Focused fluid conduits in the Southern Viking Graben and their implications for the Sleipner CO2 storage project
Focused fluid flow in marine sediments affects the evolution of sedimentary basins by altering the integrity of sealing caprocks and by transferring fluids as well as pressure. The formation of focused fluid conduits initiates, when the pore pressure within a reservoir exceeds the seal’s resistance against fracture or capillary failure. In seismic data, focused fluid conduits manifest as vertical zones of anomalous seismic amplitudes, which are known as seismic chimneys or pipes. The understanding of focused fluid flow manifestations is of great importance for hydrocarbon exploration because these structures may act as indicators for hydrocarbon reservoirs and sub-seafloor operations, as they may pose a hazard to drilling operations and influence the long-term efficiency sub-seabed storage of CO2. The geological storage of CO2 known as carbon capture and storage (CCS) is a key technology for the mitiga¬tion of climate sensitive greenhouse gas emissions recommended by the Intergovernmental Panel on Climate Change. Marine saline aquifers are a favorable storage option, because such systems have virtually infinite capacity, no influence on ground water production and bear the least conflict potential with the public. Howe¬ver, the success of CCS as a climate change mitigation technology depends on its public acceptance and builds on a reliable and transparent risk assessment, which is studied by the interdisciplinary, international ECO2 project. The project’s main aims are to investigate the likelihood of leakage from marine CO2 storage sites and its potential effect on marine ecosystems, to develop monitoring strategies, and to define best environmental practice guidelines for implementing and managing storage sites. Within this framework, my PhD thesis concentrates on the investigation of focused fluid conduits in the Sou¬thern Viking Graben and their implications for the Sleipner CO2 storage project, where CO2 is injected into a ~850 meter deep saline aquifer known as the Utsira Formation. My studies are based on the interpretation of 3D and time-lapse 4D seismic data covering more than 2000 km2 of the Southern Viking Graben, numerical fluid flow simulation, and field geological observations. My thesis builds on a detailed description of the fluid flow system in the Sleipner area, which is characterized by the interplay between deep hydrocarbon reservoirs, the Utsira Formation and the overlying strata hosting various fluid flow features. The key elements of seal-bypassing fluid flow in the strata overlying the Utsira Formation are several hundreds of meters-wide chimney structures. The study area hosts at least 46 of these focused fluid conduits, which are categorized based on their seismic appearance into three types (A, B and C). Type-A-chimney shows similarities to “blowout pipes”, which are known from different sedimentary basins around the world and generally associated with rapid expulsion of fluids. Type B is very similar to large “gas chimney” structures, which have been identified above several leaking hydrocarbon reservoirs and which are interpreted as gas filled fracture networks crosscutting a low permeable seal. Type-C-chimneys cause seismic disturbances in bands of up to 6 km length and correlate with overlying tunnel valleys. The formation of focused fluid conduits requires high pore overpressure. The analysis of the Sleipner palaeo fluid flow system indicates that there may be a link between chimney formation and the last glacial cycle. Building on this, we present a novel hypothesis, which predicts that undrained fluid flow systems could pro¬duce significant overpressure as a result of the interplay between loading and unloading during a glacial cycle and resulting gas compression and sediment compaction. The closest chimney structures with a probable connection to the Utsira Formation are 7 km away from the Sleipner CO2 injection point. We have evaluated the propensity of leakage along these structures by perfor¬ming numerical modeling of the CO2 plume evolution with the multiphase fluid flow simulator DuMux. The simulations revealed that it is not likely that the modeled CO2 plume will ever reach the chimney structures. Additionally, it is unlikely that the injection of CO2 itself may cause the formation of chimney structures at Sleipner. Although seismic pipes and chimneys are very common features in seismic data and their interpretation as focused fluid flow conduits is well-established, only little is known about the underlying geological processes and how these link up with their seismic image. The comparison of field analogues of focused fluid conduits from the Colorado Plateau (USA) and seismic chimneys revealed that specific seismic signatures correlate well with certain field-based examples. The integration of field geological observations may help to improve seismic interpretations of chimney structures and fluid flow systems in general. The importance of focused fluid flow conduits for the evolution of sedimentary basins is not sufficiently addressed yet. The results of this thesis highlight the relevance of focused fluid flow conduits for long-term integrity of sub-seabed storage of CO2 operations and that focused fluid flow conduits have to be considered for site selection of storage projects. A deeper understanding of focused fluid flow will help to understand the flux of fluids from the geosphere into the hydro- and atmosphere as well as geological and climatic processes of the past and future.
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