Understanding Natural Gas Processing
What is natural gas processing?
You might be imagining that the natural gas that we gather out of the ground can be moved into pipelines to be used straight after collection, but in fact, the opposite is true. Natural gas processing is an important process that allows us to use clean, purified natural gas in a multitude of ways. Before it can be used, it has to be subjected to a relatively complicated industrial process that is used to clean and remove all of the impurities that are present.
This purification process has to begin as soon as the natural gas has been extracted, as there are a number of different factors that will be present at this point. These can determine what additional materials will need to be removed from the gas. For example, the depth at which the gas was found, the geographical location of the gas and the geology can all affect this.
Due to this, natural gas processing plants have to remove all of the additional materials that we can find in untreated natural gas. A lot of these ‘by-products’ are quite valuable. Companies usually gather them to process them further or sell them on to another company, opposed to simply getting rid of them.
How does the type of well affect the natural gas?
Raw natural gas is usually gathered using one of three different types of gas well. They are known as crude oil wells, condensate wells and gas wells. The different by-products that we expect to find in natural gas can be based on which type of well is used. Therefore, it is something that needs to be taken into consideration during the extraction process.
The natural gas that is extracted using a crude oil well is often referred to as associated gas. This is because it exists as a type of gas ‘cap’ that can be found above areas of crude oil in underground reservoirs. It can even be dissolved in the crude oil itself.
On the other hand, natural gas that comes from either condensate wells or gas wells is often referred to as non-associated gas. This is because the reservoirs that are used for extraction tend to contain little to no crude oil. Both wells are usually used to produce natural gas exclusively.
Other terms that are often used to describe the type of gas that has been extracted include sweet gas, sour gas and coalbed gas. Sweet gas contains a low amount of hydrogen sulphide, while sour gas contains a high amount of hydrogen sulphide. This affects the way that the gas smells. Coalbed gas is a term that is used to describe the deposits of methane that can be found in or around the pores of coal seams in mining environments.
What different things can contaminate natural gas?
During natural gas processing, all impurities are removed, but what contaminates it in the first place? There are a number of different gases and materials that can contaminate natural gas. These range from acid gases to H2O. All of these need to be removed during multiple steps of the purification process. Some of the most common contaminants include:
- Acid gases: These can include carbon dioxide, hydrogen sulphide, methanethiol and ethanethiol.
- Standard gases: Gases that we are familiar with, like nitrogen and helium, frequently contaminate natural gas.
- H2O: This can be found in both gaseous and liquid forms, both of which need to be removed during different steps of purification in natural gas processing.
- Mercury: Luckily, only trace amounts of mercury are usually present in natural gas in their elemental form.
- NORM: This stands for ‘naturally occurring radioactive material’. Natural gas has the potential to contain traces of radon. The process of purifying the gas can result in water that contains traces of radium. This is rather dangerous as it can accumulate in equipment, rendering it radioactive as time passes.
What are the industry standards of purification?
Simply put, there are no industry standards for natural gas processing when it comes to the way in which gas needs to be purified. This is because the quality standards that the gas needs to meet are specified by the individual pipeline transmission and distribution companies. This means that the industry standards vary depending on which pipeline is being used to transport the gas.
Most of the time, the following standards need to be met:
- The gas must be transported at a specific temperature.
- There must be no particulate solids or liquid water present within the natural gas. This prevents a number of different things, like pipeline erosion and corrosion.
- All natural gas that is transported through the pipelines must be completely dehydrated of water vapor beforehand. The presence of water vapor can lead to methane hydrates forming.
- All of the acid gases must have been removed from the natural gas.
How does the purification process take place?
Different companies all use their own methods to purify untreated natural gas. However, the general pattern of purification and the steps that take place usually remain the same. All companies aim to gather as many of the profitable by-products as possible throughout the process. Some of the main steps of natural gas processing and purification include:
- Step 1: Immediately after the natural gas has been extracted, it needs to be processed at a collection point. This involves removing all of the liquid water and the natural gas condensate. The latter is then being transported to an oil refinery.
- Step 2: Pipelines can then be used to move the natural gas from the extraction point to the processing plant. The first step of purification here involves removing all of the acid gases. Historically, this has been done using amine treating, but that does have a knock-on effect on the environment. This has led to more and more companies instead using polymeric membranes to separate the acid gases.
- Step 3: Once the acid gases have been removed, all of the water vapor that is present also needs to be removed. Otherwise, it would have the potential to cause a lot of damage to the pipelines during transportation.
- Step 4: An absorption process using activated charcoal is then used, this removes any of the mercury that is present.
- Step 5: Finally, all of the NGL can be removed from the natural gas. This is important as it can then be transported to a final destination. If transportation via a pipeline is not available, then the NGL can be transported by ship in the form of liquefied natural gas, or LNG.
Purifying during natural gas processing is both complex and necessary. It protects our pipelines from damage. It also ensures that the natural gas that we use is free of any unnecessary or potentially dangerous components. Remember, the impurities that are present in natural gas can be different depending on a number of factors. However, it all needs to go through the same purification during natural gas processing to ensure that nothing unneeded is left.
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