Fractures in subsurface geological formations play a critical role in the development of permeability, enhancing deliverability of oil and gas to well bores. The detection and characterization of fractures -- including their orientation, density and age of generation -- represents not only an art form, but a rigorous study in structural geology.
In Alberta’s Western Canadian Sedimentary Basin (WCSB), where oil and gas companies have shifted their exploration focus to tighter reservoirs in the foothills and the deep basin, the presence of open fractures can make -- or break -- a commercial well.
Despite the economic importance of fractures, most geologists working in Calgary’s downtown oil patch practice “desk-top” geology, creating maps of the subsurface but rarely visiting the field to measure fractures in rocks outcropping at surface.
During the past two decades, the loss of in-house research and development capabilities in the global oil and gas industry has left a void in technical expertise in structural geology. However, a unique R&D partnership, based upon mutual synergies, is developing between the oil and gas sector and academic institutions across Canada.
Oil companies supply proprietary data sets and badly needed research money; in turn, universities procure masters and doctoral graduate students with professorial supervision.
Deborah Spratt is a professor of geology at the University of Calgary’s Department of Geology and Geophysics, specializing in structural geology and studying fractures from “the micro-scale to the seismic scale to the mountain-building scale.”
Her academic research is focused, in large part, on predicting where open fractures will deliver oil and gas to well bores from subsurface reservoirs.
“A lot of oil and gas companies have given up on fractures,” Spratt said, “because, with fractures, you really need time, which is what most oil companies don’t have.
“Graduate students have time.”
Freedom from Bias
Spratt’s research in fracture characterization falls under the umbrella of the Fold-Fault Research Project (FRP), which she co-founded in late 1994 with Queen’s University in Kingston, Ontario. Since its inception, the FRP has successfully attracted industry funding and participation -- in 2005, the consortium included 15 international and Calgary-based oil and gas companies and five industry software companies.
Integrating field mapping with subsurface seismic and well bore data, Spratt is studying the role that fractures play in oil and gas exploration and production in the foothills. She’s currently investigating the Pardonet, Baldonnel and Belloy formations in northeastern British Columbia, and in the Turner Valley Formation in central Alberta.
The academic environment affords both professors and graduate students the luxury of examining the big structural picture from an unbiased perspective.
Malcolm Lamb, one of Spratt’s doctoral candidates, is in his third year of research. Lamb, an AAPG member, is a part-time graduate student -- that’s because his “day job” as the geology manager for Schlumberger keeps him very busy.
With one foot planted firmly in each camp, Lamb recognizes the value of the academic research being conducted by the FRP.
“Industry has been in a holding pattern for a long time,” he said. “It’s created a very competitive playing field between all companies -- they don’t like to share information.”
Lamb describes the FRP as “an awesome venue, a really good sharing environment,” and institutions like the University of Calgary, he adds, play a huge role in dissemination of information to the oil and gas industry.
“There’s a blend of academics advancing knowledge while serving industry needs and providing economic benefits,” Lamb said of the balance that researchers continually strive to achieve.
Wearing his graduate student’s hat, Lamb likes the artistic freedom to look objectively at a structural geology problem.
“I have absolutely zero stake in whether it works,” he said. “I can be completely unbiased.”
Through joint ventures with industry, the FRP’s researchers have access to all the tools available in the structural geologist’s modern-day tool kit -- downhole wireline logs, downhole optical sensors or cameras, cores, thin sections, outcrop studies, sophisticated visualization software, aeromagnetic data and 2-D and 3-D seismic data. These diagnostic tools have been developed by the oil and gas industry, but they are being used in a slightly different way by university researchers.
“We’re not throwing out data,” Spratt said. “Even if our data comes from a dry hole, you can learn something from it ... If you only have two wells, you might think that fractures are random.”
Spratt’s investigations have led her to just the opposite conclusion; she doesn’t believe that fractures are randomly distributed.
“We’re looking for populations of orientations of fractures, and actually finding some that you wouldn’t predict,” she said.
Spratt describes discovering one extra set of fractures in the foothills of Alberta and British Columbia. In some cases, she says, this newly documented fracture set can be the dominant one in sedimentary strata, adding significantly to the permeability and commerciality of oil and gas-bearing reservoirs.
