Simulating induced seismicity using geomechanics

The best possible words in the life of an academic are undoubtably "paper accepted". Since I've recently had a paper accepted in EPSL. I thought I would add a layman's summary of it here. 

The paper is available here, and is "open access", so you shouldn't need a subscription to read it. 

Our motivation is to try to understand and model why subsurface processes cause induced earthquakes. Induced seismicity has become a controversial issue in relation to fracking, but in fact the risk of inducing an earthquake during fracking are much lower than the risk of inducing an earthquake by other subsurface activities, such as geothermal energy, waste water injection and carbon capture and storage. 

The main reason for this is simply a matter of volume - the more volume you inject, the more likely you are to trigger an earthquake. While much has been made of the water volumes used for fracking, they are actually quite small in the grand scheme of things. The volumes injected for waste-water disposal and for CCS are much larger than the volumes used for fracking. This is why we've seen such increases in seismicity in places like Oklahoma in recent years (it's got very little to do with fracking). 

We've also seen induced seismicity - albeit of small magnitude, less than mag 3 - at two pilot CCS projects, the Decatur project in Illinois, and at the In Salah project in Algeria, which is the subject of our study. 

Firstly, a brief introduction to the In Salah site. It's a gas field in the middle of the Sahara desert. 

Due to natural geological processes, the natural gas that is produced contains a relatively high percentage of CO2. This must be stripped off before the gas can be sold - there are minimum CO2 content requirements. Usually, the CO2 would just be vented to the atmosphere. However, the operators of the site, BP, Statoil and Sonatrach, decided to use the site as a pilot project for CO2. So they instead re-injected the CO2 into the water-leg of the reservoir (part of the reservoir unit that is filled with water rather than gas). The image below shows the basic principles in cartoon form.  

In total nearly 4 million tonnes of CO2 were injected between 2004 - 2011. The average car emits about 4 tonnes of CO2 per year, so that's the equivalent of the annual emissions of 1 million cars.

The site was monitored using a number of methods, but it was clear from relatively early on than the CO2 injection was producing geomechanical deformation. As a result, microseismic monitoring was used to image any small earthquakes. You can read more about the results of the microseismic monitoring here, but the main conclusions were that thousands of small-magnitude (mostly around magnitude 0.0) events had been induced. The largest event was magnitude 1.7, which is probably too small to be felt by humans at the surface (we can detect them with seismometers though of course), and definitely too small to cause damage. Fortunately, all the events were confined to the reservoir unit, so there was no evidence that the seismicity was providing a pathway for CO2 to escape.  

So, what's this latest paper all about?

The basic premise of our study was that induced events occur on pre-existing fractures. They occur because industrial activities change the state of stress in the subsurface, moving a fault from a stable to an unstable state, which allows it to move, triggering an earthquake. So in theory, if we can predict or model where the faults and fractures are, and we can predict or model the changes in stress generated by our activities, we can resolve the stress changes onto the faults, and work out when and where faults might trigger seismicity. The purpose of our paper was to assess how well this approach works in practice. 

To model the size, orientation and positions of faults and fractures I am indebted to my colleague Dr. Clare Bond at Aberdeen, who build a structural model of the reservoir, which simulates how the reservoir geometry we observe today could have formed from the originally-flat sedimentary layers. This produces a strain map, which is then converted into a discrete fracture network to account for how fractures would have accommodated the modelled strain. The resulting fracture map is shown below: you can see that fractures are not uniformly distributed across the reservoir, but there are bands of intense fracturing running through the reservoir, and zones with much fewer fractures. 

In order to simulate the stress changes induced by injection, I am indebted to another colleague, Rob Bissell, from Carbon Fluids Ltd., who built a geomechanical simulation of the injection process. More details about this model are available here. The model provides a map of stress and pore-pressure changes at monthly intervals through the injection period. 

In order to work out whether the modelled stress changes would be sufficient to induce seismicity, for each modelled fracture we resolved the modelled stress from the nearest node of the geomechanical model into normal and shear stresses on the fracture face. If the shear stress exceeded the Mohr-Couloumb criteria, then an event will occur. The size of the event will be determined by the stress drop generated by the event, which will be a function of the shear stress, and the size of the fracture, which is pre-determined in the model provided by Dr Bond. 

Therefore we have a method to simulate when and where an earthquake may occur, and how big it will be. We tested our model simulation results against the microseismic observations made by my colleague Dr. Anna Stork in this paper. 

The figure below shows that the relative rates of seismicity predicted by the model matches that observed at In Salah. CO2 injection re-starts in late 2009. However, only a small amount of seismicity is observed. Injection rates increase in summer 2010, and for 4 months the rate of induced seismicity also increases. Once injection rates are reduced, the number of events decays away as well. This behaviour is well captured by our model. 

