Improved comprehension of fracture dynamics through real-time monitoring

Fractures in the Earth’s interior play an important role in our energy systems—from providing pathways for extracting fossil fuels from rock deep underground to supporting emerging green technologies such as carbon storage and geothermal enhancement—but predicting the properties of these fractures remains a challenge. A new method developed by a Penn State-led team of scientists may paint a clearer picture of fractures as they open and close in real time.

“Fracture dynamics is a long-standing question in seismology, and how fracture opening and closing is critical to understanding this,” said Tiyuan Zhu, associate professor of geosciences at Penn State. “We have developed a new technology that extracts information in real time about the development of fractures, and this image can give us a better physical view than we had.”

The scientists made use of data from arrays of seismic sensors in wells around the hydrocracking, or fracking, site in Wyoming. Fracking involves injecting groundwater under high pressure to open cracks and create flow paths, and is often used to extract oil and natural gas trapped in rock.

One of the arrays sends a seismic signal that travels like sound waves through the rock to compress sensor arrays placed in nearby wells. Scientists said the instruments record baseline and monitoring data, which can show changes in seismic velocity that occur during fracturing.

“When you open a fracture, it will reduce the velocity of the earthquakes,” Zhou said. “Originally, you have one piece of rock, but when you open up a fracture, you’ve created a porous space and you have something filling that space like water or air that’s going to reduce the speed and make your waves a little slower.”

The scientists said the new analytical method allowed the researchers to view changes in the Earth’s interior with a higher spatiotemporal resolution than was possible with previous seismic methods used to characterize fractures.

Their findings, published in the journal Geophysical Research Lettersindicate a clear link between changes in seismic velocity recorded by the arrays and physical changes in the rock.

“The motivation for this study was to see if we could use this seismic data to distinguish when fractures open and close,” Zhou said. “We found that our algorithm can really improve the resolution of what I call this image. With this better image we can better understand the crack dynamics.”

The scientists noticed a slight decrease in velocity near the start of fracturing as pressure increased and fractures likely formed in the rock. However, they unexpectedly experienced an even greater decrease in speed later, after the fractures had already filled with fracturing fluid.

With the help of rock physics modeling, the scientists determined that gas bubbles trapped in the biotreatment agent added near the end of fracking were likely responsible for the larger drop. The compound was added to treat ground contamination at the site.

“A lot of people said when you open a fracture you’ll likely see a big change in velocity, and then when you close it, you’ll see a recovery in velocity,” Zhou said. “But what we see is when you open up the fracture, it causes a smaller change than expected, and then there is a significant decrease in velocity due to the gas bubbles.”

While more research is needed, Zhou said the method could someday help provide early warning signs of possible plume CO2 leaks from carbon sequestration projects or help increase water flow between injection and production wells in systems. Enhanced geothermal energy.

“Penn State has a lot of capacity to do lab experiments on fractures, and we have great researchers here,” Zhou said. “Our work really looks at the field scale. There is a huge gap between the lab and the field and I think our work here helps bridge that gap.”