Environmental Engineering Reference
In-Depth Information
Figure 1 Unconventional hydrocarbon systems.
All
Adsorbed
Gas
Mixed Free And Adsorbed Gas
All Free Gas
CBM
Gas Shales
Tight Gas Sands
0.01
0.1
Modified from Core Laboratories
1
10
100
% TOC
Figure 2 Definition of tight gas, shale gas and coal bed methane based on organic
matter content (modified from Core Laboratories).
environmental footprint. Both shale gas and CBM are characterised by very
low matrix permeability and natural fractures, the former generally
comprising organic-rich shales containing both free and adsorbed gas,
1
while CBM resources are limited to coal seams and only contain adsorbed
gas (see Figure 2). Both shale resources and CBM require the creation of
an artificial reservoir using hydraulic fracturing, colloquially known as
''frac'ing'' or ''fracking'', of horizontal well bores drilled within the near-
impermeable shale or coal. As this chapter relates only to shales, CBM will
not be discussed further.
Shale-based resources have a long history of exploitation.
2
The first re-
corded gas production was at Fredonia, Pennsylvania, USA, in 1821 where
Devonian-aged shale provided gas for lighting from a simple well.
3
Similarly,
in the late 19
th
century, shale-sourced gas at Heathfield, Sussex, lit the
railway station.
4
Gas-charged shales were later recognised by early oil-well
drillers in the USA as a potential hazard, often with fatal explosions and
fires. These shales, generally of Palaeozoic age, were often encountered
during drilling for conventional oils and gas reservoirs across the USA, from
