Wednesday, 24 August 2011
Coal Seam Gas Debate and Geothermal Energy
What is coal seam gas?
CSG in Australia
Water contamination issues?
Hydrofracturing Issues?
Effect on the Local Environment
Greenhouse Emissions
The recent debate on access to agricultural farmland by coal seam gas developers
has caused some people thinking about possible implications for future geothermal
energy applications. Since both access what is under the surface of the land
through drilled wells and possible hydrofracturing of their source reservoirs,
the argument has been raised that the issues raised by the coal seam gas protests
may be applicable to future geothermal power developments as well.
In the following sections, I will explain that, apart from drilling noise and
the local impact during the drilling activities, none of the concerns that have
been raised for CSG industry apply to the future Australian geothermal power
plants. Specifically,
- Geothermal power plants in Australia will be binary power plants with the
geothermal fluid circulated in a closed circuit without being exposed to the
environment. The geothermal fluid will be pumped up to the surface and it
will be reinjected into the reservoir after its heat is extracted as shown
in Figure 6. None of this water is released and there is no issue of contaminating
surface or underground aquifers.
- Geothermal reservoir stimulation occurs at depths of 4000 meters or deeper.
At such depths, there is no risk of the stimulation fluids leaking into shallow
underground aquifers that are providing water for agricultural and other usage.
- Geothermal energy is converted to electricity at the site. There are not
going to be pipelines that may have an effect on the local communities and
the local flora and the fauna.
- Australian geothermal power plants will have no emissions.
Only the temporary local impact of geothermal drilling, e.g. the drilling noise
and the disposal of drilling fluids during drilling, will be similar to CSG
drilling but such impact will be minimised because all geothermal production
and injection drilling is expected to be done from the same drill pad accessing
the entire reservoir through directional drilling underground. Therefore, the
surface footprint of geothermal drilling will be smaller than CSG and geothermal
drilling will be limited to the area where the geothermal power plant will be
built -- and therefore the access to such land will have to be negotiated with
the local owners regardless of what the law says about access rights to underground
resources. Nevertheless, geothermal power industry will be using drill rigs
and therefore they need to monitor the CSG drilling practice in terms of minimising
the local impact, even though it will be much less then the CSG drilling impact.
Let us now try to look into these issues in greater detail and hopefully convince
the reader that the above is true.
What is coal seam gas?
Coal Seam Gas (CSG) or Coal bed methane(CBM) refer to the same gas: methane.
In Australia we call it Coal Seam gas (CSG) and in other countries (e.g. USA,
Canada), it is referred to as Coal Bed methane (CBM). All coal seams have some
methane (CH4) in them. Copying from the
Geosciences Australia web site, its origin could either be biological or
thermal.
During the earliest stage of coalification (the process that turns plants into
coal) biogenic methane is generated as a by-product of microbial action (similar
to the mechanism which generates methane in council landfills). Biogenic methane
is generally found in near-surface low rank coals such as lignite.
Thermogenic methane is generally found in deeper higher-rank coals. When temperatures
exceed about 50°C due to burial, thermogenic processes begin to generate additional
methane, carbon dioxide, nitrogen and water. The maximum generation of methane
in bituminous coals occurs at around 150°C. The methane produced is adsorbed
onto micropore surfaces and, and as shown in Figure 1 (copied from the
web site of Australia Pacific LNG, an Origin - Conoco joint venture), is
stored in cleats, fractures and other openings in the coals. The gas is held
in place by water pressure and does not require a sealed trap as do conventional
gas accumulations. In other words, the coal bed is the reservoir for methane
and the water is the seal.
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| Figure
1. Coal Seam Gas(CSG)
is stored in the cleats and fractures in coal |
Figure
2. Production of Coal Seam Gas |
Since the gas is kept in the matrix by the pressure of water, the initial operation
that needs to take place is the extraction of that water. The removal of water
reduces the pressure and releases the gas from coal. The water (and then the
gas) move through the coal seam by the openings provided by the cleats as seen
in Figure 2. As in a geothermal reservoir, the permeability is of critical importance.
We will come back to this in a later section when talking about hydrofraccing.
We note here that water is co-produced with the gas and the amount of water
produced is site- and seam-dependent. Therefore, we need to examine this in
the context of the Australian CSG industry.
CSG in Australia
The GA
web site states that the CSG exploration in Australia started in 1976 in
Queensland's Bowen Basin when Houston Oil and Minerals of Australia Incorporated
drilled two unsuccessful wells. In February 1996 the first commercial CMM(Coal
Mine Methane) operation commenced at the Moura mine in Queensland methane drainage
project (then owned by BHP Mitsui Coal Pty Ltd). In the same year at the Appin
and Tower underground mines (then owned by BHP Pty LTD) a CMM operation was
used to fuel on-site generator sets (gas fired power stations). The first stand
alone commercial production of CSG in Australia commenced in December 1996 at
the Dawson Valley project (then owned by Conoco), adjoining the Moura coal mine.
