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Geological Sequestration of Carbon Dioxide in Oil and Gas Reservoirs on the Texas Gulf Coast
61%). Many recent technical advances (3D seismic for delineating through Class II wells. Class II is one of the five classes of well that
fault blocks, directional drilling for placing wells, reservoir were originally classified following the Safe Drinking Water Act
characterisation for designing flood and improvement of CO
2
(SDWA) passed in 1974 by the US federal government. A stated goal
flooding knowledge) make CO
2
–EOR a lot more attractive than when of the SDWA is to protect fresh groundwater through the
many of these operations closed down in the past decades. A recent Underground Injection Control (UIC) programme, which was
academic study
2
selected those fields with appropriate pressure and implemented mostly in the 1980s.
temperature to occur in the miscibility region and then applied a fixed
fraction of the original oil in place (OOIP) (15%) to all fields selected, In Texas, this programme is administered jointly by the RRC and the
regardless of pipeline and other needed infrastructure. A total of 360 Texas environmental agency (Texas Commission on Environmental
field candidates for EOR in the Texas Gulf Coast having >2Bbbl were Quality [TCEQ]). The US Environmental Protection Agency (EPA) has
identified. A more optimistic US DOE National Energy Technology been active recently and last year released draft regulations
Library (NETL) atlas
2
mentions ~10Bbbl of recoverable oil, although it pertaining to CO
2
injection in which a new class of injection well,
includes the East Texas Basin in its estimates. These large numbers Class VI, would be created. If it is clear that CO
2
–EOR wells are
suggest that the industry will eventually tackle CO
2
tertiary recovery permitted under UIC Class II (and many states have produced
in the Texas Gulf Coast because: legislation to make sure that CO
2
–EOR is not affected by the draft
regulations) and that anthropogenic CO
2
injected into saline aquifers
• many Gulf Coast reservoirs can be CO
2
-flooded; is permitted as UIC Class VI, the issue of transitioning to CO
2
storage
• local sources of anthropogenic CO
2
are becoming available or for EOR production has yet to be resolved and has generated
will be in the near future; vigorous discussion. A major difference between EOR (Class II) and
•CO
2
generators would benefit from having their waste become a storage (Class VI) is the elevated pressure in the storage formation in
useful commodity and would therefore support such enterprise; the latter case.
• long-term trends for oil prices are up, inevitably generating
interest in more complex reservoir operations; There has been a recent flurry of activity in state legislative bodies as
• multiple generators and multiple users would smooth out any well. Texas Bill HB3732
3
creates incentives by reducing taxes on
individual fluctuations in offer and demand (assuming a common those projects using captured anthropogenic CO
2
for sequestration
pipeline network), with dedicated saline aquifer storage sites also of CO
2
if performed along with EOR operations. To claim the tax
acting as buffers; and break, an operator must establish to the RRC that CO
2
will be
• the regulatory environment at the federal level (credits, etc.) reasonably sequestered for hundreds of years and that monitoring
might empower early entrants for leading the way. tools are in place to assess performance. The bill does not deal with
how carbon credits would be attributed (and in fact no credit may
Oil and gas reservoirs have demonstrated their ability to retain fluid be offered), but provides a tax incentive solely for use of
for geological time periods, although, admittedly, that was before anthropogenic CO
2
.
the Gulf Coast had been punctured by hundreds of thousands of
boreholes. In addition, the reservoirs may have undergone frac jobs After production has ceased, if an operator is willing to keep using
that have damaged seals. No-one knows the impact of these issues the depleted oil or gas field purely for carbon storage, many issues
on the permanence of the storage of CO
2
. After a period of grave become murkier and have not yet reached resolution. For example,
concern a few years ago, the scientific and engineering community the bill does not address the well-permitting issue, particularly the
now seems to agree that, except for some wells drilled before the transition from a CO
2
–EOR to a CO
2
storage project: would the Class
1930s – most of which are relatively shallow – leakage through wells II injection wells be grandfathered for additional CO
2
injection or
is likely to be limited. Other elements in favour of using oil and gas would the Class II wells be retrofitted to meet Class VI requirements?
reservoirs despite a higher well density are: Reducing regulatory and legal uncertainty is going to be essential to
gaining buy-in from a broad spectrum of stakeholders, including oil
• the option of avoiding pressure building above pre-development and gas operators.
conditions because most reservoirs are pressure-depleted after
the end of production; As pointed out by the State Geologist Dr Scott Tinker, Texas has
• a much better stratigraphic and structural characterisation of the higher than average CO
2
emissions, but could grow to become a
reservoir and its surroundings precisely because of the wells; leader in an emerging CO
2
management and subsurface storage
• some, if not all, of the well infrastructure needed for injecting industry. One option currently being considered is the creation of a
and monitoring the injection is already in place; and Texas repository storing CO
2
in offshore State submerged lands. Its
• the aquifer into which the reservoir is integrated will never be development could be woven into the growth of the CO
2
–EOR
considered as a source of drinking water. industry on the Gulf Coast, including offshore reservoirs. It could
become a local industry relying on a trained workforce. Such an
Despite all of these positive aspects, the regulatory field is still approach would sidestep ownership, liability and other legal issues
uncertain. CO
2
injection for the purpose of EOR is clearly made that may dampen stakeholder enthusiasm. ■
1. Carbon Sequestration Atlas of the United States and Canada, www.netl.doe.gov/technologies/carbon_seq/refshelf/atlas/ Environmental Geology, 2008;54:1695–1706.
2008, US Department of Energy Office of Fossil Fuels, 2. Nuñez-López V, Wood DJ, Ambrose WA, Hovorka SD, Quick- 3. Passed in 2007 during the 80th legislature. In Texas,
National Energy Technology Laboratory. Available at: look assessments to identify optimal CO2 EOR storage sites, legislative bodies meet every other year for a few months.
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 1
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