Environmental Engineering Reference
In-Depth Information
Authorities (generally the local authority) deal with the planning appli-
cations required for each site.
After the surface casing cement is set (and intermediate casings, if re-
quired) the well is drilled to the target shale gas formation and a ''pro-
duction casing'' is installed, either at the top of the target formation or into
it, depending on the type of well being installed (either ''open hole'' or
''through-perforated casing'', respectively). Well completions incorporate
the steps taken to transform a drilled well into a producing one. These steps
include casing, cementing, perforating and installing a production tree.
Open hole completions are the most basic type and are used in formations
that are unlikely to cave in. An open hole completion consists of simply
running the casing directly down into the formation, leaving the end of the
piping open without any other protective filters. Very often, this type of
completion is used on formations that have been fractured.
Conventional through-perforated completions consist of production cas-
ing being run through the formation. The sides of this casing are perforated,
with small holes along the sides facing the formation, which allows for the
flow of hydrocarbons into the well hole but still provides a suitable amount
of support and protection for the well hole. The process of perforating the
casing involves the use of specialised equipment designed to make small
holes through the casing, cementing, and any other barrier between the
formation and the open well. In the past, ''bullet perforators'' were used,
which were essentially small guns lowered into the well. The guns, when
fired from the surface, sent off small bullets that penetrated the casing and
cement. Today, ''jet perforating'' is preferred. This consists of small, elec-
trically ignited charges that are lowered into the well. When ignited, these
charges blast small holes through to the formation, in the same manner as
bullet perforating.
In addition to the depth of the surface casing, the regulatory authorities in
the UK are likely to put requirements on the cementing-in of the surface
casing. A method known as ''circulation'' may be used to fill the entire space
between the casing and the wellbore (the annulus or outer space between the
well casing and the rock through which it has been drilled) from the bottom
of the surface casing to the ground surface. Here cement is pumped down
the inside of the casing forcing it up from the bottom of the casing into the
space between the outside of the casing and the wellbore. Once a sucient
volume of cement to fill the annulus has been pumped into the casing, it is
usually followed by pumping a volume of water into the casing to push the
cement back up the annular space until the cement begins to appear at the
surface. Once the cement appears at the surface the pumping of water is
stopped, this ensures that the top section of the annular space is fully filled
with cement and therefore there is no leakage path between the outside of
the well casing and the rock through which it has been drilled. This method
is regarded as the highest standard of cementation compared to other
methods such as cementing of the annular space across only the deepest
groundwater zone but not all groundwater zones.
