Microseisms have a macro effect on fracking

frackingThe process of extracting energy-providing oil and natural gas from shale and sand nestled below the Earth’s surface could become a more exact science that produces more power and drives higher profits  thanks to work by Pitt researchers.

Adjunct assistant professor of geophysics Abhash Kumar and professor William Harbert of the Department of Geology and Environmental Science worked with the Department of Energy’s National Energy Technology Lab researcher Richard Hammack to develop advance geophysical workflows for detecting low-frequency earthquakes that shed light on the types of rock deformation that occur during hydraulic fracturing. This knowledge can be used to increase the amount of oil and natural gas that can be produced from unconventional shale reservoirs. 

The previously undetected resources could be a windfall for an industry expected to produce an average of 90.3 billion cubic feet per day of dry natural gas this year and an average of 92.2 billion cubic feet of natural gas per day by 2020, according to the U.S. Energy Information Administration

“This work answers a very important question, which is, when you do hydraulic fracking, where exactly is the energy going?” said Harbert. 

Hydraulic fracturing, also known as fracking, is the process where mixture of  water, sand and chemicals are pumped under high pressure into strategically drilled underground wells. Typically, explained Kumar, companies conduct the process in ways that are designed to create new fractures and increase the connectivity of preexisting fractures that leads to instantaneous improvement in fluid flow. 

“During the fracturing process, large numbers of small magnitude earthquakes are generated that are recorded by surface and/or borehole geophones. Based on the location of the microseismic earthquake, you can try to create a picture of overall volume of rock that’s been fractured. Based on that volume, companies can anticipate how much oil and gas they are going to produce,” Kumar said.

For years, conventional wisdom has been that shale reservoir rocks will show signs of instantaneous fracturing due to high-pressure fluid injection, which is known as brittle deformation. However, research by Kumar, Harbert, Hammack and former NETL intern Erich Zorn, now a Senior Geologist at DiGioia Gray & Associates,  has indicated that rocks with higher contents of clay and other soft materials dominantly undergo non-brittle deformation, which is characterized by slow rate of fracturing over longer periods of time and release ultra low-frequency earthquakes. 

They detailed the findings in the paper, “Long Period, Long Duration (LPLD) Seismicity and their Probable Role in Reservoir Stimulation and Stage Productivity” which is recently published in the journal SPE Reservoir Evaluation & Engineering. 

“We found positive temporal correlation between low frequency earthquakes and increased fluid activity during hydraulic fracturing, suggesting that these unique events are generated in response to extensive reservoir fracturing, triggered by highly elevated fluid pressure,” Kumar said. “These low frequency seismic events of long duration are likely linked with non-brittle deformation in the reservoir during hydraulic fracturing and contribute both in the overall stimulation of the reservoir and gas production,” said Kumar.

Additionally, during field research at a West Virginia hydraulic fracking site, Kumar demonstrated the low-frequency earthquakes that signal non-brittle deformation and potential stimulation of reservoir could be detected using low-cost surface seismometers placed on the ground rather than borehole geophones and fiber optic sensors, which are much more expensive and requires a well to be drilled specifically for monitoring. 

Combined, the findings could give oil and gas industry executives more precise methods to extract greater amounts of energy, cheaper equipment to get the process started and a broader view of the types of areas that are worthy of exploring.  

“Our analysis proposed that overall deformation due to hydraulic fracking doesn’t belong to one particular type of deformation, it undergoes both types, brittle and non-brittle,” said Kumar. “Taking both into account can increase overall estimates of fracturing volume of rock, therefore estimate of oil and gas productivity will be much higher.”

Hammack said the theory still needs to be proved at other well-characterized sites that feature geophysical models and he’s hoping the team will be able to monitor less-characterized areas near Pittsburgh and Morgantown sometime this year. 

And while he credited the work of Stanford University geophysicist Mark Zoback for the first work involving low-frequency earthquakes in shale gas wells, he said Kumar’s work with surface seismometers helped to advance that to new heights.

“No one else had ever gone the lengths to build a whole new science behind it,” Hammack said. 

Deborah Todd,
2/12/2020