Natural gas is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the surface of the Earth over millions of years.
This occurrence not only happens on dry land, but is found underneath the sea floor as well. Being sealed off in an oxygen free environment the organic material undergoes a thermal breakdown process because of the ongoing increasing amounts of heat and pressure converting the matter into hydrocarbons.
The lightest component of the newly formed hydrocarbon leaves the matter in a gaseous state and is known as Natural Gas. Once the natural gas is completely formed the odds of the gas being extracted is dependent on two characteristics of the surrounding rock namely: porosity and permeability.
Porosity refers to the amount of empty space that can be found within the grains of a rock. A typical example of rock with high porosity is sandstone. Sandstone has 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 a rocks permeability the easier it will allow a fluid to flow through it.
Figure 1 Illustration of Porosity vs Permeability
This leads to natural gas flowing upwards through rocks with a higher permeability, due to the natural gas’s low density compared to surrounding rocks.
The natural gas deposits found today are due to this upwards gas flow through permeable rock until it reaches rock with such low permeability it can’t flow any further, and so becomes trapped before reaching the atmosphere.
2. Conventional and Unconventional Natural Gas
There are two main categories of natural gas differentiating the origin and location of the gas: Conventional and Unconventional.
Conventional natural gas can often be found together with oil reservoir deposits and can be extracted by drilling vertical wells and making use of traditional pumping techniques. The natural gas will in many cases be found floating on top of the oil due to buoyancy or mixed with the oil.
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 can be seen illustrated in figure 3. The natural gas deposits trapped deep within these rocks are hard to extract, although recent technological advances in this field has made it possible to economically extract large amount of natural gas from these sources. Gas reservoirs are referred to as unconventional when specialized extraction methods such as hydraulic fracking and horizontal drilling must be used to extract the gas. Read more on these advances by following the links below:
Figure 2 Illustration of different layers of natural gas deposits
3. Unconventional Gas
3.1 Shale Gas
Shale gas is gas trapped deep within the earth in gas-rich shale rock layers. The gas is extracted by making use of a fracturing or Hydraulic fracturing process. Shale gas wells are typically drilled to depths of 1500m – 4000m with the average wells being 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 being 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 and this is exactly why shale gas is one of the largest natural gas resources in the world.
3.2 Coalbed Methane
Methane gas deposits is commonly found in underground coal reservoirs, this is considered a natural occurrence. Methane poses a threat to underground coal mining activities because of the volumes of methane being released and methane’s flammable nature. So, it is feasible to tap into the 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 with a variety of methods such as UCG (Underground Coal Gasification), well drilling and hydraulic fracturing similar to methods used to extract shale gas. When compared to shale gas deposits, the amount of gas to be extracted from coal beds are quite small. Coalbed methane has contributed to environmentally friendly extraction in recent times, through injecting carbon dioxide into hard to access coal seams causing the carbon dioxide into displacing the methane trapped within the coal. This means that the methane rich natural gas recovery is enhanced and the carbon dioxide stored below ground.
3.3 Tight gas Sandstone
When natural gas has flowed 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 that of a human hair, making it nearly impossible for the gas to flow freely.
Tight gas commonly requires hydraulic fracturing and horizontal drilling for the gas to be released because of the nature of the rocks the gas is stored in.
3.4. Methane Hydrates
Methane hydrates is a type of natural gas best explained as methane molecules trapped in a cage of water molecules. Methane hydrates are the most recent form of natural gas to be discovered and researched. Methane hydrates naturally occurs in a solid crystalline form which is commonly found in sediments in arctic regions and deep below the ocean floor. Methane hydrates may look like ice crystals but will start to burn when set alight.
Figure 3 Methane Hydrate set Alight
It is estimated that methane hydrates are the most abundant source of unconventional natural gas on the planet (refer to figure 5). There is, however, still much uncertainty of the exact amount of methane hydrate sources due to the technical difficulties extracting this energy source, methane hydrates are considered the most difficult natural gas resource to extract. Economically extracting the gas remains a challenge, only fractions of this resource is found in large enough concentrated volumes to make extraction feasible.
