6.3.2.4.2 Solvent Cut Test Procedure

The most reliable test for hydrocarbons is the solvent cut test. Another name for it is the cut fluorescence or “wet cut" test. If hydrocarbons are present, fluorescent "streamers" will be emitted from the sample and the intensity and color of these streamers are used to evaluate the test. Some shows will not give a noticeable streaming effect but will leave a fluorescent ring or residue in the dish after the reagent has evaporated. This is termed a "residual cut." It is recommended that the "cut fluorescence" test be made on all intervals in which there is even the slightest suspicion of the presence of hydrocarbons. Samples that may not give a positive cut or will not fluoresce may give a positive cut fluorescence. This is commonly true of the high gravity hydrocarbons that may give a bright yellow cut fluorescence. Distillates show little or no fluorescence or cut but commonly give positive cut fluorescence, although numerous extractions may be required before it is apparent. Generally low gravity oils will not fluoresce but will cut a very dark brown and their cut fluorescence may range from milky white to dark orange. An alternate method involves picking out a number of fragments and dropping them into a clear one-or two-ounce bottle. Chlorothene or acetone is poured in until the bottle is half full. It is then stoppered and shaken. Any oil present in the sample is thus extracted and will color the solvent. When the color of the cut is very light, it may be necessary to hold the bottle against a white background to detect it. If there is only a slight cut, it may come to rest as a colored cap or meniscus on the top surface of the solvent. Before a solvent cut test is to be performed, ensure the reliability of your solvent by placing a few drops of the solvent on a depression in the spot plate and observing it under UV light. If the solvent is “good”, no fluorescence should be observed.

To do a solvent cut test:

1. Place a few drops of solvent, enough to immerse the sample, on the sample in the depression in the spot plate. Be careful not to get the cutting agent into the rubber of the dropper as it might “contaminate” the solvent by giving it a pale yellowish fluorescence.

2. Observe the following:

• Cut speed: This is an indication of both the solubility of the oil and the permeability of the sample. The speed can vary from instantaneous to very slow.
• Cut nature: coloration of the solvent with dissolved oil may occur in a uniform manner, in streaming manner or in a blooming manner. A streaming cut also indicates low oil mobility.
• Cut color fluorescence and intensity: Observe the color and the intensity of the oil in the solvent under both UV and natural lights. The cut color observed under UV light could be called a cut color fluorescence (example: bright blue white cut fluorescence).
• Cut color and intensity: After observing the sample under UV light observe the sample under natural light. The cut color observed in natural light is just called cut color (example: very light brown cut color or no cut color).

The shade of the cut depends upon the gravity of the crude; the lightest crudes giving the palest cuts, therefore, the relative darkness should not be taken as an indication of the amount of hydrocarbon present. A complete range of cut colors varies from colorless, pale straw, straw, dark straw, light amber, amber, very dark brown to dark brown opaque.

• Residue color and intensity: The solvent dissolves rapidly under the heat of the UV light, sometimes leaving a residue of oil around the cutting on the spot plate. The true color of the oil can then be observed. The intensity and opacity of color, especially of the residue, is an indicator of the oil density and the quantity of oil originally in the cutting.

A faint "residual cut" is sometimes discernible only as an amber-colored ring left on the dish after complete evaporation of the reagent. The hydrocarbon extracted by the reagent is called a "cut."

3. If the sample shows all the possible signs of being oil bearing, but has no solvent cut, the sample is crushed using the metal probe and it is observed for a solvent cut. The cut is called a crushed cut.