Related to deep-seated structures that pre-exist the formation of the Rocky Mountains, this extra set of fractures is not readily predicted with seismic data. But maps produced from aeromagnetic data indicate the existence of older, structural lineaments that parallel this extra set of fractures discovered by Spratt in the WCSB.
Can You Predict Fractures?
During the summer of 2004, Spratt and Lamb conducted field studies on top of Turtle Mountain, located in the Crowsnest Pass of southern Alberta. Infamous for spawning the “Frank slide” in 1903, Turtle Mountain unleashed 82 million tons of highly fractured limestone, killing 70 people in the coal mining community of Frank.
In 2003 -- exactly 100 years later -- modern-day science was brought to bear when a chunk of rock fell off the highly fractured front face of the mountain. In response, the FRP initiated a geological and geophysical monitoring project designed to predict future slides.
“I’ve seen fractures on the top of Turtle Mountain ... ones that you could drop a mini-van into,” Spratt said.
To kick off the project, the FRP flew a drilling rig and well-logging equipment, via helicopter, to the top of the Turtle Mountain. Drilled with air and foam, the well was designed to reach a depth of 200 meters -- but drilling was stopped at 61.3 meters, after encountering lost circulation into large, open fractures. According to Lamb, the researchers feared losing the drill rig into a void space.
The Turtle Mountain well bore was logged, using an Advanced Logic Technology Obi40 digital optical televiewer. The tool consisted of a directional device and an imaging device, providing a 360-degree, continuous picture of the borehole’s surface with resolution up to 0.5 mm and 720 pixels of azimuthal resolution.
A multitude of fractures and vugs were documented in the well bore, including several large, open fractures.
The subsurface data from the well bore was correlated with seismic data and tied back to the surface, using field mapping, ground penetrating radar images and aerial photographs.
“It’s unusual to drill holes into surface structures that don’t produce (oil and gas),” Spratt said, describing the unique value of the data set collected from the Turtle Mountain project. The formations exposed at surface at Turtle Mountain -- when buried at depth -- produce prolific quantities of natural gas elsewhere in the foothills of southern Alberta.
By looking at the big picture -- and by combining her surface field studies with “real life” well bores and seismic data -- Spratt hopes to be able to predict the sweet spots for fractures.
“Rather than thinking of each well as its own case study,” she said, “is there a unifying way to predict fractures?”
The Mystery Data Sets
Don Lawton, co-founder of the FRP and a professor of geophysics at the University of Calgary, is trying to answer the same question. Lawton, who holds the chair in Exploration Geophysics, is looking for “any diagnostic, robust signatures” in seismic data in the foothills that point to fractures.
“We would like to use Spratt’s measurements and observations to validate what we see in the seismic,” says Lawton, an AAPG member. “We can only see big things; she looks at small things. But, we can overlap our scales.”
For example, a seismic wavelength is on the order of 100 meters; in contrast, Spratt measures fractures in zones ranging from less than one meter to 50 meters wide. According to Lawton, surface field measurements of fractures and subsurface measurements in well bores provide the necessary “ground truthing” for his seismic investigations.
“We see changes in seismic velocity with respect to a change in the orientation of fractures,” Lawton said.
Velocity changes, he added, originate from changes in the orientation (azimuth) of fractures relative to the source and receiver layout for seismic data acquisition in the field. Additionally, Lawton has noted changes in seismic reflection strengths or amplitudes derived from fractured layers of rock.
He calls this phenomenon “AVAZ” or “Amplitude Variation with Azimuth.”
Lawton and his FRP graduate students are testing their AVAZ theories on two “mystery” 3-D seismic data sets, acquired by the oil and gas industry somewhere in the foothills of the WCSB. A well-recorded 3-D data set, he explains, contains azimuthal information from 0 to 360 degrees. By extracting different azimuthal subsets (0 to 90 degrees versus 45 to 90 degrees), Lawton is attempting to correlate differences in seismic azimuths to fracture orientations.
To date, his research has yielded “tantalizing results.”
“It’s just at the beginning of the “S” curve of the AVAZ technology,” Lawton said. “In theory, it should work. It’s just a question of the magnitude.”