Hydraulic fracturing (fracking) is a technique used to crack rock underground, usually for the purpose of extracting oil or gas. It involves injecting a fluid down a well under massive pressure, in order to force open cracks in the targeted rock. Conventional hydraulic fracturing has been used since the 1950s to create small cracks, just around well-bores, to speed the flow of oil or gas into the well. Typically modern hydraulic fracturing of a conventional oil or gas well might use between 50,000 and 200,000 gallons of nitrogen foam or cross linked gel to propagate cracks a few feet from the wellbore. The aim is to speed up the flow where the oil or gas bottlenecks as it enters the wellbore and it usually will not have effects beyond that.
More recently “massive slickwater hydraulic fracturing” has been developed for the purpose of extracting gas from more much impermeable rock formations, such as shale. Unlike conventional extraction where the oil or gas flows though the permeable formation and one well can drain a large area, unconventional extraction requires large numbers of wells which each drain a small area (40-80 acres). Hydraulic fracturing of a typical shale gas or oil well requires the use of around 5 to 7 million gallons of slickwater (a mixture of water, sand and chemicals) to fracture the rock in up 40 stages along a mile or more long horizontal. This aims to propagate fractures hundreds of feet from the wellbore and similar parallel horizontals are usually situated to drain adjacent areas.
Unconventional gas extraction has three different processes; shale gas, coal bed methane (CBM) and underground coal gasification (UCG). While there are a lot of differing technical details these processes, all involve drilling large numbers of directional wells at regular intervals, coating the landscape.
The scale of these new more intense methods are like nothing we have seen before. Up until now the largest onshore gas field in the UK, Saltfleetby in Lincolnshire, had only 8 wells. To produce the same amount of unconventional gas would require hundreds of wells to be drilled. To temporarily replace just one offshore North Sea gas field would require thousands of unconventional wells.
As well as requiring many more wells, these methods also involve much more. Shale gas and oil require massive slickwater hydraulic fracturing to be carried out on every well. Millions of gallons of water, sand and chemicals are injected under massive pressure. CBM wells are also often fracked. UCG which involves setting fire to coal seams underground is even more extreme.
These unconventional wells also have much shorter lifespans, with production from a typical shale well declining by 70 to 80 percent in the first year alone. This means that large numbers of new wells need to be constantly drilled to maintain production, even for short periods. In many areas of the US, unconventional gas is already peaking after less than a decade of exploitation.
Find out more about the threats of unconventional gas