The crude oil assay is the collection of the results of physical tests that are performed to determine the key properties (boiling point, density, viscosity, heteroatom contents, acid number, etc.) of crude oil and its fractions. It is the procedure based on laboratory and pilot plant testing for determining the general distillation and quality characteristics of crude oil. Crude oil assay is important for determining the value and processability of crude oil. This is the preliminary step before processing the crude oil in the refinery. . In order to utilize the crude oil assay data, it is necessary to understand the results and significance of some of the laboratory tests.

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2. Introduction to Crude Oil Assay
What is crude oil assay analysis?
The crude oil assay is the collection of the results of
physical tests that are performed to determine the key
properties (boiling point, density, viscosity, heteroatom
contents, acid number, etc.) of crude oil and its fractions.
It is the procedure based on laboratory and pilot plant
testing for determining the general distillation and quality
characteristics of crude oil.
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3. Significance of Crude oil Assay
Crude oil assay is important for determining the value
and processability of crude oil. This is the preliminary
step before processing the crude oil in the refinery. . In
order to utilize the crude oil assay data, it is necessary
to understand the results and significance of some of
the laboratory tests.
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4. Crude Oil Assay
For more details about crude oil view other topcis;
1. Types of Crude Oil
https://thepetrosolutions.com/types-of-crude-oil/
2. Composition of Crude Oil
https://thepetrosolutions.com/composition-of-crude-oil/
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5. Significance of Crude oil Assay
• Determine the compatibility of a crude oil feedstock for a
particular petroleum refinery.
• Operating conditions of a refinery are adjusted according
to the crude oil assay data to control yield, quality, plant
operation, environmental and other problems.
• Gives the various components of crude oil along with the
expected percentage of recovered products for each
specific type of crude, hence the refinery configuration.
• Information obtained from the petroleum assay is used for
detailed refinery engineering and client marketing
purposes.
• The price of crude oil is determined according to the crude
oil quality determined by assay, e.g., light crudes, sweet
crudes will have different price structures based on the
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6. Types of Crude Oil Assay
1. Inspection Assay involves the determination of
several key bulk properties such as API gravity,
sulfur content, distillation range. These are routinely
performed on all crude oils received at the refinery.
2. Comprehensive Assay is more complex, time-
consuming, costly, and is usually performed only
when the new field comes into operation or when the
inspection assay indicates significant changes in
composition. Except for these circumferences, the
comprehensive assay for a specific crude oil may not
be updated for several years.
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7. Crude Oil Assay Tests
1. Specific Gravity and API Gravity
Specific gravity of crude oil depicts the heaviness and
general behavior of crude oil. Specific gravity or API
gravity both tends to show the classification of crude oil.
The API Gravity can range from 8.5 for extra heavy to 44
for light very light crude.
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8. 2. Sulfur Content
Sulfur content is expressed as the weight percent of
sulfur in oil and typically varies in the range from 0.1 to
5.0%wt. Crude oil can be referred to as Sour or Sweet,
based upon the sulfur content. Low Sulfur crudes are
high in price and better in quality.
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9. 3. Viscosity
Viscosity is the resistance to flow or indication of the
pumping ability of crude oil. More viscous oils require great
energy and power to pump, and greater pressure drop to
flow in the pipes. Viscosity measurement is expressed in
terms of kinematic viscosity in Centi Stocks (cSt) and can
also be expressed in Saybolt seconds.
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10. 4. True Boiling Point Distillation Curve
The crude oil composition is estimated by a true boiling point
(TBP) curve. The boling point range of crude oil gives
approximation of quantities of various petroleum products.
The method uses a batch distillation operation, using a large
number of stages, usually greater than 60, and high reflux ratio
(greater than 5). The temperature at any point on the
temperature and volumetric yield curve depict the true boiling
point of the hydrocarbon material present at the given volume
percent point distilled.
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11. 5. Simulated Distillation by Gas Chromatography
SDGC is another technique utilized to measure the
boiling point distribution of crude oil. This is done by
injecting the sample of crude oil in a gas chromatograph
that separates the hydrocarbons according to the boiling
point order. The results of this test can also be
compared with the true boiling point test. The boiling
point distribution of light and heavy petroleum cuts can
also be performed by gas chromatography.
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12. 6. Pour Point
The pour point is defined as the lowest temperature at
which the sample of crude oil will flow. It indicates that
how easy or difficult is to pump the oil, especially in cold
weather. It also indicates the aromaticity or the paraffinity
of the crude oil or the fraction. A lower pour point means
that the paraffin content is low.
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13. 7. Water, Salt, and Sediment
The crude oil carries small amounts of water, mineral
salts, and sediments. Most of the salts are dissolved in
water, and the remaining are present in the form of fine
crystals. The most common salts present in crude oil
are chlorides of magnesium, calcium, and sodium. The
presence of salts causes problems in oil refining
operations, such as corrosion, erosion, and plugging of
equipment, and catalysts deactivation. Sediments in
crude oil are solid materials comprised of sand, mud,
rock, or minerals, etc that are not soluble in
hydrocarbon or water.
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14. 8. Molecular Weight
Most crude oils and petroleum fractions have average
molecular weights from 100 to 500. There are several
techniques for measuring the molecular weight but the
most used method is that based on freezing point
depression.
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15. 9. Metals Content
The metals content of crude oils can vary from a few
parts per million to more than 1000 ppm and, in spite
of their relatively low concentrations, are of
considerable importance and can have damaging
effects on the downstream catalytic processes.
Amounts of trace elements in crude oil, such as
vanadium, nickel, lead, phosphorus, arsenic, and
other metals are measured.
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16. 10. Total Acid Number
Another property of crude oil is the TAN, which
describes the number of acids present in the crude oil.
Crude oils having high acid numbers have a high
potential to cause corrosion problems in the oil
refineries, especially in the crude and vacuum
distillation units. Crude oil typically has a total acid
number (TAN value) value on the order of 0.05–6.0
mg KOH/g of the sample.
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