In a recently published paper, geoscientists at Virginia Tech found that shallow sewage injection – not deep sewage injections – can lead to widespread deep earthquake activity in production fields of unconventional oil and gas.
Brine is a toxic by-product of wastewater from oil and gas production. Well drillers remove large amounts of brine by injecting it into subterranean formations, where its injection can cause earthquakes, according to Guang Zhai, a postdoctoral researcher in the Department of Geosciences, part of the Virginia Tech College of Science, and a guest assistant. researcher at the University of California, Berkeley.
The results appear in the May 10 edition of the journal Proceedings of the National Academy of Sciences. Joining Zhai on paper are Manoochehr Shirzaei, associate professor of geosciences at Virginia Tech, and Michael Manga, professor and head of the Department of Earth and Planetary Sciences at Berkeley. In the study funded by the US Department of Energy, the team focused on the Delaware Basin in west Texas, one of the most productive and unconventional hydrocarbon fields in the United States. United.
Since 2010, the basin has seen a significant increase in shallow sewage injections and widespread deep seismicity, including the recent 5.0 magnitude event near Mentone, Texas. Most of the earthquakes were relatively weak, but some were large and widely felt.
“It is quite interesting that the injection above the thick and generally low permeability shale reservoir could cause an earthquake in the deep subsoil, despite minimal hydraulic connection,” Zhai said. “What we found is that so-called poroelastic stresses can activate subsoil faults, which arise from the injection of fluid causing rock to deform.”
Poroelasticity is the resulting interaction between fluid flow and solid deformations within a porous formation, here sandstone. “This finding is significant because it puts poroelastic stresses in the spotlight as the primary driver of basin-wide earthquakes in the basin,” said Shirzaei, who is also an affiliate faculty member at Virginia Tech. Global Change Center.
Yet predicting the amount of seismic activity of sewage injection is problematic because it involves many variables, one of which is the injection depth, Zhai said. Although it is well known that the increase in fluid pressure due to deep injection is the dominant reason for the recent increase in seismicity in the central and eastern United States, it is still questionable. how shallow injections cause earthquakes.
During the study, the team examined how varying amounts of injected brine disrupted crustal stresses deep below the Delaware Basin and how these disturbances lead to earthquakes at a given fault. Zhai added, “Fluids such as brine and natural groundwater can both be stored and move through porous rocks.”
The trio used data analysis and computer modeling to mimic the large volume of fluid extraction from shale reservoirs from more than 1,500 shale production wells between 1993 and 2020, with 400 wells injecting brine. in sandstone formations from 2010 to 2020. To make the scenario realistic, the model included the mechanical properties of rocks in the Delaware Basin, Shirzaei said.
The team found that basin-wide earthquakes mainly occur where deep stress increases due to shallow injection. This means that there is a causal link between deep earthquakes and shallow fluid injection via elastic stress transfer.
“The deep stress change is sensitive to the properties of the shallow aquifer, particularly hydraulic diffusivity, which describes the ease of fluid flow in a porous medium,” Manga said. “One question to ask is why some areas that host many shallow injections lack seismicity. Our approach offers a way to study other important factors that control induced earthquakes.”
In addition to human interventions, the tectonic parameters themselves also help predetermine the magnitude and likelihood of the earthquake, Shirzaei said. This study and future work will provide a viable way to assess induced seismic risks, combining natural and human factors. The ultimate goal: to minimize the risks associated with wastewater disposal during natural gas production until long-term renewable energy technologies become available to everyone.
“As future energy demands increase globally, it remains difficult to manage the huge amount of co-produced wastewater, and shallow injection safe for disposal is more cost effective than deep injection or treatment. water, ”Zhai said. “We hope that the mechanism we uncover in this study can help people rethink how induced earthquakes are caused, potentially helping to better understand them and mitigate their risks.