Scientists show safer wastewater treatment methods to reduce the number of earthquakes in active oil fields

When humans pump large amounts of fluid underground, they may trigger potentially destructive earthquakes, depending on the underlying geological conditions. This is the case in some oil and gas producing areas, where wastewater is usually mixed with oil and treated by injecting it underground, a process that has triggered large-scale seismic events in recent years.

Now, MIT researchers, working with a team of interdisciplinary scientists from industry and academia, have developed a method to manage this man-made seismic activity and proved that the technology has successfully reduced the number of earthquakes in active oil fields.

Their results, published in the journal Nature, can help mitigate earthquakes caused by the oil and gas industry, not only wastewater from oil injection, but also wastewater from hydraulic fracturing or "fracturing". The team's approach can also help prevent earthquakes caused by other human activities, such as the filling of reservoirs and aquifers and the storage of carbon dioxide in deep geological layers.

Professor Cecil and ADA green, Department of Earth Sciences, Department of earth, atmospheric and Planetary Sciences, Massachusetts Institute of technology, and lead author Bradford Hager of the study, said that triggering seismic activity is a problem far beyond oil production. If we want to safely inject carbon dioxide underground, this is a huge problem that society must face. We show the types of research necessary to do so.

The study was co authored by Ruben Juanes, a professor of civil and environmental engineering at the Massachusetts Institute of technology, and collaborators from the University of California Riverside, the University of Texas at Austin, Harvard University and Eni, an Italian multinational oil and gas company.

Safe injection

Both natural and man-made earthquakes occur on geological faults or faults between two rocks in the earth's crust. In the stable period, the rocks on both sides of the fault are fixed in place by the pressure generated by the surrounding rocks. However, when a large amount of fluid is suddenly injected at a high rate, the fluid stress balance of the fault will be destroyed. In some cases, this sudden injection can lubricate the fault and cause rock slides on both sides and trigger earthquakes.

The most common source of such fluid injection is the treatment of wastewater brought with oil by the oil and gas industry. The field operator processes the water through the injection well, which continuously pumps the water back underground under high pressure.

Oil produces a lot of water, which is injected underground, which leads to a lot of earthquakes, Hagrid pointed out. Therefore, for some time, Oklahoma's oil producing area was more than the magnitude 3 earthquake in California, because all these wastewater were injected.

In recent years, similar problems have occurred in southern Italy. The injection wells of the oil field operated by Eni company have triggered microseisms in areas where major earthquakes have occurred before. In order to find a solution to the problem, the company consulted Hager and Juanes, both leading experts in seismic activity and underground flow.

This is an opportunity for us to obtain high-quality seismic data about the underground and learn how to safely carry out these injections, Juanes said.

Earthquake blueprint

The team took advantage of the detailed information accumulated by oil companies over the years of operation in the Val d'Agri field, which is located in a structurally active basin in southern Italy. The data include information about seismic records in the area dating back to the 1600s, as well as the structure of rocks and faults, and the underground state corresponding to the different injection rates of each well.

The researchers integrated these data into a coupled subsurface flow and geomechanics model, which predicts how the stress and strain of underground structures evolve with the change of pore fluid volume (such as injected water). They linked this model to a seismomechanical model to translate changes in underground stress and fluid pressure into the possibility of triggering earthquakes. They then quantified the incidence of earthquakes associated with different water injection rates and identified scenarios that were unlikely to trigger large earthquakes.

When they used the data from 1993 to 2016 to run the model, the prediction of seismic activity matched the seismic records of this period, which verified their method. They then ran the model in time until 2025 to predict the seismic response of the region to three different injection rates: 2000, 2500 and 3000 cubic meters per day. The simulation results show that if the operator keeps the injection rate at 2000 cubic meters per day - which is equivalent to a small public fire hydrant, a large earthquake can be avoided.

Eni oilfield operators implemented the team's recommended rate in a single water injection well in the oilfield within 30 months from January 2017 to June 2019. During this time, the team only observed some minor earthquake events, which coincided with the short period of time when the operator exceeded the recommended injection speed.

In the past two and a half years, the seismicity in the region was very low, with about four earthquakes of magnitude 0.5, while hundreds of earthquakes of magnitude 3 occurred between 2006 and 2016, Hagrid said.

The results show that operators can successfully manage earthquakes by adjusting the injection rate according to the underlying geology. Juanes said the team's modeling approach could help prevent earthquakes associated with other processes, such as building reservoirs and storing carbon dioxide - as long as there is detailed information about the underground of an area.

It takes a lot of effort to understand the geological environment, Juanes said. He pointed out that if carbon sequestration is carried out in depleted oil fields, these reservoirs may have this type of historical, seismic information and geological interpretation, which you can use to build similar carbon sequestration models. We show that seismic activity can be managed at least in the operating environment. We provide a blueprint for how to do this.

The study was supported in part by Eni.