Environmental Engineering Reference
In-Depth Information
attributed this to shale gas extraction. In active gas-extraction areas, average
and maximum methane concentrations in drinking-water wells (19.2 and
64 mg CH
4
per litre, respectively) increased with proximity to the nearest gas
well. At these concentrations there is a potential explosion risk if gas is
allowed to accumulate in a confined space. In contrast, samples from
groundwater in non-extraction areas (no gas wells within 1 km) within
similar geologic formations and hydrogeological settings concentrations
averaged only 1.1 mg l
1
. These results and conclusions were disputed by a
number of other scientists on the basis that no baseline was available and it
was known that groundwater in the area contained naturally elevated con-
centrations of methane. The study was extended by Jackson et al.,
5
with a
larger number of samples and also with additional isotopic and geochemical
analysis. This further work concluded that a sub-set of drinking water wells
were contaminated as a result of drilling operations and that this was likely
to be due to poor well completion (integrity).
Another well-publicised study at Pavillion, Wyoming,
44,45
investigated
contamination of shallow groundwater supplies in an area with a con-
siderable number of oil and gas wells (169) that had been hydraulically
fractured. The study, which was led by the US Environment Protection
Agency (USEPA), collected samples from both deep monitoring wells and
shallower domestic wells. The study found that concentrations of dissolved
methane in domestic wells generally increased the closer they were to gas
production wells. An analysis of data on hydrocarbon well completion re-
vealed a catalogue of problems with production wells having poor cement
bonding or even no bonding present at all or over considerable lengths of
the wells. Whilst these would almost certainly contribute to migration of
contaminants to the shallow aquifers, other causes for concern and alter-
native pathways were identified. It was found that hydraulic fracturing and
hydrocarbon exploitation took place at relatively shallow depths, from 372 m
below ground level, whilst some drinking water abstraction wells in the
overlying aquifer were as deep as 244 m. This means that there is only
limited vertical separation between the two. A further problem identified was
leakage and infiltration to shallow aquifers from storage pits on the surface
used for storage/disposal of drilling wastes, produced-water and flowback
fluids. The study is continuing but the USEPA have now, somewhat con-
troversially, handed over responsibility to the State of Wyoming.
It is highly unlikely that such a situation would arise in the UK because of
the strict controls that would be applied. However, the experiences from the
United States serve as a clear indication of why, and what can happen if,
things go wrong.
8 Conclusions
The UK may possess considerable reserves of shale gas with shale underlying
a significant proportion of the UK, but as yet there has been very little
exploratory drilling to confirm the resource potential.
