Background: Understanding the long term storage security of CO2 is an essential part of CCS if we want to help mitigate climate change. By studying natural accumulations of CO2 we can better understand what may happen when we inject CO2 underground on these long time scales. The Green River CO2 accumulation in Utah is an ideal place to investigate the long term effects that CO2 rich fluids (with Sulfur impurities) have on fault zone materials. The CO2 at Green River has periodically migrated up two fault zones for 400,000 years and is involved in a suite of geomechanical and geochemical processes, including bleaching of the red Entrada sandstones. Very little is known about the long term sealing and migration mechanisms of fault zones, and these faults are ideal case studies.
We applied to the International Research Collaboration Fund (Call 4) to investigate the possibility of mapping this system by drone with our US collaborators, and to sample fault zone mineral precipitates. We were lucky to get this funding, and would like to thank UKCCSRC for the initiative.
The team: Mike Bickle, Alex Maskell, Peter Scott and Hazel Chapman
GSA Fall Conference, Denver: Due to term-time commitment and weather constraints with fieldwork over winter, we decided to meet up with our U.S. hosts, Prof. James Evans and Dr. Elizabeth Petrie at the GSA conference in Denver. This enabled us to not only shorten our proposed trip but also present 3 talks, allowing us to reach a far wider audience than we would have otherwise, and additionally to attend some interesting sessions on fault-zone fluid interactions. During the conference we had several fruitful meetings with our collaborators planning further drilling of the fault zone. We also met with them in the field to revisit the fault structure and plan possible drilling sites.
Onward to Utah: After a successful GSA meeting in Denver, we headed out across to Rockies as the Aspens were changing colour, to our field site in Utah. Along the way we found a very appropriately labeled door (Fig 1), but eventually we made it to Green River. After a gap of several years it was great to see Crystal geyser still bubbling away. This man-made CO2 geyser shows the importance of properly sealing old exploration/extraction wells. However, after standing around for 10 minutes without witnessing any big eruptions we decided to get to business. Bring in mapping by drone! Not only can we accidentally produce some great arty shots (see Fig 2) but we can also collect high resolution aerial photographs and create digital elevation models from them.
Droning on: There were some initial pre-flight jitters and a close shave or two from unplanned descents early on day one, but by lunchtime we were confident in our new found pilot statuses. Taking photos every 2 seconds at a height of about 50 m, we were able to cover large areas quite quickly, and fill up our available memory on our hard drives. A screenshot for one of the mapped regions is shown in Fig 3. From this preliminary image, we can clearly see the bleached reaction fronts cutting across bedding features. Using these techniques we can pick out and measure the geometry of ancient fractures and flow pathways associated with the CO2-charged brines. Interestingly many of these vertical fractures associated bleaching are in-filled with gypsum and celestine, which we sampled with the hope of getting an approximate minimum age estimate on the closure of fluid flow (analyses to come!).
What’s next: Enlisting the expert help of Dr. Christoph Gruetzner, we’re creating higher resolution DEM’s of the three mapped study regions. We hope to combine these with reactive transport models to understand the timescales and fluxes of CO2 through the fracture networks at Green River. Watch this space or our new twitter account @UCam_C_Storage!