The geology of fracking

Just what are the geological processes involved in fracking, and does it cause earthquakes?
11 October 2022

Interview with 

Andy Woods, University of Cambridge


Oil drilling


Andy Woods works on how fluids behave at Cambridge University and so is just the man to explain what exactly fracking entails…

James - This week we are going to try and get to the bottom of one of the most divisive topics dominating the national discourse here in the UK and abroad at the moment.

Chris - No, we're not talking about Prime Minister Liz Truss per se, but in fact the cause that's being championed by the people she was interrupted by there at the Conservative party conference. They were fracking activists. Now, hydraulic fracturing or fracking is an industrial process. It's undertaken to access pockets of gas that are locked inside rock formations underground. There's reportedly a very large reserve of this fossil fuel that can be recovered this way. So it's seen by its proponents as a vital part of the solution to the current energy crisis. Particularly as ongoing tensions with Russia mean that gas supply security looks extremely uncertain and also remaining very expensive.

James - Critics on the other hand say that lifting the fracking ban is a case of robbing Peter to pay Paul. While it might defer energy concerns, although to what extent is also up for debate, it is in direct conflict with another significant impressing issue - climate change. It may also have onward environmental impacts including groundwater contamination and seismic activity. In other words, earthquakes.

Chris - And this led to some heated exchanges in the House of Commons this week recently between the energy secretary and his shadow counterpart.

Jacob Rees-Mogg - And I'm glad to be able to announce that the moratorium on extraction of Shell gas is being lifted. It is important that we use all available sources of fuel within this country. It is more environmentally friendly to use our own sources of fuel rather than to extract them from other countries and transport them here at great cost, both financially and in terms of carbon. And we need to revisit the seismic limits to ensure that shell gas extraction can be done in an effective and efficient way.

Ed Miliband - They are lifting the ban, but they can't supply the evidence. And the British Geological Survey published today certainly doesn't do it. So in the absence of the evidence, his approach is to change the safety limits. You can't escape a fossil fuels crisis by doubling down on fossil fuels.

Jacob Rees-Mogg - We know that shale gas is safe, it's safe in the United States and has been one of the biggest contributors and has been one of the biggest contributors to the decline in carbon emissions in the United States of any activity that has gone on in that country. We know that seismic activity of 2.5 and below on the Richter scale takes place millions of times a year across the world. Bringing on this supply will bring us cheaper energy, which we need.

Chris - Jacob Rees-Mogg and Ed Milliband. So today we are going to remove the party politics from the equation and we're gonna see what the science has to say. I suppose you could say we are sorting fracked from fiction and if we end up with earthquakes, then friction too.

James - Before the show this week, we asked our Twitter followers what their gut reaction was to whether fracking had their support. 68% said no. 20% said yes, and 12% were unsure. It'll be interesting to see if by the end of today's show, anyone might have changed their mind.

Chris - Well, to kick us off, I'd like to introduce Andy Woods, who is a scientist who studies how fluids behave at the University of Cambridge. He's therefore just the man to explain to us how fracking works and what it entails. Welcome to the program, Andy. What actually is shale gas? Is it the same stuff that we would get from under the North Sea to all intents and purposes?

Andy - Yeah, so shale gas is essentially a large part of it is going to be methane, but it's different from the gas that comes from some of the other reservoirs because it's trapped in shale. And shale is a very fine grain sedimentary material with grains of size of about 10 microns. And the very small pores between those grains is where the shale gas is host, and then there's also some gas adsorbed on the surfaces. And so there's organic matter inside those shale deposits which ultimately is the origin of that gas. But because these rocks are so fine grained, the gas can actually be trapped inside the rock and doesn't flow. In normal situations it'll have extremely slow flow rates.

Chris - The reason that we don't have to frack under the North Sea, for example. Presumably the geology there means you've got big pockets of gas all in communication, whereas when it's in these tiny pores in the rocks, you need to smash the rocks up a bit in order to make those little pockets of gas join up to make big ones.

