CO2 storage Environmental Baseline

Abstract title
CO2 storage Environmental Baseline
Introduction
As societies gradually shift from oil and gas to renewable energy, many offshore wells will be plugged and abandoned, while some will be transformed to facilitate carbon storage (CCS). We hypothesize that an early indicator of CO2 leakage from a CCS reservoir will be increased methane in the surface sediments, with thermogenic/long chain molecular signatures. Methane in marine sediments in the North Sea can have different origins; thermogenic gas from deep mature source rocks or biogenic gas produced by microbial degradation of organic material in more shallow and younger organic-rich sediments (Flury et al., 2016; Whiticar, 2000). To distinguish between these two types of sources the isotopic composition of carbon can be used (Schubert, 2011; Whiticar, 2000) and the fingerprint of the microbial community assemblages (Kleindienst et al., 2012 and this study).
To mitigate any methane leakage associated with abandonment and CO2 storage, it is necessary to understand whether the leakage has a natural or anthropogenic origin. Vertical migration or seepage of methane to the seabed through shallow sediments in the North Sea is often related to presence of natural faults and fractures (Andresen, 2012; Cartwright, 2007), natural vertical fluid conduits caused by localized release of subsurface overpressure (Böttner et al., 2019; Karstens & Berndt, 2015), or wells and their surrounding fractures (Böttner et al., 2020). Near wells, leakage from subsurface HC reservoirs can happen due to well integrity issues e.g. broken well casings and/or fluid flow along the outside of the well via fractures caused by drilling (Gasda et al., 2004; Vielstädte et al., 2015).
In the SEEP and SEABAS research study, we have mapped and studied recent and historical seepage in the subsurface around two producing fields (one active and one abandoned) in the North Sea and compared the data to areas with no data of prior seepage and to new possible CO2 storage sites in other areas of the North Sea. The multiproxy study includes geophysical data identifying potential leakage pathways of possible seepage, microbial and gas data characterizing the current type of seepage and sedimentary data, variations in elements and fauna characterizing the past seepage history at a storage site (figure 1). Hereby we have tested the possibilities for proxy-based identification and development of toolboxes for identifying both methane and CO2 leaks in the abandoned reservoirs and at possible CO2 storage sites in the Danish North Sea.
It is crucial to establish a baseline to understand the pathways of the natural seepage through the seabed both locally at platforms and storage sites, but also to characterize the present and past kind of seepage to evaluate whether the seepage is naturally occurring or initiated by the presence of human activities. By establishing a baseline, it is possibly to monitor and evaluate future leakage of methane, hydrocarbons and CO2 to the marine environment. The monitoring toolbox will contain many of the same tools as the baseline toolbox focusing on the environmental response of the microbial communities, bivalves and benthic foraminifera to potential seepage.
Method and/or Theory
The study is based on a broad suite of material and data:
1. Geophysical data characterizing the pathways of seepage and creating a spatial stratigraphic framework for the other data. The geophysical data are based on sub bottom profiler, multibeam echosounder and shallow reflection seismics collected during cruises in 2022 and 2023 but also based on vintage data when available.
2. Vibrocores of 3 to 6 m were drilled, they are representing the uppermost part of the sediment column below the seabed. The cores were collected during cruises in 2022 and 2023. The cores are drilled both in areas where seepage has been detected and in areas with no data of prior seepage. The positions are both close to producing fields and at new possible CO2-storage sites in the North Sea.
3. Microbial and gas data were collected in the sediments immediately after the cores were drilled and the data are characterizing the present type of seepage at a given site.
4. The cores were split lengthwise, and a sedimentary log was conducted. Part of the split core was preserved for future analysis the other part was sieved in 10 cm intervals for
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EAGE Carbon Capture & Storage Conference
palaeoecological studies of the changes in the foraminifera and bivalve composition. The cores were scanned for down-core relative changes in the bulk geochemical elemental composition using the non-destructive ITRAX X-ray Fluorescence (XRF) core scanner. Selected bivalves and foraminifera were radiocarbon (C14) dated and the isotopic composition of carbon in their shells were analyzed. This suite of sedimentary and faunal data including morphological responses to the harsh environment characterizes the past seepage at a given site.
Figure 1 Conceptual figure of the different kinds of tools in a baseline and monitoring toolbox. The tools are necessary when conducting an environmental baseline prior to CO2 storage and to monitor and evaluate future leakage of methane, hydrocarbons and CO2 to the marine environment.
Conclusions
A seabed environmental baseline for methane seepage in the North Sea has been established based on a multiproxy approach with the tools presented in figure 1. The type and pathways of present seepage at a specific locality was characterized by geophysical surveys and microbial analysis. The age and pathways of previous seepage is characterized by the presence of certain foraminifera and bivalve species and their degree of morphological malformities. C14 ages are essential to establish a clear chronology. By establishing an environmental baseline, it is possible to evaluate whether the seepage is naturally occurring or initiated by the presence of human activities. Furthermore, it is possible to monitor and evaluate future leakage of methane, hydrocarbons and CO2 to the marine environment by the tools (microbial analysis, presence of certain bivalve and foraminifera species, and the degree of malformation of some of the foraminifera species) in the monitoring toolbox.
Acknowledgements (Optional)
The authors would like to thank DHRTC/DTU Offshore for funding the project.
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EAGE Carbon Capture & Storage Conference
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