Environmental Engineering Reference
In-Depth Information
companies need to address at the early planning stage to minimise dis-
ruption and to work with the local community to prevent adverse public
relation issues which could impact on future projects.
2.3 Production
The completion of drilling and hydraulic fracturing marks the start of the
production phase of the wells. A production well head is put in place to
collect and transfer the gas for subsequent processing, either for utilising on
site for electricity production; for liquefaction and transport off site; or for
piping into the gas distribution network. Production from one well can
commence while other wells are being drilled and fractured.
In terms of production volumes, indicative figures for long-term pro-
duction for a single Marcellus well in New York State are as follows: 15
Year 1: Initial rate of 79 000 m 3 per day, declining to 25 500 m 3 per day
Years 2 to 4: 25 500 m 3 per day, declining to 15 600 m 3 per day
Years 5 to 10: 15 600 m 3 per day, declining to 6400 m 3 per day
Years 11 onwards: 6400 m 3 per day, declining at a rate of 3% per annum
As can be seen, after five years the volume of gas produced drops dra-
matically and it is at that point that the operator may decide to re-fracture
the well to extend its life. Re-fracturing can take place more than once. It is
anticipated that wells in the UK will follow a similar pattern of production
drop-off over time.
2.3.1 Decommissioning and Plugging Off of Wells. When the productive
life of a well is over, or if wells prove uneconomic to exploit, the correct
procedures need to be put in place to ensure that the wells are correctly
plugged and abandoned. Proper plugging is critical to protect the ground-
water aquifers, surface water and soil. Well plugging involves removal of
the well head and the removal of the downhole equipment. Uncemented
casings in critical areas must either be pulled up or perforated. Cement
must then be placed in the wellbore to seal the bore or squeezed through
the perforations of the casings if they remain in place to seal between the
casings, the rock formations and to fill the bore. This procedure occurs at
intervals dictated by the relevant regulatory authority to ensure a seal be-
tween hydrocarbon- and water-bearing zones. As an example of how an in-
dividual American state's regulations have evolved in specific detail,
California's plugging regulations require cement plugs to be placed in the
following locations: a 200-foot plug straddling the surface casing shoe; a
plug across oil- and gas-bearing strata that extends 100 feet above the
strata; a plug extending from 50 feet below to 50 feet above the base of
the water-bearing strata; and a 50-foot plug at the surface of the well-
bore. 33 In the UK, the Environmental Agency in England, the Scottish
Environmental Protection Agency or the Environmental Agencies of Wales
 
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