Environmental Engineering Reference
In-Depth Information
Horizontal drilling is performed with similar equipment and technologies
to that which has been established over decades for vertical drilling and, in
fact, the initial drilling of the vertical bore is almost identical to a con-
ventional well. However, the well development and gas extraction processes
differ widely between conventional and unconventional gas production.
Whilst some conventional wells have been stimulated by hydraulic fractur-
ing in the past, horizontal drilling and hydraulic fracturing are key re-
quirements to make the exploitation of shale gas deposits economically
viable.
The requirement for horizontal drilling and hydraulic fracturing also re-
sults in differences in the distribution of wells above the shale gas for-
mations. The process involves locating several individual wells on a single
''multi-well'' pad. Normally 6-10 horizontal wells radiate out from the centre
well pad; these then are drilled in parallel rows, typically 5-8 m apart. Each
horizontal wellbore is typically 1-1.5 km in lateral length, although they can
be considerably longer.
As the array of wells drilled from each pad only enables access to a discrete
area of the shale formation, several multi-well pads in a geographic area are
required in order to maximise shale gas extraction. In the US they typically
locate a maximum of nine pads per square mile. In the UK, Composite
Energy has estimated that about three pads per square mile should be suf-
ficient for the UK setting.
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However, the geological and above-ground
constraints will also impact on the location of well pads.
The differences in the production process between conventional and un-
conventional gas production also results in differences in the level of effort
required to extract shale gas. It also affects the amount of resources used and
the corresponding volume of waste products generated.
2.1.2 Well Pad Construction. As already stated previously, the pads used
for multi-well drilling require an area of land su
cient to accommodate
fluid storage and equipment associated with the hydraulic fracturing
operation. This utilises larger equipment for horizontal drilling than that
required for vertical drilling only. This results in between 0.4 and 1.2 ha
(1-3 acres) of land being required for a multi-well pad.
2.1.3 Drilling. Vertical drilling depth will vary dramatically, depending
on the depth of the shale gas strata and their location. However, it is ex-
pected that wells will be drilled through rock layers and aquifers for a dis-
tance of up to 2 km, to within 150 m of the top of the shale gas rock to be
hydraulically fractured. A more powerful horizontal drill may then be used
for the horizontal portion of the wellbore. This transition is known as
''kicking off'' and the horizontal well is then continued for an additional
1-1.5 km.
The vertical portion of the well is typically drilled using either compressed
air or freshwater mud as a drilling fluid. Once the horizontal section is ready