Andy - So, I think fracking as a process occurs in lots of different situations. But with the shale, the gas is essentially locked into those small pores in the shale. And so what you do is you drill a well down to the shale formation and and then pump up a mixture of water and sand and some chemicals and seal off the rest of the well so that that mixture can actually open up fractures in that shale. And the water carries the small sand particles with it and they end up propping open these fractures so that you've essentially opened up and provided a flow path from the rock back into the well to allow that gas to, to flow. But the gas that flows is actually gas that's very close to that fracture propped open.

Chris - How deep underground are we talking?

Andy - Well this will vary depending on the formation but we're looking at orders one, two kilometres below the surface.

Chris - And how do scientists know it's there?

Andy - If you look at the shale deposits, if you look at the geologic history of the different basins and different deposits, there's hints from that that there will be gas present in different deposits. But if you actually produce a sample of the shale and find organic matter in it and that's been through an appropriate process of being buried in the earth and coming back up again, you'd expect some of that to have produced gas.

Chris - And I follow what you say about the fact that, you break open the rocks and then prop open the gaps with the sand grains. How far can that effect go then? I mean, how far can you reach with one particular shaft, one drill shaft?

Andy - So you may actually drill a well vertically down and then it may run horizontally through the formation and then you'd prop open different parts of that selectively increasing the pressure of the hydraulic fluid in a short section of the well that'll open up the rock there and that fluid may travel a few hundred metres of order away from the well and the particles will hold open that fracture for some time. So essentially you are providing surface area for the gas near that surface area to flow into the world.

Chris - So does this mean then to get an appreciable amount of gas out, you've got to drill lots of holes or can we actually recover a reasonable amount of gas from just one drill site and then you just keep it open for a while?

Andy - So the actual gas that will flow into the fractures that you are creating that are connected to the well is only gonna come from a very small distance away from that fracture maybe of order a few centimetres typically? It depends on the time scale. That gas flows extremely slowly so it's really creating surface area and all the gas trapped in the rock around that fracture is what's going to be recovered. So you produce a very large surface area through this fracking in these deep deposits. And it's the gas around that fracture that's recovered.

Chris - The thing that people are concerned about, really, there are two aspects, aren't there? What happens to the ground? Does it destabilise it, does it cause earthquakes? That's what Jacob Rees Mogg was saying, that we should get used to a few more earthquakes. And also whether or not the chemicals that go down, the mud and the water and so on, whether they go down and stay down or whether they go elsewhere. So what, what do we know, first of all about the earthquake side of things?

Andy - So, we do know there's earthquakes that can happen in this process, because essentially when you are injecting the hydraulic fluid and lifting up fractures, that's essentially cracking open those fractures. So that will produce small earthquakes and they can be tracked by different seismic monitoring techniques. Being able to predict exactly how many earthquakes and the distribution of earthquakes is much more challenging. Because it depends on having a detailed understanding of the fault distribution, the stress state of the rocks, where you are injecting the fluids and seeing if you can actually have faults as the faults open up. So that's a very difficult thing to predict in great detail.

Chris - But will they just be minor quakes? Because the point that Jacob Rees Mogg is making is that there are these 2.5 on the Richter scale quakes happening all over the world by the million. They're very minor and therefore they are of small, of low consequence. So we shouldn't worry about them. I mean, is that true? And also given that we are, we are not in an earthquake zone here anyway in the uk, is this a fairly trivial consequence in the grand scheme of things, you're not gonna trigger a massive great earthquake just by doing this, presumably.

Andy - I think if you're injecting large volumes of fluid in the subsurface and fractures, then I think you will trigger earthquakes

Chris - And the question of what goes down, does it stay down or do those liquids smooth?

Andy - So the liquids are typically you try and recover some of these liquids because if you've opened up this flow path to actually the gas to flow back out up the, well, if that fracture when you produce it, it's full of the water you've injected with the sand and the chemicals and that's obviously going to provide some resistance to having gas coming back out through that flow path. So normally you try and back produce some of that and bring it back to the surface and then dispose of that water but some of the water may, because of effects of surface tension and a process called inhibition, some of that water may flow into other parts of the formation or may be absorbed into some of the clay materials. So some of it will remain down in the system. And some of it you can recover.

Chris - We'll look more at these environmental questions as we go on with the program. But Andy, thank you very much for giving us an introduction to it. That's Andy Woods. He is from the University of Cambridge.


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