In terms of magnitudes, our modelled largest event matched very well the observed largest magnitude of M=1.7. Magnitudes are determined by the size of the fault and the stress drop, so this indicates that Dr Bond's model did a good job of simulating the fault/fracture sizes, and that Rob Bissell's model did a good job of simulating the stress changes induced by injection. 

Overall, our model does a good job of simulating induced events at In Salah, which is encouraging in terms of our future ability to mitigate induced seismicity at future projects. We have outlined a workflow that can be followed at sensitive sites where induced seismicity may be an issue. For example, the modelling approach can be used to assess whether alternative injection strategies may lower the risk of inducing an event. 

A visit to the Heping cement CO2 capture demonstration project in Taiwan

Update (25.5.2014): Since I have arrived home and am no longer relying on dodgy hotel internet, I have been able to upload a video of the plant I took while I was there. The video gives a better idea of the scale of the factory that the picture cannot, simply because you can't fit the whole plant into the shot. The video is on YouTube, but I've embedded it here:

Also, if you want an aerial view of the plant, you can find it on google maps here.

The introduction to this post is that I'm writing it from the luxury of a rather swanky 5-star hotel in Taipei.

Taiwan is not in a great position with respect to energy. They have few natural resources of their own to utilise - only a few small natural gas fields in the north of the country. Nuclear is a troublesome subject for such an earthquake-prone island, especially after Fukushima. While you might think that such a mountainous island would be a good site for renewables, the regular earthquakes, landslides and typhoons means that maintaining a large number of wind turbines will also prove difficult. As a result, the majority of Taiwan's electricity comes from coal, most of which is imported from Indonesia.

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.
       

Induced Earthquakes in the USA, and some implications for CCS

Here's a recent BBC report on earthquakes induced by oil and gas activities in the USA. As can be expected, the twitter/blogo-sphere has been lighting up over this in the last few days. For me the biggest surprise is that this has only come up in the wider media in last few days: induced earthquakes have been a key topic of discussion among geophysicists for a couple of years now. The USGS has noted an increase in medium-sized earthquakes in the last decade:

The black line shows the total number of earthquakes in the midcontinent USA (excluding the very active San Andreas fault and other active parts on the west coast) greater than M3 since 1970: you can see the increases as the line gets steeper.

The oil industry likes to dispose of waste-water by injecting it into deep-lying saline aquifers. However, it has been well known since the Rocky Mountain Arsenal in the 1960s that deep fluid injection can trigger earthquakes. It is argued that the increase in oil industry injection activities in the last decade has been the cause of the increase in the numbers of earthquakes.

This remains under debate - could the increase be simply that, as more (and better) seismic monitoring networks are installed, we are detecting more earthquakes than we did in the past. The latest news story is a case in point. The paper in Geology attributes an M5.7 earthquake in Oklahoma to injection of waste-water. The Oklahoma Geological Survey has subsequently released a rebuttal stating that as far as it is concerned, there is not enough evidence to tie the quake to injection activities (strangely enough, the OGS rebuttal hasn't been given much of a look-in from the media).

Nevertheless, I think that it inarguable that, in certain cases at least, fluid injection has triggered earthquakes with magnitudes from about M3 to M6.

This brings me to a couple of asides. Firstly, following on from my last post about bad media reporting of these issues, many reports attributed the quake to injection of waste-water from fracking. This is not the case - the waste water in this case came from conventional oil production. This harks back to an older post I made about the relative risk profiles from fracking in comparison to conventional oil and gas. The need to dispose of large quantities of contaminated waste water is not a new, fracking-related problem in the oil industry. If you are opposed to fracking, you must presumably be opposed to all oil and gas related activity.

Secondly, M5.7 is a large earthquake. It is about 100,000 times larger than the quake induced in Blackpool by fracking. It is larger than any earthquake ever recorded in the UK. Perhaps only a few historical earthquakes in the UK have been of a similar size. An M5.7 triggered earthquake here would be serious news.

So, can we get an estimate of what earthquake magnitude might be triggered by our various activities? Art McGarr, a venerable (and venerated) and highly experienced geophysicist with the USGS has made an effort to do this. McGarr cut his teeth in the 1970s looking at mining induced seismicity, where he noticed a correlation between the total energy released during rock extraction and the volume of rock extracted. He developed the so-called McGarr equation:

Sum(Moment) = G dV

The sum of the released seismic moment equals the volume change (dV) multiplied by the shear modulus (G). It should be noted that this equation is based on empirical observation only. It has subsequently been applied to fluid injection (or mis-applied, some would say, as there is no obvious basis for arguing that physical processes during fluid injection should match those during rock removal (mining)), where dV becomes the volume of fluid injected.

