|Photo: Clare Bond|
In September 2015, a group of UK researchers sponsored by the UK CCS Research Centre travelled to South Africa to investigate a natural CO₂ seep.
We were joined by colleagues from the Council for Geosciences (South Africa) and the South African National Energy Development Institute (SANEDI). Our first publication based on fieldwork undertaken during the trip has recently been published in the International Journal of Greenhouse Gas Control, and a previous blog of our adventures can be read here.
The work outlines the nature and extent of effervescing CO₂ along a fracture corridor, and suspected deep-seated fault zone. The long list of contributors attests to the nature of such interdisciplinary short field visits, in which individual practitioners complete a specific job at each field site and the evenings and follow-up work pull the distinct elements together. Clear pre-planning and focus is the key to such fieldwork success.
This first paper, led by the structural geologists on the team, focuses on the spatial distribution of CO₂ seeps in relation to structures observed in the field. These observations are used to build a hypothetical model for fracture controlled CO₂ flux to the surface. Although a genetic link for fracture controlled CO₂ seepage was not tested, the spatial distribution of seeps and the fracture corridor have a positive spatial correlation. Rock in areas in which fracture orientations are predicted to be favorably oriented for tensile opening show significant alteration to kaolinite with reduction in porosity that leads us to suggest that fracture permeability maybe transient.
This transience may result from CO₂ reaction with fracture-wall rock, leading to precipitation of clay and a reduction in fracture porosity. The work has implications for assessing the risk of CO₂ flow through fracture permeability to the surface for geological storage sites. Fractures are generally below seismic image resolution and hence are a possible unidentifiable risk to storage security.
Further papers will take a geochemical approach to test the source of the CO₂ and to investigate the extent of CO₂, fluid and rock interaction in the sub-surface. Watch this space…
|Sorting logistics for a day in the field. Photo: Clare Bond|
|Nigel Hicks (Council for Geoscience, South Africa), investigating the white kaolinite clay resulting from fluid-rock interaction. Photo: Clare Bond|
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