Sunday, 28 April 2013

Shale gas and Cash-for-Locals?

This week the Parliamentary Select Committee for Energy and Climate Change released its assessment of 'The Impact of Shale Gas on Energy Markets'.

I particularly enjoyed conclusion 5:
One key to community acceptance will be a robust factual response by government to scare stories
I wonder who/what they could be referring to there....

More interesting, in my view at least, is conclusion 6:
Communities who are affected by shale gas development should expect to receive, and share in, some of the benefits of the development
or, as the Guardian would put it: Fracking firms should offer sweeteners to locals. It's an interesting idea, but I'm still torn between whether it is a good one or not.

In the US, mineral rights are generally owned by the person that owns the land. This means that if your farm sits on top of some shale gas, you stand to benefit directly from royalties from the gas development (try sticking the default numbers into this calculation engine). As a result, shale gas is generally wildly popular among rural American communities.

However, in the UK, in most cases the mineral rights being to the Crown Estate (i.e. 'er maj, gawd bless 'er), meaning that royalties from gas production goes straight to central government, rather than via local people.

Of course, that's not to say that shale gas development will not benefit a local community. While many of the jobs involved are high tech, and as such cannot be easily accessed by local people, there are plenty of roles for relatively unskilled workers, particularly in construction and haulage. Moreover, however the influx of skilled workers need places to stay, to eat and to drink, to do their laundry. They need to buy petrol, buy stuff from convenience stores, the list goes on. In Pennsylvania you hear of companies block-booking whole hotels for 6 month stretches to house the workers, restaurants full to bursting every lunchtime and bars full in the evenings.

However, UK public opinion continues to waver in regards to shale gas development. So, is it right to consider setting up community benefit schemes, whereby some of the profits from gas development are injected directly back into the local community? Or is this all a bribe to get people to accept something that they'd otherwise not be comfortable with?

In all honesty, I'm not sure I know the answer to this. On the one hand, shale gas development will involve some local disruption. Not the scare stories of exploding taps, blighted aquifers and general geological disruption - the so-called 'geological dread factor' - but increases in traffic, construction sites, laying new pipeline etc. Therefore it does seem reasonable that a community should receive some recompense for that. On the other hand, offering what could easily look like little more than a bung could make it look like shale gas has something to hide, when so long as the government ensures that there is 'a robust factual response by government to scare stories' it shouldn't have to.

It is worth noting at this juncture that such schemes seem to be common for wind farms (see here and here for two randomly selected examples) and nuclear power stations. I have enjoyed seeing how the language changes depending on your preferred form of energy, particularly wind farm proponents who have touted these community wind farm benefits as a great example of how wind can benefit a community, while if shale gas companies suggest the same thing then it is little more than a bribe.

So I'm still not sure whether this is a good idea or not. Regardless, in the meantime, IGas have drilled two exploration wells in Lancashire.


Tuesday, 23 April 2013

Bristol's shake table

Shake tables are used to simulate the effects of earthquakes on structures. Engineers use them to simulate the effects of earthquakes on structures, so that they can design buildings to withstand shaking.

You can program in the earthquake of your choice, put your structure on the table, and see how well it does. Today I got to visit the Bristol Engineering Dept shake table. Here's some video:


Thursday, 11 April 2013

The Maximum Magnitude Conundrum

I talked in my last post about seismicity (i.e. earthquakes) induced by subsurface fluid injection. In fact, there are many human activities that have the potential to cause earthquakes, including:
So it seems that whether you are a climate-change denier who just wants to keep on mining coal and burning oil and gas, an advocate of tech-based solutions like CCS and nuclear, or a total greenie who sees our future energy needs met by geothermal energy, hydro-electricity, and other renewables that require efficient energy storage mechanisms (i.e. lots of pumped storage reservoirs), it seems that your preferred energy source comes with a risk of generating earthquakes.

Which in turn begs a really important question - for a given operation, what (if anything) will determine the maximum possible earthquake magnitude that your activity will produce? This is a really important question both for human activities, as well as seismologists who work on natural seismicity: what is the largest earthquake possible in a given setting?

Before I get into this, a word on earthquake magnitudes (skip this if you already know about magnitudes). Most people are familiar with the Richter magnitude scale running from 0 (i.e. very very small, basically undetectable) to about 9 for the biggest quakes we've ever had. Most people are also aware that it is a logarithmic scale, meaning that an M2 event is 10 times as large as an M1 event, not twice the size. In fact, the Richter scale, which was essentially an arbitrary scaling function, has largely been supplanted by the Moment Magnitude scale, which relates more directly to earthquake physical processes. More specifically, Moment Magnitude is calculated from the seismic moment (Mo), according to:

                       Magnitude = (2/3) log(Mo) - 16.1,

This equation was deliberately scaled so that it followed the Richter scale, so that seismologists (and the public) could still understand magnitudes in the same way. In turn, the seismic moment is defined by the size of the rupture during the earthquake, according to:

                       Mo = G A D,

where G is the shear modulus of the rock, A is the cross-sectional area of the earthquake rupture, and D is the average dislacement that one side of the fault moves relative to the other. So you can see that, if there is no limit to the size of a fault (which controls the rupture area), there is no limit to the maximum magnitude that an earthquake can be. In reality, however, the the thickness of the earth's crust probably imposes an upper limit on the size that an earthquake can be, which is why even the largest earthquakes (on Earth at least - who knows, a planet with a thicker crust could probably have larger earthquakes) don't ever seem to get much bigger than M9.