More recently, McGarr has been looking at earthquakes attributed to fluid injection. This includes waste-water injection as discussed above, as well as geothermal activities and, of course, fracking. He has developed the following plot:

Each + represents an injection-induced seismic event. Unfortunately for any non-geophysicist readers, McGarr has given the earthquake sizes in moment, rather than magnitude, but 10^12 is about M2, 10^15 is about M4, 10^18 is M6. I've not found out what all of McGarr's abbreviations are, but

• BUK is the Blackpool earthquake
• RMA is the quake induced by fluid disposal at the Rocky Mountain Arsenal
• BAS is the Basel (Switzerland) earthquake caused by geothermal activity
• STZ is an earthquake caused by geothermal activity at Soultz, France
• RAT (several of them) are earthquakes in the Raton Basin (Colorado) associated with waste water injection
• POK is the Oklahoma earthquake discussed in this blog

You can see a general correlation between the maximum magnitude and the injection volume, following a McGarr-esque equation, replacing the sum of the moment by a maximum magnitude: Mmax = GdV. It should be remembered that this line appears to be describing the MAXIMUM POSSIBLE magnitude. There are over 150,000 waste-water injection wells in the USA, only a tiny fraction of them have caused detectable earthquakes.

So how does this apply to the UK? The first thing to note is that deep injection of waste fluids is not allowed in this country, so we can strike this risk off immediately. What about fracking? A typical frack stimulation uses about 1000 - 5000 metres cubed of water - that's ~10^3. This leaves us with a maximum induce-able moment of ~10^13 (or a magnitude of about M3). We get 30 or so M3 events in the UK every year, so inducing a few more due to fracking isn't going to make much difference.

What about CCS? Carbon capture and storage is a key plank in the UK's CO2 emissions reductions plan. All well and good, but CCS involves the injection of very large volumes of fluid into subsurface aquifers. Could this trigger earthquakes?

I've modified McGarr's plot to add the injection volumes of Sleipner and In Salah, two of the foremost CCS projects currently in operation (as well as changing the scale from moment to magnitude to make life a little easier for non-geophysicists):

You can see that, following the McGarr plot, Sleipner and In Salah have the potential to trigger earthquakes of M5 or larger! Of course, they haven't: Sleipner has barely done anything, while In Salah has triggered at most an M1 event (so small you can't feel it without the aid of sensitive seismometers). The McGarr plot tells you the maximum possible magnitude, not what magnitude you will get. Hence why I have shaded in the area under the line: you could get an event on the line, or anywhere under the line.

Still, I find the potential for induced earthquakes from CCS to be worrying. I think this has been under-appreciated by the UK CCS community. There is a clear need for further study on why most injection sites do not produce seismicity, but a few do? What is it that is different about these sites, and how can we identify this in advance, and only select sites that won't trigger events during CO2 injection. At the same time, we can quickly see that the earthquake risk from fracking has been hugely overplayed in comparison to the risks posed by other activites (geothermal, CCS, waste-water injection, mining, and even hydroelectric energy).

My latest outreach attempt:

Good afternoon dear readers, I trust you are enjoying your Friday afternoons. If time is passing slowly (and with the potential distraction of cricket rained off again) then you will be in need of a procrastination measure. In which case you can spend 100 seconds of your time watching my latest outreach attempt - explaining fracking and CCS in 100 seconds for Physics World's 'Physics in 100 seconds' feature. Enjoy.......

CCS in the UK

As promised, my next post will be all about carbon capture and storage. It's a little later than I intended: what can I say, I had an enjoyable Easter. As I hinted in my last post, there was big news in the pipeline for CCS, and it was this:

http://www.guardian.co.uk/environment/2012/apr/03/carbon-capture-storage-competition

The government has revived the competition to award 1 billion pounds to a power plant that can demonstrate commercial scale CCS: that is a coal or gas fired power plant where the CO2 emissions are captured and pumped offshore in the North Sea, where they are stored in depleted oil/gas reservoirs, or other suitable saline aquifers. They have also set aside £125 million for further research into CCS.

CCS has long been touted as a potential solution for reducing our greenhouse-gas emissions. It allows us to continue burning fossil fuels, without overheating the planet. However, it is not universally popular. Indeed, Greenpeace hate the idea. The principal objections are the cost, and public concerns over leakage security.

