IN WESTERN AUSTRALIA - Department of Mines and Petroleum

Barrow and Dampier Sub-basin Aquifer
Depletion Studies
Australia is potentially incurring loss of oil as
petroleum production continues. Aquifer pressure
decline is occurring not only in Western Australia
but also in the Gippsland Basin of Victoria and the
portion of the Bonaparte Basin in the Territory of
Ashmore and Cartier (administered by the Northern
Territory Government).
The Barrow and Dampier Sub-basins lie in the
Carnarvon Basin off the northwest coast of WA. First
oil from the Barrow Sub-basin came on-stream in
1986 and production from the fields in this area has
been continuous since 1986. The value of
petroleum royalties from the Barrow Sub-basin was
$56 million in 2002/03.
First oil from the Dampier Sub-basin came onstream
from the Talisman oilfield in July 1989. The
value of petroleum royalties from the Dampier Subbasin
was $262 million in 2002/03. These subbasins
lie both in WA State waters and the
Commonwealth Adjacent Area.
Originally, it was believed that the reservoirs in the
Barrow and Dampier Sub-basins had infinite aquifer
pressure support with no regional draw-down effect.
Recent observations from newly discovered fields in
the Barrow and Dampier Sub-basins, however,
indicated that there has been a significant pressure
draw down in some parts of these sub-basins. Also,
petrophysical logs from some of these discoveries
indicated a few metres of residual oil below the oil
water contact.
As a result of pressure draw down and subsequent
gas cap expansion, oil accumulations in these
reservoirs may have been forced down into the
aquifer and, where this migration extended below
spill point, it is possible that some oil may have
been lost. This, in turn, means that millions of
dollars worth of royalties could have been lost. The
conclusions that can be drawn from the above
observations would have serious implications for the
conservation of as yet undiscovered and/or
undeveloped hydrocarbon resources in the region as
well as known and developed fields.
It is crucial for the regulatory bodies to fully
understand the implications of draw down and
subsequent possible loss of oil and its impact on
State and Commonwealth revenues. To assist in this
investigation, the Petroleum Division of DoIR
engaged the services of OPES International and
CSIRO between 2000 and 2003. The main objective
was to determine the extent at which hydrocarbons
are potentially being lost in the region and possible
future losses and the affect on royalty income to the
State and Commonwealth.
Based on the results of the Barrow Sub-basin study
by the year 2030, the total oil loss can be as high
as 83 GL. At current oil prices (approximately A$50
per barrel), a loss in royalty due to oil loss amounts
to $5 per barrel and therefore the WA community
could lose as much as $2.6 billion by 2030 if
corrective measures are not taken against aquifer
depletion in the Barrow Sub-basin. Similarly, based
on the results of the Dampier Sub-basin study by
the year 2030, the total oil loss can be as high as
42 GL. Once again, this could mean a potential loss
of up to $1.33 billion to the WA community.
These studies revived the interest in industry to
implement their investigations. Major WA operators
such as Woodside Energy Limited, Santos and
Apache Energy have already recognized aquifer
depletion and its possible impact. There is increased
interest within the WA petroleum industry to take
part in a joint Government/Industry task force to deal
with the issue in a constructive manner. WA
Department of Industry and Resources has already
taken a number of initiatives to share the results of
these studies in the Barrow and Dampier Sub-basins
with the petroleum industry and Commonwealth.
Feasibility Study of Gorgon CO 2 Sequestration
at Barrow Island
The Gorgon gasfield is situated 130 km off the
northwest coast of Western Australia in 200 metres
of water and was discovered in 1981. The Gorgon
field has certified proven hydrocarbon gas reserves
of 272.69 Gm 3 (9.63 Tcf). Carbon dioxide comprises
about 14 mole % of the raw gas resource. The
Gorgon Venture asked the WA Government to
consider whether it could be acceptable, in
principle, for a gas processing plant to be located
on Barrow Island.
According to Chevron Texaco, landing and
processing gas from Gorgon on Barrow Island is the
most economically viable option for the project. The
field development concept consists of sub-sea wells
arranged in several production centres over the
field, tied back to gas processing facilities on
Barrow Island via a 70 km pipeline. Later on, a gas
connection will be installed from Barrow Island to
the mainland connecting Gorgon gas to the existing
domestic pipeline.
DoIR and ChevronTexaco Australia agreed to
regularly review the technical work being
performed. To assist in the assessment, DoIR
engaged the services of Curtin University. The Phase
1 review was completed in June 2003 and provided
technical assurance on the feasibility of CO 2 storage
beneath Barrow Island. This provided one of the
criteria for the WA State Government’s decision to
grant in-principle access to Barrow Island for the
project. The in-principle approval for access to
Barrow Island was granted on September 8, 2003
after rigorous reviews and careful consideration.
The Phase 1 review provided a comparative risk
analysis, which compared most of the injection and
storage parameters of the Utsira Formation -
Sleipner West (a site where the injection and
PWA April Edition - Resources Branch Activities 23
storage process is considered a success by the
Norwegian Government) with the proposed injection
and storage parameters of the Dupuy Formation -
Barrow Island. The key findings of the DoIR review
concluded that injection of Gorgon reservoir CO 2
into the Dupuy Formation at Barrow Island was
technically feasible, and the acknowledged risks
were to be expected and manageable. However it
was also concluded that long term monitoring of
CO 2 migration needs to be addressed.
The Phase 1 general recommendations were related
to improving the subsurface definition of the earth
model, further assessment of seal and fault
integrity, injectivity, near-well bore reactions and
CO 2 surveillance and monitoring technologies. Key
DoIR recommendations included the need for
additional geological data and a long-term
monitoring strategy for reservoir management and
contingency planning.
More specifically the study recommended that at
least one pilot well must be drilled at injection site to
acquire the necessary core and geological
information, acquire further seismic surveys in
northern Barrow Island, perform further mapping of
the seal, application of CO 2 , simulators for future
Dupuy aquifer studies, identify the optimum long term
monitoring methodology and drill an inclined injection
well with 3 km reach into the offshore areas.
Development of this world-class resource is of
national importance and will benefit Australia with
employment opportunities and infrastructure
development. It is important to note, that Gorgon may
also provide the foundation development on which
the Greater Gorgon area fields can be advanced. DoIR