Measurement and Testing

  • Quality Assurance in the Oil and Gas Industry - A Guide to Crude Oil Quality Testing

Quality Assurance in the Oil and Gas Industry - A Guide to Crude Oil Quality Testing

Apr 11 2021

From West Texas Intermediate (WTI) to Bonny Light, the quality and characteristics of crude oil directly influence market value. All crude oils have unique chemical and molecular properties, and most buyers want to have a good understanding of these before committing to a purchase.

So how does quality assurance in the oil and gas industry work? Read on for our guide to crude oil quality testing, why it matters and the key techniques used to maintain and regulate industry standards.  

Crude oil assays

Before purchasing oil, most buyers subject the product to a chemical evaluation known as a crude oil assay. Samples can be drawn from a variety of sources, including pipelines, cargo ships, reservoirs, refineries and storage tanks. The evaluation is conducted in purpose-built laboratories and offers the buyer an overview of the distinct chemical and molecular characteristics of the crude. Generally carried out by producers, refiners and traders, crude oil assays offer extensive hydrocarbon analysis data that can be used to determine suitability and value.

Without crude oil assays, buyers risk running into a slew of problems, including mechanical breakdowns, quality control issues and environmental breaches. Crude oil quality testing also ensures producers are meeting industry standards.

The importance of density and sulphur content

Density and sulphur content are two of the most important factors considered in a crude oil assay. Here’s an overview of each:


Density refers to the ratio of the mass of oil to its volume. It’s common to see American Petroleum Institute (API) Gravity used as an index to measure the density of a crude oil. The US organisation created this measure, which indicates how light or heavy a crude oil is compared to H2O.

Grades with an API Gravity of more than 10 are considered light (lower density) and generally float on water. Lighter crudes also flow freely at room temperature and are easier to extract and process. Grades with API Gravity of less than 10 are classed as heavy (higher density) and will sink when placed in water. Most crude oils have an API of between 15 and 45 degrees.

Sulphur content

The sulphur content of crude oil can vary greatly and has a significant impact on quality and price. Most grades have sulphur contents of between 1 and 4 wt%, though can sometimes be lower than 0.05 or more than 10 wt%.   

Generally, lighter and sweeter crude oils are more valuable than heavier and more sour counterparts. This is partly because light and sweet crudes are better for producing gasoline and diesel fuel, which can be sold at a higher price than residual fuel oil and other second-grade products. As the crude is lighter and less sulphur-rich the refining process is less energy-intensive and doesn’t call for sophisticated equipment. Grades containing less than 1 wt% sulphur are classed as sweet, while those with more than 1 wt% sulphur are considered sour.

Tapis, a type of crude oil found in Malaysia, is considered one of the most valuable grades in the world. It’s used as a pricing benchmark in Singapore and boasts an API gravity of between 43° and 45°. It also incredibly sweet, with sulphur content of just 0.04%.

Other crude oil quality control tests

Most crude oil assays are comprehensive as buyers want to have an in-depth understanding of the product. As well density and sulphur content, laboratories analyse many other different variables during crude oil assays. Here’s an overview of some of the most common tests carried out:

  • Amines

While amines can be an efficient way to treat acidic contaminants, they can create problems in high concentrations. Most crude oil assays test for amine content to avoid problems such as heightened salting on crude distillation units and higher nitrogen levels at wastewater treatment plants.

  • Sediment content

During the extraction process crude oil can become contaminated with sediment from the reservoir formation. While some sediment is okay excessive amounts can damage equipment and compromise the quality of the finished product.

  • Carbon residue

By weight, most oils contain between 82% and 87% carbon by weight. The ASTM D5291 test method is one of the most reliable techniques used to test carbon content and is included in most crude oil quality testing.

  • Chloride content

Chlorides are an undesirable characteristic in crude oil as they can increase corrosivity and encourage a build-up of salts in processing units. If detected, methods such as catalytic hydrodechlorination, chlorine transfer agent dechlorination and adsorption dichlorination will be used to remove chlorides.

  • Mercury content

Mercury occurs naturally in rocks and soil and as a result is present in almost all crude oil. Levels can vary significantly between grades and most buyers want to know the exact mercury content of the crude oil they’re purchasing. Too much mercury can cause major problems for petroleum refineries as the metal can amalgamate with aluminium alloys and damage equipment. Some forms of mercury are toxic and call for specific Health, Safety and Environment (HSE) protocols, while others can poison catalysts such as platinum and rhenium.  

  • Water content

Determining the water content of crude oil is an effective way to minimise the risk of corrosion and damage to infrastructure. For this reason, water content is often included in crude oil quality testing processes.

Analysing crude oil quality

Now we know more about what kinds of chemical and molecular properties laboratory teams look for, let’s take a look at some of the technologies used to carry out the tests and maintain quality assurance in oil and gas industry:

X-Ray Fluorescence (XRF) Analysis

Producing, using and recycling lubricating oils calls for a deep understanding of what additive elements are present. Fast and precise, X-Ray Fluorescence (XRF) Analysis allows quality control teams to determine the exact contents of lubricating oils. Not only does this reduce production costs, it also increases efficiency and minimises environmental impact. Fully norm-compliant with ASTM D7751-16, technologies such as benchtop Energy Dispersive X-ray Fluorescence (EDXRF) S2 POLAR are used to analyse and detect seven key additives - Mg, P, S, Cl, Ca, Zn, M. As well as being used in base oil production, the S2 POLAR promises industry-leading quality control in blending facilities and additive dosing stations.

The use of alternative calibration strategies using liquid standards

Instead of calibrating using only one certified calibration gas, kits including different liquid standards, concentrations and a constant injection volume allow quality control teams to generate blank value-corrected calibrations. This drastically improves the quality of analysis by offering results in the ultra-trace range (ppb). Not only do laboratories using this strategy enjoy faster results, they can also slash their overall costs. To leverage the benefits of different liquid standards it’s critical to invest in an analyser that digests standards and samples quantitatively, regardless of the matrix type. For example, when analysing sulphur, the detector won’t determine whether SO2 originates through liquid, solid or gaseous combustion. In a quantitative approach, one nanogram of sulphur always produces the same SO2 amount.

Portable technologies

The oil and gas industry is rapidly changing and companies are continually launching new technologies designed to keep up with progress. The ZX-101SQ is the latest ZX-101 Portable Fuel Analyser launched by US-based company Zeltex, LLC. Combining the reliability and accuracy of its predecessors with next-generation Near Infra-Red Optics (NIR) technology and an updated interface, the ZX-101SQ analyses for cetane, octane, ethanol and biodiesel percentages on the spot. Whether you’re at the pump, in the field or working from a laboratory, it ticks all the boxes showcases the exciting new technologies used to maintain quality assurance in oil and gas industry.

Boiling point measurements

Boiling point can have a significant impact on the value of crude and if it’s considered a best quality oil. Companies such as Bartec Benke have developed technologies such as the rapiDist-4 that offer fast, reliable and accurate boiling point measurements for a range of raw products, including fuel oils, middle distillates, naphtha, jet fuels, diesel and liquid hydrocarbons.

From recreational vehicles to heavy processing machinery, mechanical devices rely heavily on moving parts. Lubricants and greases play a critical role in minimising friction between these parts and maintaining operational efficiency. To ensure maximum performance it’s critical to have an in-depth understanding of the physical and chemical characteristics of these lubricants and greases, including additive content. The two key types of additives - AW and EP – are explored in this fascinating article, Development of Antiwear and Extreme Pressure Additives for Lubricants and Greases

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