Shale gas is extracted using horizontal drilling and hydraulic fracturing or ‘fracking’. None of which are particularly new technologies or shale gas specific.
During the process wells are drilled vertically to just above the known shale deposit at a depth of 1,500 to 3,000 metres. Then the drill bit is 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’ which is easily fractured and has sufficient gas saturation to make production economical. Because the gas distribution is uneven, currently reported extraction rates are between 4% and 6%. Therefore, more wells are needed to produce the same volume as would be produced for conventional gas extraction.
Increasingly hydraulic drilling is being used in the United States instead of vertical or directional drilling , despite its higher costs, because it maximises exposure to the reservoir thus the production rate is higher. This makes shale gas extraction economical whereas vertical drilling is not economical for shale gas.
Hydraulic fracturing or ‘fracking’
Productive zones are within the well are then 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 their ‘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 use to increase the efficiency of the process. Typically theseadditives comprise of around 0.5% of the total injection volume. The composition of additives used depends upon the conditions of the well such as pressure, temperature and also the quantity of proppant used.
Total estimates for its operations 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.