Final report from Call 1 Project: 3D mapping of large-scale subsurface flow pathways using nanoseismic monitoring

The project “3D mapping of large-scale subsurface flow pathways using nanoseismic monitoring” started in February 2013 and finished in August 2016. The aim of the research was twofold: (1) test the potential of a new technology for passive seismic monitoring, i.e., nanoseismics, that aims to complete the required data for three-dimensionally imaging hydraulically conductive features in the reservoir, caprock and overburden of an active CO₂ injection site: the Aquistore site, Canada and (2) use a multi-disciplinary approach to interpret passive seismic data sets obtained during operation of the same site.

A nanoseismic array consists of four short-period seismometers with eigenfrequencies equal to 1Hz and a flat response between 1 and 80Hz and it can record events of magnitudes down to -3 at source-to-sensor distances up to 10km.

The project faced a number of challenges, the main being the CO₂ injection start date being postponed for over two years (!) and when this became definite that it would start at last, our array got lost while in transit somewhere between the UK and Canada… Eventually, we were able to deploy the nanoseismic array 9 days after the CO₂ injection started at Aquistore, in April 2015. The seismometers were placed at 0.5m depth at a distance of 150m SE of the injection well. The deployment geometry was a triangle with the one-component (1D – vertical) sensors at the three vertices and the three-component (3D) sensor at the centre. The aperture of the array (distance between the 3D and each of the 1D sensors) was 100m. Data collection was continuous for 8 weeks. During that time CO₂ injection stopped at various intervals and for different durations for reasons outwith our control. In total, the CO₂ injection intervals cover 60% of our total monitoring time.

The preliminary analysis of the nanoseismic data was based on spectral techniques aiming to identify time periods where specific frequencies, potentially related to CO₂ injection, appeared. When comparing the Power Spectral Density of each hour of recordings during time periods with and without CO₂ injection, there appears to be slightly elevated levels of energy on days when CO₂ injection took place.

We further investigated this by developing an automated algorithm based on the LTA/STA approach for the detection of potential nanoseismic signals. The algorithm can automatically open and read the different folders and files of data with a single command, and took about 5 full days to scan all 8064 hourly files. More than 550 nanoseismic signals satisfied our detection criteria but almost all of them were related to sources other than CO₂ injection, e.g., mining activities.

The project finished at the most exciting stage, just when we managed to isolate a very small number of nanoseismic signals, which potentially reflect CO₂ flow. This hypothesis is so far consistent with our observations from structural geological data but a more detailed analysis of the available data is required. This means that our work is still on-going even after the official end of the project. Final conclusions will be derived only when the analysis is complete.

Updates to follow soon! In the meantime, we keeeeep (re)-searching!

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