In addition to the need to drill large numbers of density packed wells (usually 8 per square mile or more), unconventional oil and gas extraction (fracking) also involves much larger and more intense extraction processes to be carried out on each well. Regardless of the exact type of unconventional extraction being used, these processes almost always involve the use, and contamination, of very large volumes of water, often up to 10 million gallons per well, or more.
In recent years the majority of fracking wells drilled have involved long horizontal laterals, up to 2 kilometres (1.25 miles) long in the case of shale oil or gas wells. The two main processes involving are high volume hydraulic fracturing in the case shale/tight oil and gas, and dewatering of coal seams in the case of coalbed methane (CBM).
In the case of high volume hydraulic fracturing, the volume of fracturing fluid varies wildly depending on a number of factors, with the depth of the formation targeted being one of the most important. Since oil tends to be at shallower depths than gas, which means that on average shale/tight oil extraction involves the use of less fluid for hydraulic fracturing that shale gas.
The graph on the right depicts the typical range of fracturing fluid volumes per well used in a number of US unconventional oil and gas (fracking) plays. The data comes from the industry Frac Focus website, where companies can voluntarily disclose information (or not), but is likely to be relatively accurate from a fluid volume perspective. The data covers the period April-May 2014 for a number of major US unconventional plays, but with some major omission (e.g. the Haynesville Shale in Louisiana). As a comparison the US Geological Survey reports a range between 1.5 and 15.8 million gallons based on data supplied to it from the industry between 2010 and 2012.
It should be noted that while the fluid volumes in the range of 5-10 million gallons per well are typical, there are many wells in the 2-18 million gallons range. Also wells in the Utica Shale in Pennsylvania/Ohio (up to 14,000 feet deep), Horn River Shale in British Columbia, Canada (7,800-13,300 feet) and Collingwood Shale in Michigan (around 10,000-12,000 feet) regularly exceed 20 million (see graph on the left).
Some of the largest hydraulic fracturing jobs ever have occured in the Horn River Shale, mostly involving the Encana Corporation. The Encana 063-K pad, with 14 wells, reportedly used an average of 28.4 million gallons (108,000 cubic metres) per well while the Encana 001-D pad, with 7 wells, reportedly used an average of 36.4 million gallons (138,000 cubic metres) per well. It should be note that the Bowland Shale in the UK is of a similar depth (9,000-13,000 feet) to the Horn River Shale.
Be aware that fracking is an evolving extreme energy process, where the easiest to extract resources tend to get targeted first, but harder to extract resources are targeted later. Therefore the intensity of these processes, including the average fracturing fluid volumes, increases over time. In particular deeper formations, like the Utica Shale in Pennsylvania, are not being heavily targeted while there shallower formations (Marcellus Shale) in the same region, but the focus will likely shift to them in the future.