Environmental Engineering Reference
In-Depth Information
from the manufacture and transport of drilling equipment, construction
material, water, chemical additives and other physical necessities, and the
treatment of wastes, including wastewater.
Many of the emissions sources are the same for shale gas as for gas
supplied from conventional wells, for instance, compression losses in
transmission and distribution. Indeed, the largest component of the life-
cycle climate impact, the quantity of CO
2
released from combustion of the
gas, varies only slightly due to the mixture of alkanes present, which is, in
practice, regulated in the UK by National Transmission System gas specifi-
cations.
2
Additional GHG emission sources from shale gas lie in the energy
required for hydraulic fracturing; the manufacture of chemicals and trans-
portation of both water and chemicals to the well site; and their subsequent
disposal and direct releases during well completion. Emissions from flow-
back, the period following drilling when a mixture of gas, liquid and solids is
removed from the well to enable production, are the most substantial of
these but may be mitigated to a large extent by 'reduced emissions com-
pletions'.
3
Diffuse sources of emissions from sub-surface leakage, con-
sidered to be most likely the result of well casing failure, from both
conventional and shale wells, have also been identified but not quantified.
4
2.2 Quantitative Estimates of Life-cycle Climate Impacts
There are a number of estimates of the life-cycle climate impact of shale gas
available in the literature in addition to those of other energy sources. Before
presenting these figures it is worth considering some key issues in their
production. Comparison of life-cycle estimates for any product is problem-
atic due to differences in system boundaries; metrics chosen to relate dif-
ferent atmospheric effects of gases; assumed technology performance;
allocation between multiple products; treatment of uncertainties; and data
quality.
Choosing appropriate comparators and equivalent functional units is es-
sential, but is only a first step. In the case of fossil fuels, this is typically
either energy content (e.g. joules, J) or electrical energy subsequently gen-
erated (e.g. kilowatt hours of electricity, kWh(e)). For shale gas, this can
make a substantial difference in comparisons with coal due to the much
greater eciency of the conversion technology. A coal-fired electricity plant
has a thermal eciency ranging from 36% (pulverised fuel) to 47% (new
supercritical plant), whilst a gas-fired power station ranges from 40 to 60%.
5
Monte Carlo analysis,
6
informed by a number of prior studies,
7-12
suggests
that the range in possible eciency of gas power plants has a greater in-
fluence on a 'well-to-wires' life-cycle impact than the total uncertainty in
quantities of upstream emissions.
Whilst there have been efforts to harmonise life-cycle impacts for some
electricity generation technologies,
13
there is as yet no systematic equivalent
for shale gas. As a result, the data are presented in Table 2 as relative
statements and ought to be considered with some caution. Furthermore, not
