Environmental Engineering Reference
In-Depth Information
less than 20-30% of the injected water, particularly in the case of high
thermal maturity shales such as the Marcellus.
5 Geological/Environmental Considerations
5.1 Hydraulic Fracturing (Completion)
Despite the large-scale use of hydraulic fracturing across many areas of the
USA and parts of Canada since the early 1990s, there is little hard evidence
for the major environmental consequences voiced by many detractors. With
the many thousands of wells now completed by hydraulic fracturing, it is
likely that incidences of frac-related seismicity and groundwater contamin-
ation may have occurred but these appear to be related to individual well or
procedural processes rather than to the completion process in general.
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In
many basins around the world, hydrocarbon leakage to the surface and
groundwater contamination is an entirely natural occurrence, with gas being
derived from high-maturity coals and shallow reservoirs,
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and is how early
explorers located hydrocarbon-bearing basins in the 19
th
century. Many
thousands of wells have been drilled since then, the earliest with only ru-
dimentary casing, if any, and with little or no sealing-off after abandonment.
This, along with any uncased faults or fracture systems, may have allowed
sub-surface leakage of hydrocarbons from reservoir intervals up into shal-
lower levels, with potential for groundwater contamination. This situation
has been known for many years in the old production areas of the USA, such
as the Appalachians, but has only come to the forefront with the re-activation
of the petroleum industry in this areas with the application of hydraulic
fracturing for shale gas extraction,
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often being cited by detractors of the
process as evidence of environmental damage, mostly based on, at best, only
circumstantial evidence. While contamination may have occurred, the as-
sociation with hydraulic fracturing is unproven,
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and studies are being
undertaken to attempt an understanding of this highly contentious
issue,
60,61
but the lack of base-line data prior to the start of shale-gas drilling
and completion in many basins renders this dicult. Clearly, contamination
of groundwater during any stage of shale exploitation, whether by drilling,
completion, production or abandonment, is unacceptable and much effort is
made to minimise and mitigate this risk.
5.2 Groundwater Issues
A simple cross-section of the north-east Appalachian Basin (see Figure 16)
illustrates some of the groundwater issues. Most groundwater wells tap
shallow gravels and aquifers at depths of a few hundred feet. Beneath this,
between depths of around 500 and 2000 feet, lies an interval of generally
isolated sands from which early hydrocarbon production took place and now
is a potential hazard to drilling. The Marcellus Shale occurs much deeper in
the basin at depths of 3500-7000 feet, and the overpressured nature of the
