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| Titel |
Effects of faults as barriers or conduits to displaced brine flow on a putative CO2 storage site in the Southern North Sea |
| VerfasserIn |
Sarah Hannis, Stephanie Bricker, John Williams |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250080572
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| Zusammenfassung |
| The Bunter Sandstone Formation in the Southern North Sea is a potential reservoir being
considered for carbon dioxide storage as a climate change mitigation option. A geological
model of a putative storage site within this saline aquifer was built from 3D seismic and well
data to investigate potential reservoir pressure changes and their effects on fault movement,
brine and CO2 migration as a result of CO2 injection. The model is located directly beneath
the Dogger Bank Special Area of Conservation, close to the UK-Netherlands median
line.
Analysis of the seismic data reveals two large fault zones, one in each of the UK and
Netherlands sectors, many tens of kilometres in length, extending from reservoir level to the
sea bed. Although it has been shown that similar faults compartmentalise gas fields
elsewhere in the Netherlands sector, significant uncertainty remains surrounding the
properties of the faults in our model area; in particular their cross- and along-fault
permeability and geomechanical behaviour. Despite lying outside the anticipated CO2
plume, these faults could provide potential barriers to pore fluid migration and
pressure dissipation, until, under elevated pressures, they provide vertical migration
pathways for brine. In this case, the faults will act to enhance injectivity, but potential
environmental impacts, should the displaced brine be expelled at the sea bed, will require
consideration.
Pressure gradients deduced from regional leak-off test data have been input
into a simple geomechanical model to estimate the threshold pressure gradient at
which faults cutting the Mesozoic succession will fail, assuming reactivation of fault
segments will cause an increase in vertical permeability. Various 4D scenarios were
run using a single-phase groundwater modelling code, calibrated to results from a
multi-phase commercial simulator. Possible end-member ranges of fault parameters
were input to investigate the pressure change with time and quantify brine flux to
the seabed in potentially reactivated sections of each fault zone. Combining the
modelled pressure field with the calculated fault failure criterion suggests that only
the fault in the Netherlands sector reactivates, allowing brine displacement at a
maximum rate of 800 — 900 m3/d. Model results indicate that the extent of brine
displacement is most sensitive to the fault reactivation pressure gradient and fault zone
thickness.
In conclusion, CO2 injection into a saline aquifer results in a significant increase in
pore-fluid pressure gradients. In this case, brine displacement along faults acting as
pressure relief valves could increase injectivity in a similar manner to pressure
management wells, thereby facilitating the storage operation. However, if the faults act as
brine migration pathways, an understanding of seabed flux rates and environmental
impacts will need to be demonstrated to regulators prior to injection. This study,
close to an international border, also highlights the need to inform neighbouring
countries authorities of proposed operations and, potentially, to obtain licences
to increase reservoir pressure and/or displace brine across international borders. |
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