The recent release of an independent geochemical consulting firm's report concluding that carbon dioxide is leaking from one of the world's largest CO2 geologic storage projects—located at the Weyburn–Midale oil fields in Saskatchewan—has caused a public relations crisis, not only for Cenovus Energy, the oil company that operates the site, but for the CO2-alleviation strategy of carbon capture and storage (CCS). But scientists are pushing back, arguing that the evidence does not justify the report's conclusions.

Cenovus uses CO2, which arrives via pipeline from North Dakota, for enhanced oil recovery—a technique in which the greenhouse gas (GHG) is injected into an oil reservoir to coax out extra oil. Much of the CO2 (around 18 million metric tons as of July 2010) is then stored 1.5 kilometers underground in a depleted reservoir, where it is supposed to stay trapped. The International Energy Agency (IEA) GHG Weyburn–Midale CO2 Monitoring and Storage Project, a research group affiliated with the Paris-based agency has spent the past decade studying CO2 injection and storage at Weyburn, the goal being to "deliver the framework necessary to encourage implementation of CO2 geological storage on a worldwide basis," according to the group's web site.

Many scientists and engineers argue that geologic CO2 storage, which is today underway at only a few sites in the world, is a necessary weapon against planet-warming emissions. But CCS faces an uphill battle in terms of public perception. Chemically capturing CO2 is extremely expensive, the construction of a network of pipelines for its transport would also be costly, and questions remain regarding the safety of burying millions of metric tons of the gas underground. Is it certain to stay there? A confirmed leak at Weyburn, a site seen as a CO2 storage test case, would embolden critics—and initial news of the alleged breach has already inspired alarming headlines.

The report, published by a Saskatchewan firm called Petro-Find Geochem, was commissioned by a local couple alarmed by strange occurrences (ranging from gaseous bubbling in their pond to unexplained animal deaths) on their property, which sits above the Weyburn oil field. This past July the firm analyzed soil samples taken from the property.

On January 19, however, scientists affiliated with the IEA project—which is funded by 15 government and industry sponsors (including the U.S. Department of Energy) but does not receive cash support from Cenovus—released a statement saying there is "no substantiated evidence in the Petro-Find report to support [the authors'] claim that 'the source of the high concentrations of CO2 in soils of the Kerr property is clearly the anthropogenic CO2 injected into the Weyburn reservoir.'"

Data doubts
To justify their assertions, the Petro-Find consultants cite two values—a measured chemical signature for CO2 in the soil that is similar to the signature of the injected CO2, and an anomalously high CO2 soil concentration measured at a single point on the property. But Steve Whittaker, the senior project manager of the IEA project, tells Scientific American the consultants are overinterpreting their results.

Although an analysis of stable carbon isotopes showed that CO2 collected near the soil's surface had a similar chemical signature to that of the injected CO2, this is not enough evidence to claim that it is the injected CO2, and not gas produced naturally by microbes, plants, and other organic matter in the soil, Whittaker says. "We've performed similar measurements for soil gas and the numbers they have are all within our range of sampling," he notes, adding, "Multiple lines of evidence collected in the area have indicated that CO2 having that isotopic composition is of natural origin."

Petro-Find also claims that the results of its isotopic analysis represent a major shift from baseline soil carbon isotopic values for the area. But this claim is unfounded, Whittaker says, because the consultants failed to measure an appropriate baseline value. According to Wednesday's statement: "The Petro-Find report incorrectly uses the carbon isotopic values of naturally occurring CO2 in an oil reservoir at 1.5-kilometer depth to represent initial values of CO2 in soils at surface," meaning, "the argument for a dramatic shift in isotopic composition is technically flawed."

Regarding the report's finding of a "major anomaly" in the tested soil's CO2 concentration—110,607 parts per million, compared with a reported average of 23,000 ppm for 25 additional samples—the project statement holds, "The concentrations of CO2 in soil gases collected by Petro-Find are similar to those found in prairie soils in the vicinity of Weyburn." Further, Whittaker points out that one data point is not a sufficient basis for scientific conclusions, especially given the highly variable nature of soil gas composition. He notes that soil CO2 concentration depends heavily on soil features, and a lower-lying area with more moisture similar to the area from which Petro-Find collected samples would have more vegetation (especially in the summer), which would thereby produce more CO2 through respiration.

There is also the question of how the injected CO2 could have traveled to the studied property, given that the nearest active injection site is nearly two kilometers away. The Petro-Find report blames "deep-seated faults/fractures," which it says could serve as conduits for migration of the GHG. But according to Wednesday's statement from the IEA scientists, "There is no credible evidence that the surface lineaments proposed by Petro-Find indicate a 1.5-kilometer-deep open fault." The reservoir has "contained oil for tens and tens of millions of years," Whittaker notes. "These are natural containers."

Finally, the statement addresses Petro-Find's implication that high CO2 soil content could lead to unhealthy levels in the air, pointing out that "CO2 concentrations that would be of concern in the air aboveground are quite normal in soil gas at a depth of one meter," the depth from which the consultants gathered the samples, and, further, "There are no data in the Petro-Find report demonstrating dangerous levels of CO2 in the atmosphere."

More analysis required
Susan Hovorka, a senior research scientist at the Bureau of Economic Geology at the University of Texas at Austin and an expert in monitoring geologically sequestered CO2, agrees based on her reading of the report that "the conclusions are way ahead of what can be supported by the data." Before immediately drawing connections between the CO2 in the soil and the injected CO2, efforts should have been made to rule out other sources of the greenhouse gas, she says.

Hovorka says the consultants neglected to perform a relatively simple test that could have given a preliminary determination of whether or not the high CO2 concentration was due to natural processes. "We need to see the CO2 concentration versus that of oxygen and nitrogen," she explains. Because soil microbes take in oxygen from the atmosphere and release CO2, "we can deduce that this process is occurring because oxygen is consumed at a molar ratio of twice the CO2 increase." On the other hand, "if CO2 has come in from someplace else, both [the concentrations of] oxygen and nitrogen will be reduced, by dilution," she says.

Whereas this test would not necessarily confirm anything definitively, "it helps you start to pick apart the system with a large number of simple measurements, and gives you a hypothesis as to the major processes," so that scientists can know which more-expensive tests are necessary, Hovorka says. "Absence of this kind of analysis," she adds, "shows that the study presented is preliminary, and that it is premature to discuss evidence that Weyburn is leaking."