Environmental Engineering Reference
In-Depth Information
5.3 Drilling and Well Integrity
The risks associated with well design and drilling have to be carefully con-
sidered and take into account the purposes of the well, the geological for-
mations that will be drilled and the materials being used/produced during
and after drilling. Any structure that penetrates freshwater aquifers, such as
a well, has the potential to introduce a preferential pathway that could lead
to contamination if pollutants are allowed to leak.
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Control measures must
be put in place to mitigate any identified risks. These include installation of
multiple casing and cement bonded wells, blow-out preventers and en-
vironmental monitoring.
The key hazards associated with the drilling operation that could po-
tentially lead to groundwater/surface water contamination are: loss of
drilling fluids to the surrounding geological formation(s) (leak-off);
well blow-out as a result of gas or fluids under high pressure being en-
countered in the well bore; and spillages of wastes and chemicals on the
surface.
During drilling of the shallow geological formations, drilling fluids
which aim to minimise the risk of groundwater contamination are gener-
ally used. Examples include the use air and/or water. The well casing and
cement that holds it in place provide the seal(s) that is (are) of vital im-
portance both during the drilling phase and then for maintaining the in-
tegrity of the well during its lifetime. Failure of the cement or casing
surrounding the wellbore poses a significant risk to groundwater. If the
annulus is improperly sealed or the seal fails, natural gas, fracturing fluids
and formation water(s) containing high concentrations of pollutants may
be communicated directly along the outside of the central wellbore be-
tween the target formation, drinking water aquifers and layers of rock/
groundwater in between.
Studies that have looked at well integrity failure reveal considerable vari-
ation in failure rate. Work by Schlumberger estimated that by the time a well
is 15-years old there could be a 50% chance of failure.
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CIWEM reported
that 6 to 7% of new wells in Pennsylvania have compromised structural
integrity,
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and a more recent review showed that rates of wells with integrity
issues ranged from 2.9 to 75%, with the lower rates reflecting wells drilled
since 2010 in Pennsylvania.
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Another potential cause of well integrity failure is as a result of ground
movement, which includes damage induced by seismic activity triggered by
hydraulic fracturing. Casing deformation is relatively common in deep
hydrocarbon wells due to geological processes and differences in the
properties of adjacent geological formations. This may result in horizontal
shearing and subsequent deformation (buckling) of the well casing. At
Preese Hall in Lancashire the seismic events which were triggered by the
hydraulic fracturing of the exploratory well are also believed to have led to
deformation of the well casing as a result of seismically induced rock
shear.
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