1. Natural Gas Formation
Natural gas is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the Earth’s surface over millions of years.
This occurrence occurs not only on dry land but also underneath the seafloor. Being sealed off in an oxygen-free environment, the organic material undergoes a thermal breakdown process because the increasing heat and pressure convert the matter into hydrocarbons.
The lightest component of the newly formed hydrocarbon leaves the matter in a gaseous state known as Natural Gas. Once natural gas is completely formed, the odds of the gas being extracted depend on two characteristics of the surrounding rock: porosity and permeability.
Porosity refers to the amount of empty space within a rock’s grains. Sandstone is a typical example of high-porosity rock with large amounts of storage space for fluids such as oil, water and gas.
Permeability, on the other hand, is the measure to which the pore spaces in a rock are interconnected. The higher the permeability of the rock, the easier it will allow fluid to flow through it.
Figure 1. Illustration of Porosity vs. Permeability
This leads to natural gas flowing upwards through rocks with higher permeability due to its low density compared to surrounding rocks.
The natural gas deposits found today are due to upward gas flow through permeable rock until it reaches rock with such low permeability that it can’t flow any further and becomes trapped before reaching the atmosphere.
2. Where Natural Gas Is Found and How It Is Obtained: Conventional and Unconventional Natural Gas
There are two main categories of natural gas based on origin and location: conventional and unconventional.
Conventional natural gas is often found together with oil reservoir deposits and can be extracted by drilling vertical wells and using traditional pumping techniques. Due to buoyancy, the natural gas will often be found floating on top of the oil or mixed with the oil (Student Energy, Conventional gas).
Unconventional natural gas deposits include shale gas, tight gas sandstone, coalbed methane and methane hydrates, to name a few. Unconventional natural gas is mainly formed deep within the earth, as illustrated in Figure 3. The natural gas deposits trapped deep within these rocks are hard to extract. However, recent technological advances in this field have made it possible to extract a large amount of natural gas from these sources economically. Gas reservoirs are considered unconventional when specialized extraction methods such as hydraulic fracking and horizontal drilling must be used to extract the gas.
Read more about these advances by following the links below:
Figure 2. Illustration of Different Layers of Natural Gas Deposits
3. Where Natural Gas Is Found and How It Is Obtained: Unconventional Gas
3.1 Shale Gas
Shale gas is trapped deep within the earth in gas-rich shale rock layers. It is extracted using a fracturing or hydraulic fracturing process. Shale gas wells are typically drilled to depths of 1500m – 4000m, with the average wells estimated at 2500m.
Only drilling a vertical well into the shale layer will not release enough gas to make the process economical. This is mainly due to the gas trapped in the low-permeable shale rock. This is why specialized drilling is the only way to extract large amounts of shale gas. Gas-rich shale rock layers make up a large area of the earth’s rock layers, which is exactly why shale gas is one of the largest natural gas resources in the world.
3.2 Coalbed Methane
Methane gas deposits are commonly found in underground coal reservoirs, which is considered a natural occurrence. However, methane poses a threat to underground coal mining activities due to the large volumes released and its flammable nature. Therefore, it is feasible to tap into coal seams and extract the gas in a controlled manner, known as coalbed methane, which is a form of natural gas.
Coalbed methane is also referred to as sweet gas, coalbed gas and Coal Mine Methane (CMM). The gas can be extracted using various methods, such as UCG (Underground Coal Gasification), well drilling, and hydraulic fracturing, which are similar to the methods used to extract shale gas. Compared to shale gas deposits, the amount of gas extracted from coal beds is quite small. Coalbed methane has contributed to environmentally friendly extraction methods by injecting carbon dioxide into hard-to-access coal seams, which displaces the trapped methane. This process enhances the recovery of methane-rich natural gas while also storing carbon dioxide underground.
3.3 Tight gas Sandstone
When natural gas flows into rock reservoirs with high porosity but low permeability, it can be referred to as tight gas. Typically, tight gas is held in rocks with pores up to 20,000 times smaller than a human hair, making it nearly impossible for the gas to flow freely.
Because of the nature of the rocks in which it is stored, tight gas commonly requires hydraulic fracturing and horizontal drilling to be released.
3.4. Methane Hydrates
Methane hydrates are the most recently discovered and researched form of natural gas. They are formed by methane molecules trapped within a cage of water molecules. Methane hydrates occur naturally in a solid crystalline form, commonly found in sediments in Arctic regions and deep beneath the ocean floor. Although they may resemble ice crystals, methane hydrates will ignite when set alight.
Methane hydrates are estimated to be the planet’s most abundant source of unconventional natural gas (refer to Figure 5). However, there is still much uncertainty about the exact amount of methane hydrate sources. Due to the technical difficulties of extracting this energy source, methane hydrates are considered the most difficult natural gas resource to extract. Economically, extracting the gas remains challenging; only fractions of this resource are found in large enough concentrated volumes to make extraction feasible.
