Sources and Process Overview of Natural Gas |ChemFam #26|

Greetings to everyone! In my previous posts, we have studied some various processes for the purification and separation of natural gas. Today, we shall dive into the basics of those that I should have wrote earlier. This brings us to the various sources and a brief process overview of the natural gas.

WHAT WE SHALL LEARN?

In this particular post, we shall learn about the various sources of natural gas, the distribution of natural gas and the overall processing of natural gas.

Sources of Natural Gas

Let us first come to the various sources of natural gas. Generally we have two prime sources of natural gas, which could either be conventional or non conventional sources. By conventional sources we mean the natural gas could be extracted economically and commercially and these sources are easily accessible. The non conventional sources are being developed as a result of non commercial uses of natural gas by the former one. These type of sources are normally uneconomical to extract and are difficult to access.

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☑ Conventional Sources
Conventional sources are further divided into two types -

✓ Associated Gas : These are gases that are in close contact with the petroleum in a petroleum reservoir.

✓ Non-associated Gas : These gases are produced in the reservoirs that do not have crude oil or higher hydrocarbons. They may contain non hydrocarbon gases such as carbon dioxide, hydrogen sulphide etc.

☑ Non Conventional Sources

In the non conventional sources, we have 5 different types of gases -

✓ Shale Gas : A shale is a very fine-grained sedimentary rock and is easily breakable into thin, parallel layers. They have been formed about 350 million years ago. The oil and gases are extracted by destructive distillation. By destructive distillation we mean that we are destroying particular shale and this destruction is brought about by applying high pressure and high temperature load. These shales exist quite below the sea level which is about 1000 meters.

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✓ Natural Gas Hydrates : Hydrates are formed of a lattice of frozen water molecules as a result of hydrogen bonding and forms a cage like structure around molecules of methane. These are formed under high pressure and low temperature conditions. This is basically termed as host-guest chemistry in the field of supramolecular chemistry, where the water molecules acts as host and the trapped gases acts as the guest molecule.

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These hydrates are formed within a fixed sone inside the subsea sediment because as we go deeper and deeper into the sediments we will be reaching towards the center of the earth where the temperature starts increasing. So, the conditions are not favourable for the formation of the hydrates.

✓ Coal Bed Methane : Many coal seams contain natural gas either within the seam itself or the surrounding rock. The coal bed methane is trapped underground and is generally released into the atmosphere only during coal mining activities.

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✓ Tight Gas : These gases are stuck in a very tight formation underground. As they are held very tightly so a great deal of effort is required to extract natural gas from a tight formation. These reserves are generally put under some kind of fracturing or acidifying agents to extract the natural gas. But the cost of the technique is quite high.

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✓ Geopressurized Gases : The geopressurized gases are supposed to have substantial amount of natural gas. The geopressurized zones are natural underground formations that are under unusually high pressure for their depths. In that high pressure zines, we find that layers of clay get deposited and compacted very quickly on top of more porous, absorbent material such as sand or silt.

Processing of Natural Gas

There are various processes for various components in natural gas. We shall just quickly have an overview of the important processes in converting the raw natural gas into LNG.

☑ Dehydration : Dehydration generally means removal of water from water saturated natural gas. The natural contains lots of water as it comes out from the sea and the water needs to be removed to get high calorific value of the natural gas. If there is goof amount of water, it can form hydrates and these hydrates may then clog the pipelines. So we need to prevent the formation of hydrates and also in presence of water the acid gases forms some weak acids which will lead to the corrosion of the pipelines and other equipments.

☑ Hydrocarbon Recovery : Here, our main aim is to remove higher hydrocarbons like ethane, propane etc. These recovered hydrocarbons are used for fuel fas conditioning and to alter gas condensation temperature although they may not have higher fuel values. We have to remove these hydrocarbons so that we can adjust the condensation temperature of the natural gas. Generally this kind of recovery process involves the cryogenic separation.

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☑ Nitrogen Rejection : Nitrogen is an inert gas as such it doesn’t react with other components but it reduces the calorific value of the natural gas and that is why it is important to remove for low quality gas feedstock. There are various technologies involves for the removal of nitrogen. These includes adsorbent technology, membrane technology and cryogenic technology

☑ Helium Recovery : Helium is a very important gas and the whole super conductivity is dependent on helium. One of the most common applications of He is MRI. The He recovery is uncommon, unless the He content is above 0.5 vol %.

☑ Outlet compression : Most plants must compress the gas before it goes to the pipelines. Majority of the plants that have cryogenic hydrocarbon recovery use turbo-expanders to provide refrigeration in the cryogenic section. The work generated in expansion is used to recompress the outlet gas.

Liquid Processing

By liquid processing, we are getting the natural gas liquid (NGL). It is important to note that this is not same as LNG. LNG is basically methane and some other higher hydrocarbons but NGL means some higher hydrocarbons other than methane which can be used as feedstocks for making other chemicals. This depends on the liquid content of the inlet gas and the desired product drying.

Summary

Natural gas is available via various sources and all these sources are equally important. After finding the desired sources, these have to processed well to eventually drain out the required gas. Sometimes removal of some trace components are essential for processing and production of natural gas. After a series of processes these can only be then marketed commercially.