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Facilities Sand Management: Calculations – Oil-on-Solids Content (B-FSM025)

An oil droplet about to release from the end of a syringe needle into saline water

In article B-FSM-20 it was shown that sand is oil wet at normal oilfield conditions. What happens when a sand particle interacts with an oil droplet? – an agglomerate forms, with an effective density between the solid and liquid phase.

How does adsorbed oil layer affect sand particle density?

The figure below shows the effective density of the oil-sand agglomerate. The x-axis shows increasing oil layer, with the y-axis as net density. The effect is shown for sand with particle diameter from 10 to 150 µm. This simple calculation assumes a monolayer of oil fully encapsulating a spherical sand particle. For example, a 100 µm sand particle with 10 µm oil layer shows ~14% drop in density. Oil coating will have a strong effect on particle movement in a fluid system.

What is the thickness of oil layer on “cleaned” sand?

As an example of calculation for the preceding graphic, we will calculate the thickness of the oil layer on cleaned sand. The base assumptions are;

  • Oil content is 1 weight percent oil on dry sand – OSPAR regulation for overboard discharge of solids
  • Sand density = 2650 kg/m³, oil density = 850 kg/m³
  • All particles in system are equally coated with monolayer of oil encapsulation

The calculation steps for 100 µm sand particle are shown in the graphic below.

Clean sand has an average oil layer of one-half micron – which is very small – but the net density still drops ~2%. Anytime sand is encountered in oil & gas facilities, the effect of oil coating must be considered.

The next series of articles will cover particulate solids transport – in piping and separators.

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