Earthquakes in Lac: Wastewater injection into the basement will be responsible

What is the seismic situation in Lac?

The first earthquake in the Lac basin was detected in 1969. These events have become more regular since 1976. We record, on average, a few dozen earthquakes of magnitude greater than 1 per year, and a few events of magnitude 2 to 3 per year. Six earthquakes have exceeded the 4 mark (in 1972, 1978, 1978, 1981, 2013 and 2016). We have no historical traces of ancient earthquakes felt by the population. It is therefore important to understand the origin of this phenomenon that seems very recent.

Can we rule out the natural origin of these earthquakes?

Lacq is located near Pau and not far from the Pyrenees. It is legitimate to ask whether tectonic activity associated with the Pyrenees Mountains is the cause. This was formed when the Iberian plate collided with the Eurasian plate 49 to 28 million years ago. It then came under further pressure 16 million years ago, including the African Plate. However, such a collision creates faults, which are areas of weakness in the Earth's crust, and earthquakes occur at the level of these faults.

But in the case that interests us, we no longer observe any movement towards the Iberian and Eurasian plates. And even on the contrary, the earthquakes that we record in the Pyrenees are due to the dynamics of plate separation. Erosion and gravitational action lead to a new distribution of the mass of the mountain range, this is technically called equilibrium.

Lacq is too far away to have earthquakes associated with the Pyrenees balance. On the seismic gradient map, we see a clear separation between Pyrenees earthquakes and those in the Lac basin. The faults in Lacq, traces of plate collisions, are considered inactive.

However, there must be a trigger, i.e. variation of stresses in the Earth's crust, to explain the presence of earthquakes in Lac. In this region, long-term tectonic or isostatic movement of the Pyrenees leads to only minimal pressure changes. On the other hand, some activities of human origin can more strongly influence the distribution of these constraints.

Are these activities related to the industrial history of the Laq basin?

Very likely. Large reserves of oil and gas have been discovered in Lak. Oil has been exploited since 1950, coming from a shallow reservoir (700 meters from the surface). Gas extraction began later, in 1957, in a deeper reservoir 4-5 kilometers deep. Gas exploitation continued until 2013, and since then has been negligible.

After the first earthquake in 1969, extraction was quickly suspected as the cause of the earthquakes. It should be clearly understood that the exhaustion of the gas reservoir leads to a decrease in pressure in the basement being restructured. The gas pressure rose from 66 MPa (650 times atmospheric pressure) when it began operation in 1957 to 1.5 MPa at the end of its operation in 2013.

Is there another hypothesis?

To reduce pressure loss, operators inject wastewater into the reservoir. These fluids come partly from extraction. When we exploit a hydrocarbon reservoir, we also extract large quantities of fluids that we do not need. These are reinjected into the reservoir. In addition, several chemical industries specializing in thiochemistry (chemical conversion of sulfur-containing products) have been established in the Lac region. Some use gas, especially sulfur, extracted from the reservoir. After treatment, the sulfur compounds remain as “wastewater” called effluent. Instead of storing or reprocessing this water, it is injected into the ground. Sometimes, if the quantities are insufficient, part of the water is also injected from the Dry De Pau River.

We are beginning to better understand the relationship between fluid injection into the Earth's interior and earthquakes. Many studies have been conducted, most notably in the state of Oklahoma in the United States, which in 2015-2016 became the state with the largest number of earthquakes in the country, due to industrial activities that used wastewater injection into the basement. Then seismic activity decreased from 2017, along with a decrease in the amounts injected and the closure of many sites.

Can we tell if the earthquakes are coming from gas extraction or water injection?

If extraction is the cause of earthquakes, we would expect to see two specific signs: the epicenter (location at depth) is conveniently located above the reservoir from which the gas is extracted, and as the latter is exploited it recedes and recedes over decades (we speak of depletion), and the epicenters migrate from the center to the periphery.

Did you notice these signatures?

Specialists who studied the Lac site for fifty years faced a difficulty. Thanks to industrial surveys, they have a good model of the Earth's interior and the speed of wave propagation, which is essential for correctly locating earthquakes. But the existing network of instruments did not make it possible to perform a detailed analysis of the different types of seismic waves (longitudinal P-waves and transverse S-waves), which led to large uncertainties in the location of the hypocenters. Not all results led to the same result, but some suggested that extraction was indeed the cause.

But the extraction stopped in 2013 and the earthquakes continue, so what can we conclude?

To find out what's really happening, since 2020, in partnership with the German Research Center for Geosciences, in Potsdam, Germany, we have installed a denser and more sensitive network of sensors that allow us to analyze earthquakes more closely to better pinpoint their location.

An important part of our work also consisted of retrieving all the data from fifty years ago, and analyzing it in the light of our current knowledge to understand why it arrived at one conclusion rather than another.

We divided the data into three categories: those for which we had no location constraints (which accounted for 84% of the data), those for which the accuracy was less than 2 to 3 kilometers, and those for which the accuracy was less than 1 kilometer.

This new analysis allowed us to obtain a more coherent set of data and conclude that the hypothesis of earthquakes primarily related to extraction was not correct. The origin of the earthquakes appears to come from the reservoir at depth and did not follow the effect of oceanward migration. What is even more interesting is that we obtained temporal correlations.

What do you correspond to?

Since 2016, we have had access to daily data on the volume of water injected. We found that there is a threshold effect: if strong fluid injections continue over several days, we observe significant seismic activity. This response is not always immediate and can occur within several weeks to a few months. The relationship is statistically robust. For example, during the winter of 2018-2019, the number of days with strong injections was lower than in other years and the seismic activity that followed decreased.

Despite its strength, we have only found a relationship between these factors. To firmly establish causality, it will be necessary to build a complete model of this phenomenon, in particular explaining how fluid injection, through slow diffusion into the ground, causes earthquakes with a delay.

What are the challenges in understanding these anthropogenic earthquakes?

In recent years, industrial activities that exploit the earth deep within have multiplied: extraction of gas and oil whose reservoirs closer to the surface have been exhausted, hydraulic fracturing, wastewater injection, geothermal energy, and carbon dioxide storage.₂etc. Therefore there are security issues for the local population and economic issues. As such, three sites in Europe have been extensively studied: the gas exploitation site in Groningen, the Netherlands, the geothermal project in Basel, Switzerland, and the Strasbourg site. These three sites caused earthquakes and material damage that led to the cessation or reduction of activity, with financial consequences sometimes amounting to millions of euros. It is important to understand the mechanisms by which disturbances associated with human activity lead to better risk assessment.

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