Hydraulic fracturing occurs when high pressure fluids primarily consisting of water and sand are pumped at high pressure into subsurface formations, typically shale that contains natural gas and/or oil. The high pressure fluid causes the rock to fracture. The new fractures increase the surface area of the shale and better interconnect previously existing fractures, allowing more natural gas and/or oil to be pumped from the formation. Modern hydraulic fracturing, referred to as “fracking,” is an evolving technology that largely began after 2000 and has significantly increased natural gas production in the United States in the past five years with corresponding decreases in natural gas prices.
The revolution in hydraulic fracturing has been made possible by technological advancements in directional drilling. In the past, wells were drilled vertically and sometimes passed only briefly into the producing formation. Shale is a sedimentary rock that is initially formed underwater as a horizontal layer containing compacted mud that is cemented into rock. Intact shale has a low permeability, making fluid movement slow except along natural or artificial fractures in the rock. In the case of the Marcellus Shale in Pennsylvania, the shale is approximately 100 to 250 feet thick. The Barnett Shale in Central Texas is between 100 and 500 ft, averaging about 300 ft; Eagle Ford Shale in South Texas is very variable with an average of about 250 ft; Fayetteville Shale in Arkansas is between 60–575 ft, average of about 200 ft; Haynesville Shale in Northwest Louisiana averages about 250 ft. Tectonic activity may later deform the initially horizontal layer into different angles and shapes, but the fundamental problem remains of how to most efficiently extract fluids from relatively thin and deep rock layers that have low permeability. Directional drilling allows a well to be oriented in a vertical direction until the shale layer is approached and then turned in to the approximately horizontal direction needed to follow along the shale layer ( Figure 1). The wells can be turned in any compass direction, allowing multiple wells from a single pad to reach areas of several square miles in the producing shale location, thus significantly reducing their surface footprint and disturbance when compared to vertical drilling. Wells have been drilled more than a mile deep and a mile in horizontal reach. Furthermore, the Marcellus Shale is underlain by the thicker Utica Shale, making it likely that the same pads may years later be used to drill wells into the Utica Shale as well when gas prices rise sufficiently to make deeper drilling cost effective. Information on the extents of the Marcellus and Utica Shale formations is available in USGS reports listed in their Energy Resources Program (USGS, 2013) and at the website geology.com ( Geology.com, 2013).
The second key to increased access to energy resources is hydraulic fracturing. Shales naturally have a low permeability, meaning removal of resources is slow and inefficient. High pressure fluids containing proppants (e.g., sand), biocides, friction reducers, corrosion inhibitors, iron control, scale inhibitors, surfactants, and acids are injected into the wells to cause fracturing of the shale. The proppants move into the newly created fractures and expanded natural fractures and then prop the fractures open when the fluids are removed. The injected fluid is primarily (~99.5%) sand and water and the additive mixtures change by location, company, and as the technology evolves. Much of the injected water is later extracted from the well during production, along with formation waters.
The exact spacing of pads, number of wells per pad, orientation of wells drilled from each pad, and well distance in the horizontal and vertical directions, depends upon surface access, drilling rights, formation properties, and economics. Leasing is an issue. Leases typically expire after a fixed period of time (typically about 5 years) if there is no drilling activity. Companies can drill but choose not to hydraulically fracture or produce from the well prior to lease expiration in order to lock up the resources until the economics of production improve.
Hydraulic fracturing has revolutionized the energy field, causing the United States to switch from a need to import natural gas to a situation where export of natural gas is being considered. Energy independence, low energy costs, and economic development are clear positives that have been advanced by the revolution in hydraulic fracturing.
This paper will summarize some of the environmental impacts associated with fracking. The order of subjects is approximately from the most obvious and certain to more subtle impacts.