Environmental Engineering Reference
In-Depth Information
to be drilled, then this normally requires drilling mud for powering and
cooling the down-hole motor that is used for the directional drilling. The
drilling mud also provides stability for the horizontal drilling and for using
the navigational tools, which require mud to transmit the sensor readings
that enable the bore to be accurately traced. The mud also enables the re-
moval of the cut material from the drilling operation. The drilling mud is a
heavier mud than the freshwater mud used in the vertical drilling due to the
need to prevent hole collapse of the horizontal wellbore where the earth's
vertical stress is much greater than in the vertical bore.
Developments are being undertaken to drill the horizontal bore pneu-
matically, using specialist equipment to control fluids and gases entering
the wellbore. It will be interesting to see which method will come to dom-
inate the horizontal drilling operation, although it could be the geology or
water availability that determines the approach adopted.
In terms of waste material generated from the process, if the vertical well
was 2 km deep with a 1.2 km lateral well the volume of waste would be in the
region of 140 cubic metres, whereas a conventional well drilled to the same
depth of 2 km would generate about 85 m
3
of waste material. Therefore a
10-well pad would generate 1400 m
3
of waste.
2.1.4 Well Casings. Well casings
17
are installed to seal the well from the
surrounding formation and to stabilise the completed well. A number of
these may be installed to meet a variety of circumstances and are typically
concentric steel pipes lining the inside of the drilled hole, with the annu-
lar spaces filled with cement. There are four casing ''strings'', each in-
stalled at different stages in drilling. The first is the ''conductor casing''
this is installed during the first phase of drilling; it is a shallow steel con-
ductor casing installed vertically to reinforce and stabilise the ground sur-
face, the depth of the conductor casing is typically 40-300 ft. This is
followed by the ''surface casing''. After the installation of the conductor
casing, drilling continues to below the bottom of the freshwater aquifers
(depth requirements for groundwater protection are likely to be the sub-
ject of approval from the Environment Agency in the UK), at which point
a second casing of smaller diameter (the surface casing) is installed and
cemented in (see Figure 3).
Intermediate casings of still smaller diameter are sometimes installed
from the bottom of the surface casing to a deeper depth. This is usually only
required for specific reasons such as additional control of fluid flow and
pressure effects or for protection of other resources such as minable coals or
gas storage zones. It could, of course, form part of the requirements from the
regulatory authorities in the UK.
Regulation in the UK is controlled by a variety of national and local gov-
ernment departments with a variety of different responsibilities. In the UK
the environmental considerations are controlled by the following orga-
nisations: The Environmental Agency (EA)
in England,
the Scottish
