Natural gas is a fossil fuel created by decomposing plant and animal material under immense pressure and heat deep within the earth. The gas they produce can be extracted for commercial use and the extraction method is determined by the location of the gas within the earth as well if the reservoir is on- or offshore. But how is the natural gas extracted?
Before a company can start drilling for gas they have to ensure that the identified reservoir is commercially viable by drilling an exploratory well and determining the quality and quantity of gas available. Once the viability has been confirmed drilling and extraction can begin in earnest.
Conventional natural gas is found in reservoirs where a layer of rock keeps the gas trapped within the earth. These can usually be accessed through vertical drilling as the gas can be extracted directly below the end of the well. The well is drilled straight down and gas usually flows easily to be extracted.
Unconventional natural gas reserves need different methods to extract the gas and are more complex. From the established vertical well the area from which gas is extracted can be increased by further horizontal drilling. Geologists determine the best depth for horizontal holes to be drilled and gas accessed.
Shale gas, tight gas and coal bed gas are all types of unconventional gas and other methods of drilling need to be used to access these reservoirs. Shale is a fine-grained, sedimentary rock of many layers compacted together. As the rock is compacted gas can become trapped within the layers and the gas is difficult to be extracted through traditional drilling methods. To extract the gas in shale horizontal holes are drilled and fracking takes place. Fracking, or hydraulic fracturing, is executed by pumping a high-pressure stream of water along with chemicals and sand into the drill with the purpose of fracturing the shale. The sand keeps the newly fractured rock ‘open’ allowing the gas to escape to the surface. Similarly to shale gas, tight gas is also trapped in impermeable rock and also need to be extracted through fracking. Similar to fracking drilling companies can make use of acidizing where acid is pumped into the well to dissolve the rock rather than just breaking them apart. The two methods can also be combined into a process called acid fracking. The exact location of the mine and sediment profile will determine which method is used.
Another unconventional gas is coal bed methane. This gas is trapped within the coal seams that run underground. The gas can be extracted as a by-product of coal mining but when a well is dropped for the specific extraction of gas it is achieved by depressurizing the seam. Water is pumped from the seam and the gas is able to desorb and flow to the surface as a gas.
All of these onshore drilling methods can also be employed at offshore drilling sites. Offshore drilling is significantly more dangerous due to the isolated nature of the drilling operations.
While all of these methods are used in gas extraction methods like fracking and acidizing have come under immense scrutiny for their negative environmental effects.
