Page 15 - Treating Oilfield Emulsions

Segment III
Chapter 10

Testing for Sediment and Water
Petroleum Extension-The University of Texas at Austin

A fter a representative sample has been ob­ water, to prevent water from clinging to the walls
rt tained, the next operation is to detennine of the centrifuge tube, and to enhance the dis­
the percentage of S&W present in the sample, tinctness of the water-oil interface.
thus ensuring that the oil meets pipeline specifi­ API recommends that toluene and xylene be
cations. Or, as stated in API, MPMS, chapter 10, saturated with water, since water in the oil sample
section 4, Detennination of Sediment and Water in can go into solution with these two solvents if
Crude Oil by the Centrifuge Method (Field Proce­ they are not water saturated. If water in the oil
dure), second edition, May, 1988, "A detennina­ sample goes into solution, erroneous S&W read­
tion of sediment and water content is required to ings result. To saturate a solvent with water, the
detennine accurately the net volumes of crude oil method API suggests follows.
involved in sales, taxation, exchanges, invento­ 1. Fill each of two centrifuge tubes to the 2-ml
ries, and custody transfers. An excessive amount (4-part) mark with water and then to the 100­
of sediment and water in crude oil is significant ml (200-part) mark with the solvent to be
because it can cause corrosion of equipment and saturated.
problems in processing and transporting and 2. Stopper the tubes and shake vigorously for
may violate federal, state, or municipal regula­ 30 seconds (sec), holding the tubes below eye
tions." The testing method presented in this level to protect the eyes should solvent come
chapter is paraphrased or quoted from MPMS, out of the tubes. Loosen the stoppers.
chapter 10, ,section 4. 3. Place the tubes containing the water-solvent
The most common method of detennining the mixture into a sample preheater or heated
percentage of S&W in treated oil on a lease but nonspinning centrifuge maintained at a
employs a centrifuge. In brief, the procedure is to temperature of 60 0 e (140°F) for a minimum
place known volumes of crude oil and solvent of3O min. Be sure the stoppers in the tubesare
(water-saturated, if required) in a centrifuge tube loosened to allow vapors to escape.
and to heat them to 60 0 e ±3°e (140°F ±S°F). For 4. Inspect the water-solvent mixture for sus­
some waxy crude oils, temperatures of 71°C pended water droplets before use. Ifany sus­
(16()OF) or higher may berequired to melt the wax pended water is visible, the tubes must be
crystals completely so that they are not measured centrifuged at a temperature of 60 0 e (140°F)
as sediment. If temperatures higher than 60 e for S min at a speed sufficient to give a rela­
0
(1400F) are necessary to eliminate this problem, tive centrifugal force of 500 at the tube tip.
they may be used with the consent of the parties Table 3 shows rotation speeds necessary to
involved. If water saturation of the solvent is produce the required relative centrifugal force
required, it must be done at the same tempera­ for centrifuges of various sizes. An equation
ture. After centrifuging, the volume of the sedi­ may also be used. If the diameter of the swing
ment and water layer at the bottom of the tube is is in mm (swing diameter is the distance be­
read. tween the tips of opposite tubes when the
tubes are in their rotating position), theequa­
Solvents tion is
Solvents decrease the viscosity of the sample to rpm = 1,135 + ref + d.
assist in the separation of S&W from the oil dur­ If the diameter of the swing is in inches, the
ing centrifuging. Solvents that API recommends equation is
include kerosene, Stoddard solvent, toluene, and rpm = 265 + rcf+ d.
xylene. Toluene and xylene are recommended for 5. Use the top 50 ml (100 parts) of the mixture
S&Wdeterminations involvingasphaltenic crude from each tube for test purposes. Take par­
oils. Also, if necessary, a demulsifier may be ticular care not to pour any of the free water
added to the sample to promote separation of in the tip of each tube into the sample.

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