Shale gas is extracted using horizontal drilling and hydraulic fracturing or ‘fracking’. Neither of these technologies are particularly new or specific to shale gas, but they can make up anything from 20% to 47% of gas producers’ capital expenditure.
Extracting any resource from the ground is likely to raise some environmental considerations. For shale gas extraction, the main environmental concern is contamination of the water supply. Carbon emissions are also a concern, although less is known about shale’s likely carbon footprint at present.
During this process wells are drilled vertically to just above the known shale deposit at a depth of 1,500 to 3,000 metres. The drill bit is then deviated and drilled horizontally through the shale at an angle to maximise horizontal stress for fractures.
Typically 15 to 16 wells need to be drilled to find a ‘sweet spot’ of rock that is easily fractured and has sufficient gas saturation to make production economical. Because the gas distribution is uneven, current reported extraction rates are between 4% and 6%. More wells are needed to produce the same volume as would be produced for conventional gas extraction.
Despite its higher costs, hydraulic drilling is being increasingly favoured over vertical or directional drilling in the United States because it maximises exposure to the reservoir, thereby increasing the production rate. This makes shale gas extraction more economical.
Hydraulic fracturing or ‘fracking’
Productive zones within the well are isolated for fracturing where water and chemicals are injected under high pressure into the wells to fracture the rock. ‘Proppants’, usually sand or ceramics, in the injected water solution hold the fracture crack open to prevent it from ‘healing’ and allow the continued release of natural gas. This gas is in two forms: ‘free gas’, which is released first, and ‘adsorbed gas’ on the surface of organic matter, which is released when the pressure in the well drops. The solution injected into the well also contains a very small quantity of additives such as gelling agents to cause the rock to crack, biocides to kill contaminating micro-organisms and surfactants to sterilise the well. Additives are also used to increase the efficiency of the process. Typically these additives comprise around 0.5% of the total injection volume. The composition of additives depends on the conditions of the well such as pressure and temperature and on the quantity of proppant used.
The gas company Total estimates for its operations that an average of 30 ‘fracs’ are performed for each 1,000 metre well and each ‘frac’ uses 300 m3 of water, 30 tonnes of sand and 0.5% additives in the solution mixture. Therefore, the process is very water intensive, which is a big issue for water-stressed states where gas shale plays are located, such as Texas. This water needs to be extracted from aquifers or trucked in to the site on access roads.