What happens in the reservoir following hydrofracturing? Microseismic monitoring is providing some important answers.
Wouldn’t it be great if we understood everything that was happening in hydraulic fracturing?
We aren’t there yet.
“We’ve made tremendous progress. There’s no question about that. But I don’t think you’ll find anyone who would say we’ve optimized what we do,” said AAPG member Mark Zoback, professor of geophysics at Stanford University’s School of Earth Sciences.
Recent work by Zoback and his colleagues found that slow slip along misoriented or poorly oriented faults can contribute to high production rates in very low permeability reservoirs.
That response to hydraulic fracturing doesn’t show up in routine microseismic monitoring. Understanding this overlooked slippage is key to knowing what occurs in the reservoir following hydrofracturing, Zoback said.
How important is it?
“I don’t think shale gas could be produced in many of these reservoirs if this wasn’t happening,” he said.
In March, Zoback will be the kickoff speaker in Denver at the 19th annual 3-D Seismic Symposium, jointly sponsored by the Rocky Mountain Association of Geologists and the Denver Geophysical Society.
He will speak on “Reservoir Geomechanics Applied to Stimulation of Shale Gas/Tight Gas/Tight Oil Reservoirs.”
Zoback’s highly regarded text, “Reservoir Geomechanics,” is now in its fifth printing from Cambridge University Press, and he brings a reservoir perspective to the development of unconventional resources.
Geologists have long believed that the presence of existing faults and the orientation of those faults can contribute to high production rates in shale gas plays.
Zoback’s work indicates those considerations can be even more important than most geologists expected.
More Than Micro
According to Zoback, improved knowledge about hydraulic fracturing’s effects on the reservoir can be seen as a third wave of understanding in unconventional resource development.
At first in shale gas plays, “the concept was to make the biggest fracs possible,” Zoback noted. “High gel content fluid was used to carry as much sand as possible as far as possible.”
Later, more hydraulic fracturing jobs utilized “slickwater,” or low viscosity fracturing fluid with friction-reduction additives.
“People then realized you’re not making that big of a frac, and you’re not using that much sand,” he said.
The burst of stimulation from hydraulic fracturing was compared to a micro-earthquake around the well bore, and technicians used monitoring of the microseimic activity to image fracture growth and subsurface response.
That captured the immediate effects of the hydrofracturing. But in addition to induced fractures, other faults in the reservoir can and do become active, according to Zoback.
“I’m saying there are other faults that are slipping slowly, and they are contributing to the production,” he said. “More was happening than the microseismicity.”
Faults misoriented for slip in the stress field usually would not be expected to be capable of slipping on their own, Zoback said. High pore pressure from hydraulic stimulation can induce slip, however.
He called the misoriented faults “old and dead” faults.
“This is the story for these misoriented faults: The stresses in the Earth are pressing them shut,” Zoback explained. “The high fluid pressure in the hydraulic fracture can reawaken or reactivate these faults.”
Zoback said the effect of fault slipping on production helps explain why microseismic has not been a good predictor of production rates resulting from successive hydraulic fracturing stages.
“The conventional model of what happens in hydraulic stimulation is that you’ve got these traditional planes surrounded by microseismic events,” he said. “But it’s actually very difficult to account for the gas production based on microseismic.”
All In the Timing
In a paper he prepared with Arjun Kohli, Indrajit Das and Mark McClure from Stanford University, Zoback wrote:
“The fact that elevated pore pressure initiates slips on misoriented planes is well known from fault mechanics.
“What is not well known is that while slip on a critically stressed fault could propagate rapidly as a micro-earthquake when triggered ... induced slip of misoriented planes will propagate slowly and go undetected during normal microseismic surveys.
“Simply put, the reason for this is that slip on a portion of a misoriented fault will only occur when the pore pressure is anomalously high. Thus, slip will propagate along a misoriented fault only as rapidly as the pore pressure propagates along it.”
In contrast to the fracture growth measured by microseismic, the pressure-induced slow slippage of misoriented faults appears to persist for tens of seconds over tens of meters, he said.
Improved knowledge of reservoir changes from hydraulic fracturing brings several possible implications. One is that shale gas development should proceed from a predictive perspective, rather than hydrofracing with regularized spacing, volumes and rates, Zoback observed.
Zoback’s studies drew on data from hydraulic fracturing in the Barnett Shale and laboratory friction measurements on samples from the Barnett, Eagle Ford, Haynesville and Fort St. John shales. The principles of slow fault slippage generally apply everywhere, he noted.
“I think it’s a fairly ubiquitous phenomenon. It’s not limited to this one case,” he said.
Composition of shales does make a difference, with higher clay content being associated with slower slipping, Zoback said.
Unique Challenges
Just as high pore pressure can reactivate misoriented faults, pore pressure increase has been cited as a cause of induced earthquakes from disposal of wastewater in injection wells.
Hydraulic fracturing is different, “because in any given hydrofrac you’re only pumping for about two hours and you’re affecting only a small volume of rock,” and noticeable tremors from hydrofracturing are very rare, Zoback explained.
The combination of horizontal drilling and hydraulic fracturing led to a revolution in developing low-permeability reservoirs in the United States. In other parts of the world, the same concepts haven’t always brought success.
Zoback said reservoirs and production challenges in unconventional resource development are unique and take time to understand, anywhere.
“In the United States, the Floyd Shale was kind of a bust in Mississippi and Alabama. Not too many people know about it,” he noted.
Success should spread as our understanding of specific resource plays increases and our knowledge of what results from hydraulic fracturing improves even more, according to Zoback.
“I’m very optimistic,” he said.