Environmental Engineering Reference
In-Depth Information
that the tremors could affect well integrity,'' Frohlich says. ''In my business,
you never say never. That said, most of the time these earthquakes are not
right near the well. But it's possible an earthquake could hurt a well,''
though he knows of no instances where that has occurred.
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In order to minimise the risk of future seismic events the companies now
review local geology for potential fault lines prior to drilling. In addition,
they monitor the process with very sensitive instruments so that the oper-
ation can be halted if there are indications that an earthquake is likely to be
triggered.
2.2.2.5 Disposal of Waste Fluids
Once the hydraulic fracturing is completed, fluid returns to the surface in a
process known as ''flowback''. The US Environmental Protection Agency
(EPA) estimates that fluids recovered range from 15-80% of the volume in-
jected, depending on the site conditions. Therefore, each well will generate
1.5-16 million litres of flowback fluids which contain water, sand, methane,
fracturing chemicals and contaminants released from the rock being frac-
tured. These contaminants could include heavy metals, organic compounds
and naturally occurring radioactive materials. Approximately 60% of the
flowback fluids occur within the first four days, with the remaining 40%
occurring in the next ten days.
The fluids that are not recovered remain underground and concerns have
been expressed that these could become a source of contamination to
underground aquifers in the future. Concerns have also been expressed as to
the environmental risk as a result of either waste fluids disposal or a leak
from the waste fluids storage facilities. The waste fluid from hydraulic
fracturing can be managed in a variety of ways, including re-use for further
hydraulic fracturing, but this is more practical in multi-stage hydraulic
fracturing. However, the re-use may concentrate the contaminants in the
fluid, making it harder to dispose of or to remove in water treatment plants.
Waste fluid can be disposed of through injection into deep underground
wells, it can be treated at local water treatment facilities to make it accept-
able for returning to the environment provided it meets the water treatment
standards, or it can be stored in tanks or deep lined pits. The size of these
pits could be substantial: to accommodate up to 160 million litres of fluid
from a multi-well pad would need a pit of 160 000 cubic metres per pad.
Underground injection is the primary disposal method for most shale gas
projects worldwide, but whether this will be acceptable to the United King-
dom's relevant Environmental Agencies has yet to be determined. Where
injection is not an option, new wastewater treatment facilities are being built
in some parts of the world. The funding for building these plants varies, with
some built using local taxpayers' money, with the gas companies paying for
the volume of wastewater treated; in other cases the gas companies are
paying for the construction of the plants while some are built under joint
ventures with both the taxpayers and the gas companies funding the
