Hydraulic Fracturing and Shale Gas Production
Travis Hudson, American Geological Institute, 4220 King Street, Alexandria, VA 22302
Shale is a rock formed from mud and silt. In many places this sediment is rich in the remains of tiny organisms that sank to the seafloor. These organic remains become part of the shale and if sufficiently heated change to natural gas. In many places the gas-bearing shale is so fine grained and lacking in internal openings (porosity) and connections between them (permeability) that the natural gas is trapped in the shale. This is shale gas.
Shale gas is found in many parts of the United States (Figure 1) but it is only since the 1990s that technology has enabled it to be economically produced in large amounts. In 2007 shale gas supplied 7% of US natural gas consumption but a boom is underway in developing these resources and three to four times as much is expected to be produced by 2020 (DOE Primer). Advances in how wells are drilled and completed are enabling this expanded shale gas production.
Technology now enables wells to curve and advance horizontally. This horizontal drilling technology enables up to several thousand of feet of the shale gas reservoir to be penetrated by a single well (OERB animation).
Hydraulic fracturing technology is used to fracture the shale and create pathways that enable the trapped gas to migrate to the well. The fractures are created by pumping large volumes (up to several millions of gallons) of fluid at high pressure down the well bore and into the gas-bearing shale (Figure 2). These fluids are commonly water that contains chemicals to control the fluid’s physical properties and possible reactions with the shale. In addition to chemicals, the fluid carries small solid particles such as sand grains or ceramic pellets known as “proppants”. These prop open the fractures and enable gas to flow through them after high pressures have dissipated.
Environmental issues associated with hydraulic fracturing and shale gas production are mostly related to protecting surface and ground water quantity and quality (EPA Studies). They include:
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Figure 1: United States Shale Gas Basins (Source: DOE).
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Figure 2: A large hydraulic fracturing operation underway in a Marcellus Shale gas well (Source USGS).
The US is the world leader in shale gas production technology (TAMU Engineer). An example of exporting this technology is the US-China Shale Gas Resource Initiative established in 2009 (White House Press Release). A type of hydraulic fracturing has been used for many years to assist water well production. This hydraulic fracturing is very different from that used in shale gas production and it is not accompanied by the same environmental concerns. (NGWA).