Figure 4. World Map Showing Estimated Methane Hydrate Locations
3.5. Biogenic Gas
Certain types of bacteria can produce methane in large quantities through the breakdown of organic matter in an oxygen-free (anaerobic) environment.The bacteria are commonly known as methanogens. With methane being the main component in natural gas, the produced methane-rich gas is considered a type of natural gas referred to as biogenic gas or biogas. Biogenic gas must be differentiated from Thermogenic gas (fossil gas), which is produced from organic material deep in the earth and subjected to high pressures and temperatures. Biogenic gas typically forms closer to the earth’s surface than other unconventional natural gas sources. Thermogenic and biogenic gases have identical properties, but their compositions may differ in some cases.
Figure 5. Typical Composition of Biogenic Gas
Any organic matter is considered a potential source for producing biogenic/bio-gas. Food waste, livestock manure and sewage are just a few examples and can all be broken down into smaller categories. Biogas is considered a major renewable energy source. A popular method for producing biogas is using an Anaerobic Digester system. The gas produced by the digester can then be used to produce electricity from gas-powered generators. It is common practice for farmers around the world to produce biogas from an anaerobic digestion process by using livestock manure or vegetable/food waste as feedstock. Landfill sites can be considered as another major production source of biogas. Large amounts of municipal waste are buried in landfills, which will then start producing biogas, much like an anaerobic digester. The methanogens will start breaking down the organic matter in landfills, such as food waste and newspapers, producing gases such as methane and carbon dioxide. These gases can then be captured and separated before being implemented productively in the energy sector. When implementing biogas, a reduction in greenhouse gas emissions can be expected.
4. Where Natural Gas Is Found and How It Is Obtained: Extracting/Harnessing Natural Gas
Before implementing any extraction method, the location of natural gas deposits must be established. This can be done using seismic testing, similar to the methods used for locating petroleum deposits. Gas prospectors use seismic trucks or more complex three-dimensional tools to set off a series of small charges close to the earth’s surface, generating seismic waves thousands of meters below the surface in potential underground rock formations such as shale rock and coalbeds. Geophysicists then interpret the results of the seismic waves by using acoustic receivers known as geophones. They then measure the travel times of the waves through the earth, after which a picture is constructed of the subsurface structure and potential natural gas deposits are identified. An exploratory well must be drilled to establish whether the identified gas deposits contain economically viable amounts of natural gas. Once the amount of natural gas is determined, the following extraction methods can be implemented.
Rwanda has implemented the first natural gas extraction method in the world. A natural gas extraction barge is located on Lake Kivu and is the largest extraction plant of its kind in the world. Gas bubbles are extracted from the water and processed accordingly. For further insight into this fascinating development, watch the video and explore the detailed article via the link below:
4.1 Where Natural Gas Is Found and How it is Obtained: Hydraulic Fracturing (Fracking)
Hydraulic fracturing, widely referred to as fracking, was developed to extract gas from very low-permeability rock, such as shale rock. The process consists of injecting large volumes of water mixed with sand and various fluid chemicals at high pressure into a well to fracture the above-mentioned rock. This increases the rock’s permeability and the production rate at which the specific well produces gas.
The typical procedure for extracting shale gas through fracking follows these steps:
Once a gas-rich shale rock location is identified, a production well is drilled vertically until it reaches the shale formation. The wellbore then turns horizontally to follow the shale rock. Advances in horizontal drilling have made it possible for a single well to pass through larger volumes of shale, allowing more gas to be extracted.
A steel casing is inserted into the well to ensure it remains open and to protect the wellbore’s integrity. Cement is then pumped into the well and forced up the outside of the casing to seal the well, preventing fracking fluid, natural gas, chemicals, and produced water from leaking into surrounding groundwater supplies.
Once the well is sealed, small explosives are detonated in the horizontal section to create holes at specified intervals where fracking will occur. Fracturing fluid is pumped into the well at controlled pressures to fracture the rock several hundred meters from the well. Sand mixed with the fluid holds the cracks open when pumped out. After fracturing, gas flows freely to the surface, where it is collected in a controlled manner.
4.2 Where Natural Gas is Found and How it is Obtained: Underground Coal Gasification (UCG)
Underground coal gasification is an industry process that converts coal into gas while it is still underground. It involves drilling wells into the coalbed layer. Oxygen and water are then injected into the well. Once the well is injected with the appropriate amounts of oxygen and water, the coal is partially buried underground. The burned coal then produces a type of natural gas that starts flowing towards the surface. An animation of the USG process can be seen in the video below:
4.3 Where Natural Gas Is Found and How It Is Obtained: Traditional Well Drilling
Vertical well drilling is a common method for extracting natural gas. After exploration, well locations are determined. Many natural gas reservoirs, particularly conventional ones, are found at relatively shallow depths. Horizontal drilling or hydraulic fracturing is not necessary to extract natural gas from these reservoirs because the gas pockets are typically found close to the earth’s surface and not in tight rock layers. These natural gas deposits are primarily of bacterial origin, continuously producing gas at relatively shallow depths, and are classified as biogenic gas. Vertical wells are drilled at depths varying from 300m-800m until a gas pocket is reached and gas starts flowing toward the surface. Horizontal drilling may be considered in some cases to interconnect wells and gas pockets in an underground system to increase the flow of wells. These natural gas wells usually produce gas at low pressures varying from 0.2 to 4 bar.
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