Here’s an important point about unconventional reservoirs: They’re unconventional. So when it comes to identifying and characterizing those reservoirs, conventional geophysics has bumped up against some locked doors.
Research into seismic anisotropy offers an important key. To understand why, think about the nature of anisotropy and unconventionals.
Anisotropy is directionally dependent variation, so anisotropic reservoirs have different properties in different directions. When the variations are large, that can have a significant effect on seismic velocity.
According to a definition from Schlumberger, “in rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy.” And unconventional reservoirs tend to be very anisotropic.
Shale reservoirs, especially, are anisotropic as all get-out.
“What we’re finding is that the variation (in velocity) can be quite surprisingly large, as much as 30 percent,” said Doug Schmitt. “That has a lot of implications in respect to how you explore, how you produce.”
Schmitt has been involved in seismic anisotropy research as a professor of geophysics and physics at the University of Alberta in Edmonton.
He’s currently on leave and serving as Karen and Stephen Brand professor of unconventional resources in the Department of Earth, Atmospheric and Planetary Sciences at Purdue University in West Lafayette, Ind.
The Perils of Ignoring Anisotropy
By now, most geoscientists have seen one of those microseismic frac-monitoring maps with the clusters of little circles or two-dimensional spheres, Schmitt noted. If anisotropy isn’t taken into consideration, those little spheres might not be where the operators think they are, he said.
“If you’re doing seismic imaging and you’re ignoring anisotropy, things can be in the wrong place,” Schmitt said. “You have all these guys doing microseismic and they have no idea that these rocks are so anisotropic.”
Making the picture even murkier is natural fracturing in unconventional reservoirs, which can “complicate the anisotropy with more anisotropy,” Schmitt observed.
Bottom-line, geoscientists need to be aware of the potential effects of anisotropy on seismic when assessing all reservoirs, not only unconventional shales.
That’s especially important in seismic interpretation, said Xiang-Yang Li, director of the Edinburgh Anisotropy Project (EAP), a research team working under the Seismology and Geomagnetism Programme at the British Geological Survey.
“Shale is anisotropic, the earth’s crust is anisotropic. In the presence of anisotropy, mis-positioning of faults, sand channels, is common if ignored. Therefore, these (geoscientists) should be made aware when interpreting seismic data,” Li said.
“In the presence of anisotropy, if ignored, seismic inversion results would also suffer and therefore, again, care should be taken when integrating seismic inversion during interpretation and reserve evaluation,” he added.
The EAP, based at the University of Edinburgh in Scotland, investigates the application of seismic anisotropy to the needs of the oil and gas industry. A consortium of oil companies, service-and-supply companies and software firms funds the project.
Anisotropy research in shale gas and other unconventional reservoirs comprises one of EAP’s principal areas of investigation, along with converted wave analysis for improved seismic imaging, amplitude-versus-direction technology for fracture characterization, and rock physics for frequency-dependent anisotropy.
“For unconventional reservoirs, such as shale gas and tight gas, shale is the rock with perhaps the strongest anisotropy, and velocity anisotropy can be up to 30-40 percent,” Li noted.
“Therefore, any seismic techniques for predicting shale gas sweet spots has to consider the effects of anisotropy, both in processing and inversion,” he said.
Schmitt said his own interest in seismic anisotropy includes multiple areas, from earth stresses to geomechanical processes to “fluid flow and how the fluid moves through the rock, which would be affected by anisotropy,” he said.
At this point, Schmitt thinks, “It would be good somehow to take a step back and ask, ‘How much error is involved in the location of things if you don’t get the anisotropy into it?’”
Researchers already have made advances in “seismic imaging in the presence of anisotropy, widely acknowledged by the industry, including vertical transverse isotropy (VTI) for shales, tilted transverse isotropy, etc.,” Li said.
Organic-rich shales normally exhibit strong VTI, also known as polar anisotropy, where bedding-perpendicular/vertical seismic velocities are slower than bedding-parallel/horizontal velocities.
The industry’s ability to understand the effects of anisotropy in rock physics models has also been advanced by ongoing research, and work in seismic anisotropy can help in the characterization of fracture systems, Li observed.
That includes research into azimuthal velocity analysis to correct for anisotropy, important to understanding fracture density and distribution and to defining unconventional reservoir properties.
Generally speaking,” Li said, anisotropy studies benefit the oil and gas industry by “improving seismic imaging, help in fracture characterization or in reservoir characterization.” He sees quantitative seismic interpretation based on anisotropy rock physics as an important future area for research.
One of the problems in analyzing anisotropy is a lack of multi-directional seismic data sets, with more than just a down-and-up bounce. Horizontal, orthogonal, azimuthal data would help researchers when reservoir properties differ in multiple directions.
“We don’t get a lot of angles,” Schmitt lamented.
That could mean a bigger role for borehole geophysics, for cross-well seismic tomography and other forms of non-vertical signal capture in future anisotropy research.
With advanced seismic helping so much in offshore exploration and production, and also in characterizing onshore conventional reservoirs, why hasn’t geophysics been able to contribute more to the exploration and development of shales and other unconventional resources?
Seismic anisotropy research could hold one key to a breakthrough. Other advances in geophysics are already pointing the way to a better understanding, according to Schmitt.
“The processing algorithms are getting better and better. Seismic processing is now going into an evolution,” he said.
But with all the recent improvements in data collection and manipulation, there’s still that devil-in-details hurdle for seismic interpretation: You have to understand what the data is telling you.
“They get the numbers out, but what do the numbers mean?” Schmitt said. “Anisotropy has to do with that.”