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Once wells are no longer economically viable or fuel flow stops, production ceases and they are decommissioned.
At present there appear to be no new government legislations or recommendations regarding the decommissioning of new onshore wells since the 2013 documentation has been withdrawn.
However, in 2016 the OGA published a 'Decommissioning Strategy' which is focussed mainly on offshore wells. The document admits
"The oil and gas industry in the UK is largely unfamiliar with large scale decommissioning projects,
but much can be learned and transferred from other sectors and industries."
Based on best practise from the USA this is a brief outline of the fundamentals of decommissioning a well.
All steel and cement structures and any remaining fuel remains undergound.
Up to 60% of fracking fluids from hydraulic fracturing remain in the well.
In some cases drilling fluids from other wells can be reinjected under pressure to fill the voids created by extraction in the strata.
Abandoned wells may also be filled with waste mining materials, fluids, cement and gravels as a form of production waste disposal, before being sealed.
The well is sealed with a cement plug approximately 1 metre below the surface.
All fencing and surface structure of the wellpad is removed but the concrete pad may remain.
The site should be returned to its original state, including the planting of shrubbery and trees.
Statistics and scientific analysis from 2011 by Professor Terry Engelder revealed a risk factor of 98.5% of the occurrence of serious incidents of contamination from poorly constructed or poorly regulated unconventional wells in the USA. This represents 1 in every 150 wells.
The Industry admits that all wells will ultimately leak due to decompostion of structures below the surface, allowing gas and contaminants to migrate into strata and possibly to water supplies. (See graph below showing leakage rates)
Images below are from this video link. Facts on Fracking by Dr Anthony Ingraffea
Graph of leakage rates of wells under sustained casing pressure.