Now, back to human activities triggering earthquakes. It's unlikely that any of the activities listed above could trigger a crustal-scale fault capable of triggering an M9 earthquake. So what controls the maximum magnitude that could be triggered by mankind's operations? In my last plot I showed the graph used by Art McGarr to explain induced seismicity in the USA and around the world:
BUK is the famous fracking-induced Blackpool earthquake. RMA is the Rocky Mountain Arsenal. YOH is Youngstown Ohio. PBN is Paradox Basin, Colorado. GAK is Guy, Arkansas. BAS is Basel, Switzerland. GAR is Garvin County, Oklahoma. STZ is Soultz, France. RAT is the Raton Basin, Colorado.

McGarr adapted the long established McGarr equation, which originally stated that the total seismic moment released is proportional to the volume of rock extracted during mining (multiplied by G):

            TotalMoment = G x VolRockMined.

This equation was soon adapted to cover seismicity induced by fluid removal and/or injection, with VolRockMined being replaced by dV: the volume of fluid injected. The physical basis for doing so has always been slightly dubious, but empirically it seems to do a reasonable job. After the recent induced seismicity incidences, the McGarr equation has been modified again: now G dV gives not the total seismic moment released, but the magnitude of the largest event alone. This modification should only be strictly true in situations where the single largest event completely dominates the total seismic moment released. So we end up with the modified McGarr equation for injection-induced seismicity:

          MaxMo = G dV.

This is the equation plotted in the above figure, and you can see that it does a good job of fitting the data. But the key thing to note with the McGarr equation is that it is empirical, it does not have any real physical basis - i.e. it is something that is observed, but it doesn't really explain WHY the maximum magnitude should be controlled by the volume of fluid injected (instead of, say, the rate of injection or the change in pore fluid pressure).

A new model for Mmax has been developed by Serge Shapiro of the Freie University, Berlin, which I find very interesting. The Shapiro model suggests that Mmax should be controlled by the size of the fluid-affected zone.

A large earthquake can't just happen anywhere - a pre-existing fault plane must be present on which the quake happens. As an approximate scaling, the maximum earthquake magnitude created by a circular shaped fault with radius r can be approximated as:

          Mmax = 2 log(2r).

So an M3 quake needs a fault of radius 15m, and an M6 quake needs a fault of radius 500m. So, could the size of the area stimulated by fluid injection control the size of the earthquake? Lets consider fluid injection into a completely homogeneous porous rock. The fluid-saturated zone will spread as a sphere (assuming it has similar density to the in situ fluid). The radius of this sphere can be easily calculated from the volume injected. Shapiro argues that the size of the largest fault that can be triggered must scale with the radius of the injected fluid volume.

If a fault is significantly larger than the radius of the injection zone, only a small portion of this will be influenced by it, and this will not be sufficient to trigger rupture. Failure will only occur on faults where the majority of the fault is influenced by injection. As a result, we have a reason to scale Mmax with injection volume. Do the maths and you end up with:
           RADIUS = (3/4)*VOLUME^(1/3)

           Mmax = 2 log((3/4)*VOLUME^(1/3)).

In the plot below, the green circles show Mmax for the McGarr data, which I have calculated using the Shapiro model. You can see that, much like the McGarr model, they fit pretty well. So do we now have a better model to explain Mmax?

Unfortunately, I don't think we do. When you look in more detail at the induced events, you can see that some of the key assumptions of the Shapiro model are not met. The Shapiro model requires that events occur within the immediate radius of the injection zone. The figures below show induced seismic events from Arkansas, Oklahoma and Colorado, with the injection wells marked.

A common theme is that the majority of events occur well below the injection point. Perhaps some of the initial seismicity is triggered in the injection interval, but the majority of the triggered faults lie outside the zone that is directly influenced by injection. The Shapiro model explicitly assumes that the triggered faults lie almost completely within the injection-influenced interval. I think that the Shapiro model is a great attempt to simplify a difficult problem, but to me it seems that more complicated effects involving stress transfer through many layers of rock are acting, and need to be taken into account, to understand the triggering of these faults.

This takes us back to the title of this post: predicting Mmax is still a conundrum. The McGarr model seems to fit the data, but it is only empirical, there is no real physics behind it. The Shapiro model also fits the data well, and has a physical mechanism of control. However, the suggested controlling mechanism doesn't stack up when the events are studied in more detail.

A physically realistic, empirically verified model to predict Mmax still eludes us. We are usually able to  explain post hoc why a particular operation triggered an event. However, we are still not very good at predicting in advance whether a project will induce larger seismic events. If you can come up with a better method, then do get in touch, because a solution will be extremely valuable in a range of industries as discussed above.

In the meantime, we are left with the empirical McGarr equation as our main guide. It should of course be remembered that the McGarr equation does not tell you the maximum magnitude you will get in an operation. The maximum magnitude produced during most operations fall well below the McGarr line. The McGarr line tells you the maximum magnitude you could get if you are very unlucky.

Monday, 1 April 2013

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).