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Seismic activity occurs regularly, primarily due to Earth’s natural movement but sometimes due to the deep injection of wastewater for disposal or other valuable purposes. Far from the catastrophic images of devastating earthquakes that may come to mind, the most frequent seismic activity is imperceptible and even beneficial to the average American. When it comes to hydraulic fracturing operations to recover deep shale gas, seismic activity provides vital information to scientists about the subsurface stresses and permeable fractures. Seismic activity also enhances production and boosts efficiency.
Hydraulic fracturing involves rapidly pumping vast amounts of fluid underground, creating and enlarging cracks that provide pathways for natural gas recovery. Hydraulic fracturing operations are designed to take advantage of existing natural cracks – routes that are easily reopened by fluid under high pressure. During the process, scientists record microseismicity – unfelt tremors caused by the movement of brittle rock – to estimate the production capacity, or stimulated reservoir volume (SRV), of a shale gas well.
However, oil and gas production estimates based solely on microseismicity often under-predict actual production, in part because there are other deformation mechanisms involved in hydraulic fracturing that increase production but do not cause microseismicity. To improve SRV predictions, NETL researchers are performing a comprehensive analysis of all recorded seismic activity.
In particular, NETL researchers are recording and analyzing low-frequency seismic events of long duration – a type of seismic tremor previously ignored as “noise.” NETL researchers have recorded low-frequency seismic events during hydraulic fracturing at shale gas wells in Pennsylvania, West Virginia and Texas. Scientists now believe that this low-frequency activity reflects the deformation of weak, non-brittle rock within formations of shale, the type of rock predominantly targeted by unconventional oil and gas operations today.
Previous studies have associated low-frequency seismic events with either the slow deformation of weak rocks – typically shale with clay content greater than 30 percent – or slow slippage along pre-existing cracks that do not align with Earth’s natural stress field, caused by gravity and surrounding rock layers. Because these unfavorably aligned cracks are subjected to less shear stress, rock deformation induced by hydraulic fracturing takes place at a slower rate.
The slower rate of rock deformation causes seismic activity that is easily distinguished from the brittle rock microseismic events: Microseismic events caused by the fracture of brittle rock last less than one second and have a frequency greater than 100 Hertz (Hz), whereas seismic events caused by the deformation of weak, non-brittle rock last longer (10 to 100 seconds) and have a lower frequency (80 Hz or less). Analyzing both types of seismic activity offers a more complete understanding of subsurface rock deformation to inform shale gas recovery efforts.
With that in mind, NETL researchers are working to develop a more comprehensive methodology for calculating SRV that incorporates low-frequency seismic events caused by non-brittle rock deformation. “When your picture is more complete in terms of rock deformation, you can have a much better estimation of how much oil and gas can be produced,” said Abhash Kumar, a researcher for AECOM working with NETL’s Field Monitoring Team. He added that improved seismic analysis would allow natural gas producers to create 3D models of subsurface rock cracks, enabling them to boost efficiency by focusing recovery efforts where the greatest deformation occurred.
Enhancing efficiency of natural gas operations helps to cut costs for consumers while increasing production to meet the nation’s growing energy demands. Seismic monitoring is essential for modern energy operations, as it provides crucial information for reservoir management.
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