Summary
Two main types of reservoirs are considered for geological storage of carbon
dioxide (CO2): deep saline formations and depleted oil and gas
(O&G) reservoirs. The former offers very large potential capacity and a
more-even distribution at the expense of high uncertainty because of the very
poor characterization of their properties to date, including their sealing
capacity. The latter offers smaller overall capacity, but with a reduced risk
because of better reservoir knowledge. Gas reservoirs have also provided a
proven seal to pressurized gas.
However, reusing depleted O&G reservoirs presents challenges that must
be considered in the evaluation of performance factors and the associated
risks.
Depletion can cause pore collapse in the reservoir, with an associated loss
of capacity and injectivity and can weaken caprock and bounding faults or even
well completions, leading to possible containment losses because of mechanical
failure. O&G reservoirs are also intersected by many wells, and it is
likely that stricter regulatory requirements on well integrity and the quality
of zonal isolation will force operators to recomplete or work over wells that
will be exposed to CO2, with an obvious impact on cost.
Low reservoir pressure may also mean that injection of CO2 in a
dense phase would result in reservoir fracturing and very strong thermal
effects that may lead to injectivity problems. In the reservoir, chemical and
physical differences in behavior between CO2 and methane may
adversely affect geological containment and injectivity.
Analyzing the benefits and challenges with respect to all performance
factors (capacity, injectivity, containment) shows that depleted O&G
reservoirs and deep saline reservoirs both offer potentially attractive targets
for geological storage of CO2, mostly for complementary reasons.
Uncertainty on capacity and injectivity is clearly lower for depleted
reservoirs, giving them a potential net economic advantage, whereas uncertainty
on well containment favors saline formations, which are intersected by fewer
wells. Injectivity in depleted reservoirs may be much more difficult to ensure
than for saline formations or O&G reservoirs where pressure has been
maintained. Saline formations have a lower, mostly unproven, safety margin
between injection and fracturing pressure, resulting in a potential advantage
for depleted reservoirs where repressurization will lead to a final pressure
lower than or equal to the original value. Each reservoir type has a different
risk profile, different advantages, and a rightful place in a portfolio of
injection sites.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
12 June 2009
- Meeting paper published:
9 September 2009
- Revised manuscript received:
10 October 2009
- Manuscript approved:
29 October 2009
- Published online:
13 September 2010
- Version of record:
13 September 2010
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