Environmental Engineering Reference
In-Depth Information
result, treatment with solutions containing 1 to 3% salt is generally used as a
base liquid when clay swelling is probable. Potassium chloride is the most
common chemical used as a clay stabiliser due to its ability to stabilise clay
against the invasion of water to prevent swelling.
In some operations, a 'breaker' is also added to the fluid in later stages of
the process to reduce the viscosity of the gelling agent to help release the
proppant from the fluid once the fractures have been created and to increase
the volume of flowback water returned to the surface. Chemicals used as
breakers may include magnesium peroxide, peroxydisulfate, sodium per-
borate and glycol. Breakers are normally mixed into the fracturing fluid
during the pumping operation to give enough time to transport the proppant
to the fracture zone.
At the present time there has only been exploratory drilling (and hydraulic
fracturing) for shale gas in the UK, carried out by Cuadrilla at Preese Hall in
Lancashire. For this they received approval for a limited suite of chemical
additives which comprised: polyacrylamide (friction reducer), salt (fracturing
fluid tracer), hydrochloric acid and glutaraldehyde (biocide).
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At the con-
centrations used, these were classed as non-hazardous by the Environment
Agency. However, this was only for exploratory purposes and operational wells
will probably need a different suite of chemicals. These would also need to be
approved by the relevant regulator, e.g. the Environment Agency.
4.3 Flow Back and Produced (Formation) Wastewater
A considerable proportion of the fracturing fluid injected into the well re-
turns to the surface as 'flowback'. Flowback starts immediately and can
continue for anything from a few days to a few weeks following hydraulic
fracturing. The length of time depends on the geology and geomechanics of
the formation. The highest rate of flowback occurs on the first day, and the
rate diminishes over time; the initial rate may be as high as 1000 m
3
per
day.
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Depending on the geology and extent of fracturing, the volume of
produced water may range from between 30 and 70% of the injected frac-
ture-fluid volume.
A certain amount of fluid will continue to emerge from the well over its
entire lifetime. This on-going discharge is termed 'produced water' and its
composition increasingly reflects that of the geological formation water ra-
ther than the injected fluid. The rates of produced-water discharge are
generally low and the volumes can be relatively easily handled on the sur-
face. The principal problem is the potentially large volumes of flowback
water during the period immediately after hydraulic fracturing.
The composition of flowback water will be similar to the injected fluid(s),
modified by the fracturing process and exposure to formation water. It will
include the chemicals injected, their transformation and/or breakdown
products and formation water. As an example, concentration ranges for the
main components of produced water from Marcellus Shales in the United
States are shown Table 6. A much larger range of trace elements will also be
