Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

1.3. THESIS OVERVIEW
for CO 2 to be injected into depleted hydrocarbon reservoirs is already in place. By changing the injection
and/or production schemes in such situations, it will be possible to increase hydrocarbon output
while ensuring that the injected CO 2 is stored underground. This is currently being conducted at In
Salah in Algeria, and at the IEA GHG Weyburn-Midale CCS/EOR project, which will be discussed
at length in this thesis.
Current estimates suggest that there is significant storage space available in such reservoirs, and the
advantages of using such reservoirs are threefold: the economic benefits of increased oil production may
offset some of the storage costs; depleted oil reservoirs will have been well mapped, so potential storage
volumes will be known, and much of the infrastructure required will be present already. However,
there are concerns that abandoned production wells could provide a pathway for CO 2 escape, so
these must be sealed effectively, and that production activity may have damaged the caprock through
fracturing. Despite these disadvantages, it is likely that most storage operations will initially focus on
these targets, before moving on to saline aquifers when larger storage volumes are required.
1.2.4 Saline aquifers
Injected CO 2 is a buoyant fluid, so it will be trapped in porous reservoirs that are overlain by impermeable
layers. Such stratigraphic arrangements abound in most sedimentary basins. These rocks are
only occasionally filled with hydrocarbons - usually they remain filled with brine. These saline aquifers
represent by far the largest volumes of storage available. Torvanger et al. (2004) estimate that there is
storage potential for 8×10 11 tonnes of CO 2 in saline aquifers in the North Sea, representing hundreds
of years of European emissions. However, saline aquifers have no economic value, so are not usually
well mapped. As such, estimating possible storage volumes and guaranteeing storage security will be
more difficult. Storage in saline aquifers is currently being demonstrated at Sleipner and Snøhvit in
the North Sea.
1.3 Thesis Overview
If CCS is to have a positive environmental impact then the injected CO 2 must be stored in the
subsurface for as long as it takes for anthropogenic output rates to drop to acceptable levels and for
the carbon cycle to have recovered and stabilised (Holloway, 2001). This constraint requires that CO 2
be stored for timescales of the order of 10 4 or even 10 5 years. To meet this requirement we must ensure
that it is not possible for injected CO 2 to migrate large distances either vertically or horizontally away
from the target reservoir. This compels us to answer several fundamental scientific questions for CCS
to become economically and politically acceptable: can we develop models that can predict both how
injected CO 2 will migrate through the subsurface and the effects on the subsurface of the CO 2 , and can
we monitor CO 2 migration in the subsurface using geophysical (and geochemical) methods Finally,
can we link model predictions to field observations to ensure that modelled behaviour matches the
actual behaviour Consideration of these fundamental research questions will strengthen the scientific
foundations for CO 2 storage, and form the focus of this thesis.
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