According to Baker and Slater, CSG was supplied to the eastern Australian market
at a rate of 419 TJ per day (153 PJ per year) in 2008. Queensland produced 96%
of this total. This is a small fraction (about 5% of the Australian conventional
natural gas production) of the total Australian primary energy production in
2006-07 as shown in Figure 3 (which incidentally gives 87 PJ as the CSG production
instead of 153 PJ of Baker and Slater but it is for 2006-07).
Figure 3. Australian primary energy
supply and utilisation (http://www.australianminesatlas.gov.au/mapping/files/australian_energy_flows_2006-07.pdf)
The Australian proven and probable CSG reserves in 2009 were reported as 21180
petajoules (Underschultz et al, AAPG Conference, Calgary, 2010). I use the term
"reserves in 2009" because new exploration is adding to the reserves
every year. Therefore, it is reasonable to expect that, although it is only
a small fraction of the total Australian energy supply today, the share of CSG
will grow exponentially in future years. The expectations of such growth probably
have been the driver for some of the anxiety expressed in recent months. A summary
of some of these concerns can be found on the Queensland
Conservation web site. The main environmental issues associated with the
CSG industry according to the postings on the Queensland
Conservation web site are
- water - contamination of adjacent aquifers and removal of too much water
from underground storages
- local environment - the impact on farming and local environment from wells
and pipelines
- greenhouse gas emissions - during the extraction and transportation
Let us give a closer examination to these issues.
Water Contamination Issues
We have already seen above that a CSG well produces both water and gas. More
water than gas is produced in the initial stages of production as seen in Figure
4.
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 |
|
Figure 4. Typical gas and water
production profile for a CSG well.
From Helmuth(2008). CH4 P/L Arrow Energy is the original source.
|
Figure
5. Gas production scenarios for period 2008 – 2020 (A); and corresponding
estimates of CSG water produced (B) based on possible production figures
of 10, 28 and 40.8 Mt/y and production for domestic consumption only |
According to Helmuth(2008), the amount of water to be produced from CSG wells
in Queensland could be between 100 and 450 gigalitres per year. The reason for
the wide range is the uncertainty for the CSG sector growth and the gas and
water relationships across the target Basins. Figure 5 plots the CSG production
scenarios up to 2020 based on three possible gas production figures of 10, 20,
and 40.8 MT per annum. The yellow line in Figure 5 represents a scenario in
which CSG is produced only to meet domestic consumption. The vertical bars indicate
the range in sensitivity of water production estimates for 2020. The water coming
out of the CSG well is not potable water and the concern raised by the CSG opponents
is on an environmentally safe of disposing such water.
IMPORTANT : The issue of water contamination is irrelevant
to geothermal energy. Geothermal power plants in Australia will be binary power
plants with the geothermal fluid circulated in a closed circuit without being
exposed to the environment. The geothermal fluid will be pumped up to the surface
and it will be reinjected into the reservoir after its heat is extracted as
shown in Figure 6. None of this water is released and there is no issue of contaminating
surface or shallow underground aquifers.
 |
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| Figure 6. In a binary-cycle
geothermal power plant, the reservoir fluid is circulated in a closed piping
circuit and returned to the reservoir after its heat is transferred to the
power plant working fluid (http://www1.eere.energy.gov/geothermal/powerplants.html) |
Figure 7. A typical
geothermal well design (Paralana 1 by Petratherm). The well is cased using
a steel liner through the entire depth of the well. It is uncased (or partially
cased) in the reservoir 4 kilometers below the surface so that it can collect
hot water from the reservoir. |
Geothermal wells are cased with steel liners that are cemented into the well
as shown in Figure 7. The casing prevents the geothermal fluid getting in contact
with the aquifers that may be present at shallower depths. In the unlikely event
of a casing failure, the situation would be noticed by the operating company
and the well can be plugged without any significant leakage. Since production
pumping will stop immediately, the leakage from the casing breach will be minimal
during the time it takes to plug or repair the well.
CSG Hydrofracturing
In addition to the concern about the native coal seam water coming to the surface,
concenrs also have been raised against the CSG industry about the environmental
effects of the fluids used furing the hydrofracturing operation. This is needed
in some CSG sites to increase the gas production. The higher the permeability,
the greater the flow from a CSG reservoir. If the permeability is too low, the
gas does not come out. What is too low? Most commercial CBM plays in USA have
permeabilities in the range 3 to 30 mD (1 MD or millidarcy is roughly equal
to 1 μm2). But there are many seams at lower permeabilities. Different
reservoir engineering techniques are applied depending on the permeability of
the seam as summarised in Figure 3 (Palmer, 2010).
 |
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| Figure 8 - Completion methods
based on permeability (SIS=Surface to In-Seam horizontal well). Palmer(2010) |
Figure 9. Depths and lengths
of horizontal wells drilled for CSG harvesting in Australia (Palmer, 2010) |
There are many Australian CSG reservoirs with permeabilities in the range 20
- 100 Md. The use of horizontal wells is therefore reported to be standard practice
in Australia and, from these horizontal wells, the coal seam is hydraulically
fractured to increase the flow rate. This is done by pumping large volumes of
water at high pressure down the well into the coal seam which causes it to fracture
for distances of up to 400m from the well. Sand particles (called proppants)
are included in the water and they move into the fractures created and keep
them open once the pressure is removed. Palmer(2010) reports that Australian
CSG practice commonly uses proppants of 16/30 size (where the numbers refer
to the mesh sizes). As reported in the same paper, gels and similar fluids can
also be used to initiate the fractures.
