Abstract
Evaluation of seismic reflection data has identified the presence of fluid escape structures cross-cutting overburden stratigraphy within sedimentary basins globally. Seismically-imaged chimneys/pipes are considered to be possible pathways for fluid flow, which may hydraulically connect deeper strata to the seabed. The properties of fluid migration pathways through the overburden must be constrained to enable secure, long-term subsurface carbon dioxide (CO2) storage. We have investigated a site of natural active fluid escape in the North Sea, the Scanner pockmark complex, to determine the physical characteristics of focused fluid conduits, and how they control fluid flow. Here we show that a multi-scale, multi-disciplinary experimental approach is required for complete characterisation of fluid escape structures. Geophysical techniques are necessary to resolve fracture geometry and subsurface structure (e.g., multi-frequency seismics) and physical parameters of sediments (e.g., controlled source electromagnetics) across a wide range of length scales (m to km). At smaller (mm to cm) scales, sediment cores were sampled directly and their physical and chemical properties assessed using laboratory-based methods. Numerical modelling approaches bridge the resolution gap, though their validity is dependent on calibration and constraint from field and laboratory experimental data. Further, time-lapse seismic and acoustic methods capable of resolving temporal changes are key for determining fluid flux. Future optimisation of experiment resource use may be facilitated by the installation of permanent seabed infrastructure, and replacement of manual data processing with automated workflows. This study can be used to inform measurement, monitoring and verification workflows that will assist policymaking, regulation, and best practice for CO2 subsurface storage operations.
Highlights
Carbon dioxide (CO2) capture and subsurface storage (CCS) within sedimentary basins has been identified as an effective solution for reducing anthropogenic CO2 emissions in the atmosphere (IPCC, 2005; The Global CCS Institute, 2019)
We have investigated a site of natural active fluid escape in the North Sea, the Scanner pockmark complex, to determine the physical characteristics of focused fluid conduits, and how they control fluid flow
This permits integration into a multi-scale approach, and demonstrates how the different techniques may be employed in combination, to ensure appropriate constraint and calibration of the different methods, for complete characterisation of the fluid escape structures (Section 4). (3) Thirdly, based on our findings from this study, we describe a framework which can be used to determine the approaches that are needed to un derstand potential fluid flow structures in marine environments, in the context of the risk assessment of potential future CO2 geological storage sites (Section 5)
Summary
Carbon dioxide (CO2) capture and subsurface storage (CCS) within sedimentary basins has been identified as an effective solution for reducing anthropogenic CO2 emissions in the atmosphere (IPCC, 2005; The Global CCS Institute, 2019). Potential pathways may include anthropogenic sources, such as aban doned wells (e.g., Watson and Bachu, 2009), formation level inherent structures, including natural migration up-dip along permeable strati graphic horizons (e.g., Toth, 1980; Hindle, 1997), and the formation or reactivation of fluid escape structures (e.g., Nichols et al, 1994; Frey et al, 2009). Such fluid escape, or seal bypass, structures permit pressure-driven, focused fluid flow, which hydraulically connects deeper strata with the seafloor through inter-connected faults, fractures, and porous-permeable sediment layers (Cartwright et al, 2007). The combined un derstanding of the presence of fluid pathways and their fluid flow regime is critical for the risk assessment of potential subsurface CO2 escape
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