Friday, 15 April
HSA or EGS ?
The term Hot Sedimentary Aquifer or HSA has been coined together in recent
years in Australia. I am not sure who is the first user and so has the naming
rights. In spite of the increasingly common usage of the term by the industry
practitioners, a formal definition does not exist. For example, the Australian
Geothermal Lexicon, which is an industry initiative to standardise the way resources
and reserves are reported, uses the term HSA but does not offer a definition.
The name implies a geothermal resource that has natural permeability, as opposed
to an EGS reservoir that needs to be stimulated. I reported in an earlier blog
(15 March) that the proposed US Geothermal Technologies program 2012 budget
is allocating $6m to "Permeable Sedimentary Resources", which very
much sounds like our HSA.
In Australia, the argument has been that an HSA development will yield lower
temperatures compared to a deeper EGS reservoir but this will be compensated
by much higher flow rates.
The conventional wisdom on upper crust permeability is summarised by the Manning-Ingebritsen
Geothermal/Metamorphic curve:
Permeability [m2] = -14-3.2 log z[km]
which I plot in the following chart (below on the left):
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Measured EGS and estimated HSA permeabilities and the
general crust permeability curve
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The general crust permeability curve by Ingebritsen
and Manning (1999)
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According to this curve, the permeability of the upper crust is 10-16
to 10-15 m2 in the depth range 1 km to 5 km. On the same
curve, I also plotted the permeabilities measured at some past EGS experiments.
These are values measured during hydraulic tests and induced seismicity experiments
in those sites (cited in Townend and Zoback, 2011). There is no data on HSA
permeability. Barnett(AGEC 2010) offers some estimated values for the Koroit
reservoir based on the petrophysical relations between rock porosity and permeability.
These are plotted as squares in the above chart. They suggest permeabilities
much higher than what can be predicted by the Manning-Ingebritsen curve at those
depths.
The fact that the expected HSA permeabilities are two orders of magnitude higher
than the general crust permeability curve should not mean that the expectations
are unreal. The data used in generating the Manning-Ingebritsen curve has a
relatively large spread. I copy from one of their papers above (above on the
right) where the data in the first five kilometers of crust cover a wide range
from 10-13 to 10-17 m2. In fact, compared against
this spread, the three EGS data points shown in the left-hand figure are astonishingly
close to the Ingerbritsen-Manning curve. In any case, the deviation from their
best-fit curve is no reason to suspect the Barnett(2010) predictions or high
HSA permeabilities expected by other players in this area.
So, the answer to the question in the title has to be both.
However, and this is the main point of this blog entry, it does not look like
there will be many HSA reservoirs with such high permeabilities and for companies
chasing after HSA resources it will be important to locate such resources before
they start drilling. This is in contrast to the EGS reservoirs which suggest
a reasonably close fit to the curve as shown above and have higher predictability.
If the geothermal electricity is to make significant penetration into the electricity
generation, it will have to be through EGS. This is why it is important to solve
the three challenges facing the EGS today as identified in the Hedberg Geothermal
Conference last month:
- Drilling costs must be reduced by up to 20%
- Production flow rates must be doubled to 60-80 kg/s
- The power conversion efficiency (kWe per kg/s of brine) must increase by
20%
Click here for the rest of the blog