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Assay
Simulator - Causes of Crude Quality Variation
Crude
from a single field can vary as different strata of the field are
produced. In addition, there are several production, shipping and
delivery practices that can cause an assay to be inaccurate in representing
a given days supply of crude.
- Variations
in crude quality caused by production company practices:
- Intentional
blending or "spiking" a crude with condensate or light
material that make it appear lighter (higher API) in order to
"upgrade" the poor quality material into a "light"
crude.
- Intentional
blending of a heavy crude to "upgrade" a poor quality
crude, while staying within overall quality constraints.
- Intentional
blending of several crudes to match a given set of quality constraints.
- Variations
in crude quality caused by the shipping/delivery process;
- Contamination
from injections of crude from small production fields along
a crude pipeline
- Contamination
from moving the crude through tanks with varying tank heels
either along a pipeline or at the refinery charge tanks
- Contamination
caused by making imperfect cuts into tanks between batches of
varying qualities along a pipeline route
- Contamination
during transport by barge or ship where line-fills or dregs
from previous shipments are mixed with the new crude
Although
this list sounds ominous, the effect will often be reflected in
the whole crude properties and allow the assay to be accurately
predicted with the Assay Simulator. If the crudes in a blend are
from a common region, then models created for the Assay Simulator
will be quite accurate. Use of light ends properties corrects for
much of the effect of spiking of very light material. Blending of
very dis-similar crude oils can sometimes create a "dumb-bell"
crude that will not be accurately modeled by the Assay Simulator.
In that case a simulated distillation would give better results.
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