Field studies have shown that detergent surfactants can significantly reduce the accumulation of fouling deposits from crude oil and suspended solids, often keeping monitors clean for months without operator intervention.1 Surfactants also condition the water sample for better measurement by releasing oil from suspended solids and breaking up large oil droplets (particle diameter 1 to 200 μm) into tiny micelles (particle diameter 0.003 to 0.2 μm) that naturally disperse themselves evenly throughout the water stream. The increased homogeneity of the sample and the radical reduction in the size distribution of the dispersed oil particles amplifies the amount of fluorescence emitted by the dispersed oil. The degree of amplification depends on the oil type. Amplification factors of ~2X have been observed with light oils (>20°API). Amplification factors for heavy crudes are typically 20X or greater. The fluorescence emitted by water-soluble organics (WSO) is not affected.
In practice, the surfactants are injected at constant rate into a side-stream carrying produced water to the instrument, at a concentration in excess of the surfactant’s critical micelle concentration (CMC). Since they are either viscous liquids or waxy solids in their 100% active forms, they must be diluted with water before use. Additional ingredients (e.g. methanol, acetic acid, etc.) may also be added to prevent the blend from freezing or to help break up precipitated solids that trap oil droplets.