Induced seismicity refers to typically minor earthquakes and tremors triggered by human activity, deliberate or not. Rarely are humans aware of these mostly low-magnitude events occurring deep underground. However, some regions have over the past decade shown quite significant earthquakes such as a few larger than magnitude 5 events in Oklahoma, which have been linked to wastewater injection in deep disposal wells.
Researchers at Lawrence Berkeley National Laboratory are among the many geoscientists worldwide who are taking a closer look at the possibility of creating unwanted seismicity from injecting fluids into deep rock formations within the Earth’s crust, a routine practice associated with energy extraction or storage in the subsurface, including enhanced geothermal energy production, CO2 sequestration, and wastewater disposal. Their investigations have the potential to improve understanding of the hazards and risks associated with induced seismicity. Because of its similarity to natural seismicity, research into induced seismicity can also be relevant to the study of natural earthquake triggers and processes.
Yves Guglielmi, staff scientist within Berkeley Lab’s Energy Geosciences Division, has co-authored a paper on the subject published yesterday in the journal Science Advances. In their paper, “Stabilization of fault slip by fluid injection in the laboratory and in-situ,” an international team of researchers from the United States, France, and Italy describe their work to replicate the impact of fluid injection on fault slip both in a natural fault in the deep subsurface and on core samples in the research laboratory.
“The paper is about how injected fluids will activate or reactivate a fault and create an earthquake. As geoscientists, we of course want to make sure that fluid injection will not trigger a high-magnitude earthquake, so we are interested in how induced seismicity can be better predicted and potentially avoided.” Guglielmi said. “The standard approach to monitoring fault slip is to measure micro-seismicity via acoustic emission, which is narrowly focused on quantifying the rate at which grains within the Earth’s crust move in response to fluid injection. Our study results brings light to some limitations with this approach, and makes a case for complementing micro-seismicity with methods like ours which are capable of monitoring pressure and strain within the fault in real-time as fluids are being injected.”
For this study, the research team led by Frederic Cappa of the Universite Cote d’Azur investigated what effect fluid injection had on an about 10 meter by 10 meter patch of a natural subsurface fault located in southern France near Marseilles and on rock samples collected from this same fault within the laboratory. Their study was similar to previous studies of earthquake physics in that it used laboratory experiments to characterize friction created within a fault from increasing fluid pressure. But in contrast to previous studies, the research team was able to conduct the same high-resolution measurements of fault slip from increasing fluid pressure in a natural subsurface fault deep underground. Their results show that measurements of fault slip and frictional properties induced by fluid injection in a natural fault that is kilometers in length are consistent with those from equivalent laboratory experiments at centimeter scale.
The team followed exactly the same protocol in the two settings: to slowly increase the pressure within the natural fault underground and fault rock samples in the laboratory to the point of creating seismic instability. In the laboratory, the scientists used a three-dimensional hydromechanical model to test if these frictional properties were consistent to those found in-situ.
“We were examining how high pressure fluids can initiate seismic instability,” Guglielmi said. “We’re most interested in the nucleation – or initial part – of the earthquake.”
A fault is a fracture between two blocks of rock which allows the blocks to move relative to one another. When these two blocks of Earth rapidly slip past one another, an earthquake occurs, whereas when the movement happens slowly it’s known as creep. Faults can be a few millimeters to thousands of kilometers long.
Studies like these are increasingly relevant to geoscience and energy sectors, especially as interest grows around practices like carbon sequestration, the process through which carbon dioxide is removed from the atmosphere for storage in underground reservoirs. According to Guglielmi, this study uniquely takes into account the impact of fluid injection on strain and pressure within the fault in addition to the speed at which fault slip occurs, which is the sole focus of the standard method used to investigate fault slip, micro-seismicity.
“For the geosciences, this research is novel in a number of ways,” says Guglielmi. “For one, this is the first study of its kind to evaluate the influence of fluids on faults at two scales, in the laboratory and in the classical setting. Second, this research is unique in that we are studying the role pressure from fluid injection plays on fault slip characteristics,” said Guglielmi.
The next step for research of this type is to measure fault displacement during industrial large-volume and deep-fluid injections. This can help address the crucial need that exists to explore fault movements in reservoir conditions in order to better calibrate micro-seismicity monitoring.
“A key question is how to consider the aseismic energy produced by fluid injections in the total energy budget, in order to improve prediction of the magnitudes and occurrence of induced earthquakes,” Guglielmi said.