Energy return on energy invested (‘EROEI’, or ‘EROI’) is a significant issue, but one not receiving attention in nearly all global oil models (and not in the mainstream ‘all-energy’ models, for that matter). The data are probably still not adequate to give a comprehensive picture, but none the less are sobering, as shown in Table.
The recently rapidly falling EROI ratio for conventional oil, if
confirmed by other studies, is of concern as independently Hall and co-workers
have suggested that modern society needs a minimum EROI ratio of perhaps
10–15:1 from its fuel sources to function in its current form (e.g. Hall 2008;
Lambert et al. 2014). And even where EROI ratios are higher than this, being
lower than in the past reduces society’s overall wealth if not compensated for
by productivity or efficiency gains elsewhere.
Recently, Campbell (2015) has incorporated EROI ratios into his
global oil forecast, multiplying his forecast production levels for the various
categories of oil
(‘Regular conventional’, Deepwater, tar sands, etc.) by the
corresponding ‘net yield’ ratio to turn gross barrels of production by category
of oil into a corresponding forecast of ‘net-energy’ barrels. Because the
non-conventional oils have—in general— lower EROI ratios than conventional oil,
the overall decline in global production is steeper in net-barrels terms than
in gross barrels.
And in terms of investment required to produce future oil, as
mentioned earlier note that the need for increasing quantities of a resource to
extract that resource was one of the drivers of system collapse in the original
Limits to Growth modelling (the other driver being rising costs from increasing
pollution).
Finally, an often-overlooked aspect of EROI data is net-energy rate
limits. These are limits to the maximum rate that an energy-producing
technology (in this case, a new fuel source) can be usefully introduced, and
reflect the fact that if the technology is introduced faster than the embodied
energy required for new plant, the overall net-energy yield during the growth
phase is negative. (For example, photovoltaics, with about 200 GWp installed,
have to date yielded no net-energy to mankind. This is partly due to having a
moderate EROI ratio, but mainly to their rapid uptake, and even where
technology is introduced at a slower pace, the net-energy yield can be
significantly less than the energy yield as usually calculated. Most current
global energy models do not take into account either EROI ratios or net-energy
rate limits.
A major cause of confusion in oil statistics is that there is no
standard definition of the boundary between the so-called Conventional and
Unconventional oil and gas. It is clearly important to make clear distinctions
because the different categories have different distributions, rates of
extraction, costs and other characteristics. In this study, it has been found
expedient to recognize what is termed Regular Conventional Oil (>17.5 o
API) and Gas, defined to exclude the following categories, which are
designated as non-Conventional:
1. Oil from
coal and organic-rich clays (kukersite), commonly termed oil shale
2. Oil
extracted by the artificial fracturing of low permeability reservoirs (Shale
Oil or Tight Oil)
3. Extra-Heavy Oil (<10 o API)
and bitumen
4. Heavy Oil (10–17.5 o API)
5. Deepwater Oil and Gas (>500 m
water depth)
6. Polar Oil and Gas
7. Liquids from gas plants
8. Gas from
coal (coalbed methane), tight reservoirs (shale gas), deep brines and
hydrates. (Note: °API is a measure of density, with water having a density of
10 API)
The resources of Non-Conventional Oil and Gas are large, but
extraction is normally difficult, costly, environmentally damaging and, above
all, slow. The entry of supply from these sources in the future is clearly
important, serving to ameliorate the post-peak decline, but it is doubted if
they will have much impact on the date or height of the overall peak itself.
They are described briefly below.
A conventional reservoir is naturally pressurized, or more
commonly, over-pressurized for the formation pressure of a hydrocarbon-free
sediment at the same depth and geological situation. Conventional gas deposits
will flow spontaneously to the surface when a reservoir is drilled, and the
risk of blowout venting is pervasive in all conventional hydrocarbons,
particularly those found at greater depths and higher pressures.
In contrast, an unconventional deposit is one that must be
stimulated in some way in order to cause the hydrocarbon to flow. In a
conventional gas deposit these processes (steam or hot water injection,
chemical solvent injection, gas injection, etc.) would be considered to be a
secondary recovery technique that are applied to recover hydrocarbons that
would otherwise have to be left in the reservoir.
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