SHALE gas is usually located more than 1000 metres to 3000m below the surface. It has long been known to exist, but new developments in hydraulic fracturing and horizontal drilling, since the 1990s, has made it the preferred recovery method in the United States.
The methods used:
• Because of its depth, shale gas requires hydraulic fracturing, or fracking, to extract the gas – mostly from horizontal wells.
• Fracking fluid is typically between 96 per cent and 99pc water and sand with the remaining percentage comprised of chemical additives used to reduce friction, inhibit bacterial growth and prevent corrosion.
• A well cased in cement and steel is drilled through the overburden and the aquifer down into the shale where horizontal drilling may take place.
• Water is pumped down at a pressure high enough to fracture the shale.
• The flowback water, or produced water, is extracted from the well for treatment on the surface and sand granules remain in the fractures to hold them open and allow gas to flow.
• More than 1 million litres of water is required for each fracture and about 10-20 fracture stages are employed for each well – spaced out in the horizontal part of the well, which is usually about 1 kilometre to 2km long, but can be up to 4km.
• The SA government estimates that between 8 megalitres and 16mgL of water is required to frack a vertical well five to 10 times, while up to 24mgL is required for horizontal wells, utilising up to 15 treatments per well.
• When production is finished, sections of the well are filled with cement and the well is capped.
• A well can remain in production anywhere from a few years to 20-30 years.
• Not all CSG wells require fracking because they are typically shallow, between 300-1000m, and gas can flow freely once the well penetrates the coal seam.
• Fracking is required about 30pc of the time to stimulate CSG and the fractures are generally smaller than those used for shale or tight gas production.
• Coal seams are usually found within an aquifer, unlike shale gas and tight gas, which is typically located deep beneath the acquifer.
- Full report in Stock Journal, June 6 issue, 2013.