Karl Fisher titrations. Possible explanations for this discrepancy are discussed. The developed NMR techniques allows to determine low water contents in 6 to 14min and have a detection limit between ≈0.3 and ≈0.5–1.0v/v%, depending on the method used.