As seen in Figure 9, the hydrofracturing of coal seams may be used at depths
200-700 meters. Concerns have been raised about the possibility of the hydrofracturing
fluids bursting our of the coal seam reservoir and invading nearby aquifers
providing water for agricultural usage.
IMPORTANT: There is no possibility of geothermal reservoir stimulation fluids
contaminating aquifers providing water for agricultural or other usage. A typical
hot fractured rock (EGS) reservoir in Australia is below 4 kilometers. Hot Sedimentary
Aquifers are shallower but even they are at least 3-km deep or deeper. There
is no risk to shallower aquifers.
Effect on the Local Environment
Concerns have been raised about the effect of drill rigs and the gas transporting
pipelines on the local flora and fauna.
The temporary local impact of geothermal drilling, e.g. the drilling noise
and the disposal of drilling fluids during drilling, will be similar to CSG
drilling but such impact will be minimised because all geothermal production
and injection drilling is expected to be done from the same drill pad accessing
the entire reservoir through directional drilling underground. Therefore, the
surface footprint of geothermal drilling will be smaller than CSG and geothermal
drilling will be limited to the area where the geothermal power plant will be
built -- and therefore the access to such land will have to be negotiated with
the local owners regardless of what the law says about access rights to underground
resources. Nevertheless, geothermal power industry will be using drill rigs
and therefore they need to monitor the CSG drilling practice in terms of minimising
the local impact, even though it will be much less then the CSG drilling impact.
The issue of pipelines does not apply to geothermal power because the energy
is transmitted as electricity.
Greenhouse Emissions
Since methane is a potent greenhouse gas, the issue of gas leakage during CSG
production and gas transportation has been raised as a Greenhouse Gas impact
that somewhat mitigates the relatively low (compared to coal) greenhouse impact
of burning gas to produce electricity.
In contrast, there are no greenhouse emissions from a geothermal binary
power plant. In such plants, the fluid is restricted to within a closed
circuit as shown in Figure 6. There are a few places in the world, e.g. some
Turkish and some Tuscan fields, where the geothermal fluid contains substantial
amounts of noncondensable gases like CO2, which may be cheaper to
vent. These usually apply to geothermal reservoirs heated by a volcanic heat
source. This is not applicable in Australia where the heat source is radiogenic
granites and therefore future Australian geothermal power plants will have no
emissions, greenhouse gas or otherwise.
Conclusion
Except for drilling noise and the local impact during the drilling activities,
none of the concerns that have been raised for CSG industry apply to the future
Australian geothermal power plants. Specifically,
- Geothermal power plants in Australia will be binary power plants with the
geothermal fluid circulated in a closed circuit without being exposed to the
environment. The geothermal fluid will be pumped up to the surface and it
will be reinjected into the reservoir after its heat is extracted as shown
in Figure 6. None of this water is released and there is no issue of contaminating
surface or shallow underground aquifers.
- Geothermal reservoir stimulation occurs at depths of 4000 meters or deeper.
At such depths, there is no risk of the stimulation fluids leaking into shallow
underground aquifers that are providing water for agricultural and other usage.
- Geothermal energy is converted to electricity at the site. There are not
going to be pipelines that may have an effect on the local communities and
the local flora and the fauna.
- Australian geothermal power plants will have no emissions.
REFERENCES
Baker, G, and Slater, S. The increasing significance of coal seam gas in eastern
Australia. PESA Eastern Australasian Basins Symposium III, Sydney, 14-17 September
2008.
Helmuth, M. Scoping Study: Groundwater Impacts of Coal Seam Gas Development
– Assessment and Monitoring, report prepared by the Centre for Water in the
Minerals Industry, The University of Queensland, for Queensland Government Department
of Infrastructure and Planning (December 2008)
Palmer, I, Coalbed methane completions: A world view. International Journal
of Coal Geology, 82:184-195 (2010).
Underschultz, J, Connell, L, Jeffery, R, and Sherwood, N. Coal Seam Gas in
Australia: Resource Potential and Production Issues. Search and Discovery Article
#80129 (2011) - Adapted from oral presentation at AAPG International Conference
and Exhibition, Calgary, Alberta, Canada, September 12-15, 2010.
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