Figure 4 World map showing estimated Methane Hydrate locations
3.5. Biogenic Gas
Methane can be produced in large amounts by certain types of bacteria in a process of breaking down 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 and referred to as Biogenic Gas or Bio-Gas. Biogenic gas must be differentiated from Thermogenic gas (fossil gas) produced from organic material deep in the earth subjected to high pressures and temperatures. Biogenic gas is typically formed closer to the earth’s surface than other unconventional natural gas sources. Thermogenic and Biogenic gas has identical properties but the composition 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 is just a few examples and can all be broken down into smaller categories. Bio-gas is considered a major renewable energy source. A popular method for producing bio-gas is by making use of 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 bio-gas from an anaerobic digestion process by making use of livestock manure or vegetable/food waste as feedstock for the process. Landfill site can be considered as another major production source of bio-gas. Large amounts of municipal waste are buried in landfills, the landfill will then start producing bio-gas much like an anaerobic digester. The methanogens will start breaking down the organic matter in the landfill 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 in a productive manner in the energy sector. When implementing bio-gas a reduction in greenhouse gas emissions can be expected.
4. Extracting/Harnessing Natural Gas
Before implementing any sort of extraction method, the location of the natural gas deposits firstly has to be established. This can be done with seismic testing similar to that of petroleum deposit locating methods. Gas prospectors make use of 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 making use of 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. To establish whether the identified gas deposits actually contains economically viable amounts of natural gas an exploratory well must be drilled. Once the amount of natural gas is determined the below methods of natural gas extraction can be implemented.
A natural gas extraction method which is the first of its kind has been implemented in Rwanda. 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. Watch the video and read more on this interesting occurrence by following the link below:
Hydraulic fracturing widely referred to as fracking was developed in order to extract gas from very low permeable 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 in order to fracture above mentioned rock, this increases the permeability of the rock and the production rate at which the specific well produces gas.
The typical procedure for extracting shale gas by means of fracking will be take place in the following order; once gas rich shale rock location has been established a production well is drilled vertically until it reaches the shale rock formation, once penetrated the wellbore turns to follow the shale rock formation horizontally, advances in horizontal drilling has made it possible for a single well to pass through larger volumes of shale rock which in turn means more gas volumes to be extracted. Steel tube casing is inserted into the well to make sure the well remains open and to protect the integrity of the wellbore. Next up cement is pumped into the well and forced up the outside of the steel tube casing, this is done with the aim to seal the well to prevent any fracking fluid, natural gas, chemicals and produced water to leak into surrounding ground water supplies. Once the well is sealed and casing is completed small explosives are detonated in the horizontal section of the well to create holes in the cases at specified intervals where fracking is to occur. The fracturing fluid is then pumped into the well at controlled pressures to fracture the rock several hundreds of meters from the well. The sand mixed with the fracturing fluid acts in a way to keep the cracks open when the fluids are subsequently pumped out. After the fracturing process is done gas will start flowing freely towards the surface where it is collected in a controlled manner. An animation of the process can be seen when following the link below:
4.2 Underground Coal Gasification (UCG)
Underground coal gasification is a process in industry which consists of converting coal into gas while it is still underground. It involves drilling wells into the coalbed layer. Oxygen and water is then injected into the well. Once the well is injected with the appropriate amounts of oxygen and water the coal is partially burned underground. The burned coal then produces a type of natural gas which then starts flowing towards the surface. An animation of the USG process can be seen at the link below:
4.3 Traditional Well Drilling
Vertical well drilling is another method of extracting natural gas. Gas deposit exploration is done and well locations are determined. Various natural gas reservoirs are located at quite shallow depths, typically conventional natural gas reservoirs. 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 mainly of bacterial origin that keep on producing the gas at relatively shallow depths and can also be 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 flow of wells. These natural gas wells usually produce gas at low pressures varying from 0.2Bar to 4Bar.