BS and W Measurement Principles and Practices
Authors: Warren W.J. 
Published: December 2007

This paper first briefly reviews principles on which equipment and procedures for the measurement of BS and W content of crude oil are based. One of these procedures, the dielectric-constant technique, is examined in some detail. Finally, some limitations in practical applications of the technique are reviewed.

Basic Principles of BS and W Measurement

The technique of physically separating an oil and water mixture by centrifuging ("shake-out") is familiar to all (ASTM D96–60T) and still forms the basis for evaluating other methods of determining the basic sediment and water (BS and W) content of crude oils. Its use is fraught with the usual problems of obtaining a truly representative sample. BS and W determinations from a given sample are repeatable under typical field conditions to within an accuracy of approximately 0.1 per cent. The practice of allowing a tank-size quantity to settle and then determining the total liquid level and level of the water-oil interface is an application of the physical-separation technique, although somewhat less accurate and certainly on a larger scale. Careful measurement of the density of the oil and water mixture also allows one to determine the water content. A field system has been implemented in which the total hydrostatic head of a 50-bbl volume of the wet oil is compared with the hydrostatic head of a reference material. From the pressure difference, one can infer the water content after correcting for the salinity of the water and for the liquid temperature. Density measurement by gamma radiation techniques has been proposed. Under controlled conditions the attainable resolution is 0.001 density units. This corresponds to approximately 1 per cent BS and W for a 30 degrees API oil containing water of approximately 50 per cent salinity. Differential thermal capacity of the oil and water has been proposed as a technique for measuring BS and W. The specific heat of water is approximately twice that of oil. An instrument based on this scheme is being tried at a refinery in England. The dielectric technique measures, by electronic means, the capacitance of a pipe-like cell filled with the wet oil. This capacitance increases as the volume fraction of water (or other materials of high dielectric constant) increases. The attainable resolution of approximately 0.02 per cent BS and W stems from the well known contrast of dielectric constants of typical crude oils and water.

Dielectric-Constant Instrumentation

The now widely used dielectric-constant instrument incorporates a coaxial cell similar to that shown in Fig. 1. Ordinarily, the entire wet-oil stream flows through the annular space. The electrical parameters that figure into the performance of the measuring scheme can be represented as shown in Fig. 2(a). Cc represents the unchanging (stray) capacitance and Co represents the capacitance of the active portion of the cell when it is empty. Rp is the alternating- current resistance corresponding to electrical losses in the imperfect dielectric. This resistance will depend upon the volume fraction and salinity of the contained water, and will include the effects of leakage through or over insulating sections of the cell assembly. This resistance is not to be confused with the resistance Rd-c obtainable by conventional direct-current measurement on the oil-filled cell. When the cell is filled with fluid of dielectric constant E, its total capacitance is. The opposition (impedance) to AC flow and phase angle between the current to and voltage across the cell will vary with cell capacitance as shown in Fig. 2(b).Two circuit schemes for converting changes of cell capacitance to corresponding electrical signals are shown in Fig. 3. These techniques are incorporated in instruments currently available. In Fig. 3(a), the cell is one arm (DG) of the bridge. This circuit is customarily used with an oscillator and a phase-sensitive detector circuit to preferentially convert the total signal to corresponding cell capacitance changes. It lends itself to low-cut (0 to 5 per cent) measurement, particularly if the comparison arm (EG) of the bridge circuit includes means for compensating for cell capacitance changes due to stream temperature changes. The effect of temperature on wet-oil dielectric constant is discussed in a later section. In Fig. 3(b) the cell capacitance is incorporated in an R-C oscillator, whose output frequency varies as the cell capacitance varies.

JPT

P. 1207^