I'm not best placed to comment on costs. Yes, CCS will be expensive in comparison with just burning coal with no efforts to moderate their pollution. But then, every alternative energy source, from renewables to nuclear, is a damn sight more expensive than just burning coal. That's why we're so addicted to fossil fuels - they're so remarkably cheap and easy to burn!!! So if we want to mitigate global climate change, whether we get there with nuclear, renewables or CCS (or a combination of all 3), it's going to be more expensive than just burning fossil fuels.

Secondly, public concerns over the risks of leakage. As a geologist/geophysicist, I'm better qualified to comment on this. Public outrage about a potential CCS storage site in the Netherlands has already been sufficient to see the cancellation of Shell's Barendrecht CCS project.

CCS is seen as an unproven technology. This is one of the first things Greenpeace will bring out as the main criticism of CCS. And, once you start telling people that an unproven technology will be deployed near their town, of course they're going to get upset about it. But is CCS really unproven? When the Norwegian government announced an offshore CO2 tax in 1995, Statoil decided it would be cheaper to capture and store the CO2 emissions from their Sleipner drilling rig. CO2 has been injected at Sleipner at a rate of 1 million tonnes per year for over 15 years now, with no sign of leakage. Similar stories can be found at Weyburn, Canada, and at In Salah, Algeria. Unproven? Not really.

However, by far the best way I've found, when talking to non-geologists, to convince them that storing CO2 in subsurface reservoirs is, from a geological perspective, not really that challenging, is to point out that we've been storing gas underground for decades and noone's had a problem with it. The gas we do store is highly explosive, so it's pretty dangerous. It is, of course, natural gas.

Natural gas is produced from reservoirs at a relatively constant rate all year round. However, we use a lot more gas in winter than we do in summer. So the gas we produce in summer has to be stored until winter. The most common storage sites are either depleted gas reservoirs, or saline aquifers, near to the population centers that will be needing the gas.

Now, how is it that we (and the residents of Barendrecht) are ok with storing an explosive gas in geological formations near their towns, yet we have problems if we want to store an inert, non-toxic gas (CO2) in the same formations? This makes no sense to me. But of course, we've been storing natural gas for decades, so it's a proven technology, and it enables the gas companies to lower our bills, while CO2 storage is a 'new' technology, and obviously it's more expensive to capture the CO2 at power plants rather than just burning the coal and polluting our planet.

What's more, while some people are also getting upset at the thought of CO2 pipelines running near their neighbourhoods, they seem totally fine with having explosive gases pumped through pipelines right into their kitchen.

It's the kind of problem that should (and, I believe, does), get psychologists and social scientists all excited about why it is that we often perceive risky things to not be risky, and non-risky things to be dangerous. As a geophysicist, understanding people has never been my strong point. 

Anyway, good news that the government has decided that it will put some more effort into encouraging CCS.

<insersts tongue into cheek>Time to dust of some funding proposals and see if I can't get my fingers into some of that £125 million designated for CCS research </removes tongue from cheek>

Perspective

In writing my last post, I began thinking about how important perspective can be in our view of a given topic. Some instances:

• if you came from a planet that had never burned fossil fuels (or from our planet but 300 years ago) you'd probably find the whole notion of burning fossil fuels on the scale we do to be abhorrent, what with the pollution, oil spills, global warming etc. However, we are accustomed to it, so it doesn't bother (most of) us.
• If you're used to your hydrocarbons coming nice and easily from a Saudi field (where the method of extraction is pretty much stick a hole in the ground, let the light, sweet crude oil flow up with hardly any effort at all) then things like shale gas, hydrofracking and tar sands seem crazy.
• However, if someone is considering burning coal from underneath your feet (UCG, see my previous post), suddenly shale gas doesn't seem so bad after all.

During my PhD I worked on Carbon Capture and Storage (CCS). In this process, CO2 is captured from coal power plants and pumped to appropriate sedimentary basins where it is injected into deep lying saline aquifers, preventing CO2 emissions which cause global warming. Some people think this is a crazy idea - why don't we just not burn the coal in the first place, and use renewable energy instead? Here's Greenpeace on the matter.

However, a sense of perspective can be achieved by considering the work of some colleagues of mine who are working on a project called 'SPICE' - Stratospheric Particle Injection for Climate Engineering. We are currently looking at the possibility of constructing a huge pipeline into the stratosphere, through which we can pump sulphate particles that will help absorb the sun's rays and reduce global warming to manageable levels. When you consider that plan B is a 25km space-pipe injecting sulphate into the stratosphere, wouldn't it be a better idea to start burying CO2 emitted from power plants now, rather than hoping that we will switch to renewable energy at some unspecified point in the future. Perspective people......