Wednesday, 10 July 2013

Breaking news: Geomechanical effects crucial for secure CO2 storage, experts warn!

Breaking news: Geomechanical effects crucial for secure CO2 storage, experts warn!

My paper, published this week in the Proceedings of the National Academy of Science, compares the geomechanical response to CO2 injection at 3 commercial-scale CO2 injection operations. This is the first time (after about 20 papers), that one of my publications has received press attention of any kind (so thanks very much to the good folks at PNAS for their work in putting together a press release).

Since this seems to be generating a modicum of interest, I thought I'd better run through a quick layman's summary of what we've found (also, I'm nervous about how the paper will be written up by non-specialist journalists, so this blog affords my a chance to put myself across in my own words).

The term 'carbon capture and storage' covers the whole process by which CO2 is captured at fossil fuel power stations, and rather than being emitted to the atmosphere where it will contribute to global warming, it is compressed and pumped to geologically-suitable places where it can be injected into deep rock formations, where it is trapped by overlying impermeable layers and permanently trapped. As an Earth Scientist, my particular focus is on the last, 'storage', phase of this process - ensuring that the injected CO2 stays buried in the ground.

The key is the aforementioned 'impermeable caprock'. Even if we start with the assumption that site operators have chosen a formation with a suitable caprock, the concern is that pressure increases caused by CO2 injection will begin to fracture the rock, ultimately creating enough fractures running through the caprock that the CO2 can escape. Such 'geomechanical' effects have been the focus of previous papers that are critical of CCS, most notably last year's Zoback and Gorelick paper.

In my paper we make observations of geomechanical deformation at 3 large-scale CCS sites: Sleipner, in the North Sea, Weyburn in Central Canada, and In Salah, Algeria. I say 'we' because I am hugely indebted to my co-authors from the BGS, from the GSC, and from BP, who helped to compile this comparison paper.

We found that the 3 sites exhibited substantially different behaviours. At Sleipner, the target formation is huge (it extends under much of the North Sea) and has excellent permeability, meaning that it soaks up CO2 like a spunge. As a result, there has been almost no pressure increase. As such, there is little risk posed by geomechanical deformation

At Weyburn, the field has experienced a long history of stress change from 50 years of oil production prior to CO2 injection. Geomechanical effects at Weyburn have been monitored using microseismic: geophones placed near to the reservoir that pick up the 'pops' and 'crackles' as the rock fractures. A total of ~100 events have been detected over 6 years, a very low amount. These are all located around the reservoir, suggesting little possibility of leakage. They were in fact mainly located around the production wells, an initially counterintuitive observation that can be explained by the long and complicated stress history of the reservoir.

In Salah has been the most geomechanically active of the 3 sites we looked at: over 1000 microseismic events were detected in only a few months. Deformation at In Salah was initially detected using satellite methods that picked up the fact that the ground surface had been uplifted by a few centimeters because of the pressure increase in the reservoir. The flow properties at In Salah are not great, (only 10 millidarcy or so),so injection has lead to substantial pressure increases, hence the surface uplift and the microseismic activity. We believe that the fracture at In Salah has extended 100-200m into the caprock. At In Salah the caprock is ~1km thick, so it's not posing a risk to storage security at this point, but it's an important lesson for future storage sites that might only have 100-200m of caprock.

Our key finding is the huge differences in geomechanical response at the different sites. This shows the importance of carrying out detailed geomechanical appraisal prior to injection at every future CCS site, and putting monitoring programs in place during injection to ensure that adverse geomechanical effects are not posing a risk to secure storage.

I'll finish with my thoughts on CCS more generally. Many people love to declare that CCS is an 'untested' technology, which, frankly, is rubbish. Statoil have been storing CO2 at Sleipner since  1995. EnCana have been storing CO2 at Weyburn since 2000. We know that it is technically feasible to store millions of tonnes of CO2 in the subsurface.

However, the problem is one of scale - it's a similar problem to that facing renewables - burning hydrocarbons with unabated emissions is so very good and providing a huge amount of energy for not very much cost, and no other method can really compete with this as yet. We know we can stick a wind turbine on top of a hill and it will generate electricity. But, as we saw in a previous post, you have to plaster a hole mountainside with them to generate as much energy over 20 years as you can get from a single shale gas well pad.

So it is with CCS: we know that we can put a million tonnes under the North Sea without too much  difficulty. But we need to be storing billions of tonnes every year to make a difference with respect to climate change. And if we are to do that, we may not have the option of being too fussy about where we store it. Given this, it seems inevitable that at some point a future CCS site will run into geomechanical difficulties. We should be prepared for this eventuality.

1 comment:

  1. I appreciate you and I would like to read your next post.
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