Groundbreaking: Earthquakes and oil extraction

In recent years Oklahoma City has been subjected to hundreds of earthquakes. A decade or so ago though it was recording only one or two. Why? The answer, a new study suggests, is that it’s down to the oil and gas industry. Contaminated water that’s been used to force out oil under pressure from below ground is being disposed of by injecting it back deep into the bedrock where it’s causing things to slip and slide about. Izzie Clarke...

Izzie - Oklahoma: a state that’s home to four million people, where the voicemail was invented and, in the 20th century, produced the most oil of any state or territory in the United States. In fact, oil was first discovered there by accident in 1859 but now, they’ve got another accident on their hands that’s shaking things up…

Tom - My name is Tom Gernon and I’m an Associate Professor in Earth Sciences at the University of Southampton.

Oklahoma has experienced some pretty intense seismicity over the last decade. Seismicity is the frequency at which earthquakes repeat through time and, in the case of Oklahoma, if you think back to 2008, they had one earthquake which was over magnitude three - by definition, an earthquake that you can feel. In 2015, the number of earthquakes was 903, so that just gives you some sense of the scale of the problem here. These earthquakes are linked to the injection of water underground into deep rock formations.

Izzie - The reason for this process is to extract oil from the ground. Across Oklahoma there are 10,000 or so wells where water is injected at high pressure deep underground to help release oil and gas which is locked up in the rocks. This contaminated water and oil combination then comes back out of the well, is separated and yes, the oil companies have their oil... but what about all that waste water?

Tom - If you think about it, you’ve got 2.3 billion barrels of waste water and you’re in a sort of landlocked state in the centre of the US. Very expensive to ship it elsewhere to be processed and treated and disposed of, so it’s economically more efficient to inject this deep underground because it’s otherwise very expensive to get rid of.

Izzie - This constant cycle of injecting wastewater into the ground not only weakens the rocks but acts as a lubricant, which is part of the problem. The University of Bristol and Southampton University developed a statistical model that was able to analyse a combination of impacting factors…

Tom - What we looked at was data from these injection wells: the injection volumes, injection depths, and injection locations through time. We looked at all these different parameters and the correlation between those different parameters and the seismicity - the earthquakes that are happening - within a given radius of the injection wells. And, I should say, typically the injection depth is about one or two kilometres beneath the surface.

Izzie - The injection depth a few kilometres underground was a huge factor for the frequencies of these earthquakes due to the type of rock lying there…

汤姆-废水通常注入sedimentary rocks which are overlying much deeper crystalline, very old rocks - you can think of granite maybe, or metamorphic rocks which are heavily fractured and so on. Liquids can then find a way downwards basically into the boundary between these two different rock types, and we know that most of the earthquakes in Oklahoma are happening in these deep, what’s called crystalline basement rocks. These are, presumably, very heavily fractured; there’s lots of old ancient faults in there which allows the stress to build up and release.

We have most of the biggest earthquakes which have happened over the past decade have actually been in this rock. The closer you inject to that level, the more earthquakes you’re going to experience. This is incredibly important; it’s not been shown before; it carries a lot of implications for regulation in Oklahoma, and potentially elsewhere. The operators could use this information to change their practices and, potentially, raise well levels to reduce the earthquakes that are happening.

Izzie - Another key finding was that there’s even a time lag. Wastewater would injected into the ground and the resulting earthquake would then hit a few months or even years afterwards. With over two billion barrels of wastewater injected into the ground every year, for Oklahoma worse may be yet to come…

Tom - Another thing we need to consider is that the oil demand hasn’t really been very high over the last two years. The number of earthquakes has dropped slightly since 2015, although we have had some of the biggest earthquakes - I think magnitude 5.8 Pawnee earthquake which struck in September 2016. This is the largest magnitude earthquake that has ever happened in Oklahoma and this caused injury and damage to buildings. This happened at a time when injection had actually been reduced by the regulators. This shows that there are these potentially significant time lags between injection and seismicity.