Petroleum companies aiming to pump more oil out of depleted reservoirs while also reducing greenhouse gas emissions blamed for climate change now have a new tool to help them do the job.

Scientists from the University of Calgary and the Alberta Research Council have successfully tested a new method for "fingerprinting" and tracking carbon dioxide (CO2) pumped deep underground.

Such technology will be necessary for using CO2 to efficiently recover more oil from aging reservoirs while also verifying the permanent and safe storage of this greenhouse gas in deep-rock formations, says Bernhard Mayer, a U of C geology and geophysics professor involved in the project.

"This project has shown that this technology can be applied to tracking CO2 in enhanced oil recovery and in carbon sequestration projects," he says.

Bernhard Mayer

The Alberta and federal governments see geological storage or "sequestration" of CO2 as a key technology to keep the greenhouse gas out of the atmosphere, where it accumulates and contributes to global warming and climate change.

At the same time, the oil and gas industry also wants to use CO2 in enhanced oil recovery operations to boost oil production from otherwise depleted oilfields.

Mark Raistrick, a PhD student with Mayer at the U of C, says the tracking method could be coupled with other technologies for long-term monitoring of CO2 storage in oil reservoirs to ensure that the gas isn't leaking through production wells or rock fractures and escaping to the surface.

"The importance of the work is that we are increasing confidence in one of the best mitigation technologies for greenhouse gas emissions," he says.

Raistrick is the lead author of a study applying the method that was recently published in the top-ranked Environmental Science & Technology journal.

"If we can track what happens to injected CO2, then we have the ability to assess whether it's staying in place or not," he adds.

The research team was able to chemically fingerprint and track carbon dioxide injected more than a kilometre deep in EnCana Corp.'s Weyburn oilfield in southern Saskatchewan.

They were able to show that the CO2, as it loosens and 'sweeps out' oil remaining in the reservoir, also dissolves into the brine or salty water within the rock formations - essentially being trapped underground.

"The research confirmed that Weyburn is a good storage reservoir (for CO2)," says Dave Hassan, EnCana's acting vice-president of the Weyburn business unit. "It also confirmed that the CO2 was sweeping into the reservoir" and moving more oil into production wells.

The research is funded as part of the International Energy Agency's Weyburn-Midale CO2 monitoring and storage project - the largest such project in the world.

EnCana injects about 1.8 million tonnes of CO2 per year into the reservoir. The gas is purchased from an American coal-gasification plant in North Dakota and shipped by pipeline across the Canada-U.S. border.

EnCana's $1.5-billion US operation is aimed at recovering an additional 155 million barrels of oil from the Weyburn field over 30 years.

Last fall, Calgary-based oil exploration and development firm Apache Canada also began injecting CO2 bought from the North Dakota plant into its Midale oilfield, next to EnCana's Weyburn field, in a $95-million enhanced oil recovery project.

Apache plans to inject 25 million cu. ft. of CO2 per day into the reservoir over the next 20 years to recover an additional 45-60 million barrels of oil, while also storing the greenhouse gas underground.

The oil found in EnCana's Weyburn field sits in saline water enclosed in carbonate rocks. When the CO2 is pumped more than a kilometre deep into the reservoir, the gas mixes with oil and the saline water and partially dissolves into a bicarbonate compound.

Raistrick and the research team measured the chemical composition and carbon isotopes in the fluids and gases in a total of 40 wells in the oilfield between 2000 and 2005.

The novel part of their technology involved using carbon isotope analysis to track the telltale isotopic "fingerprint" of carbon dioxide injected from the surface into the reservoir.

They observed a decrease in the carbon isotope ratios - indicating that the industrially fingerprinted CO2 being pumped into the reservoir was dissolving into the saline water and converting into bicarbonate.

The concentration of bicarbonate in the reservoir increased, and the CO2 levels also rose by about 10-fold.

"We have the ability to 'see' that the injected CO2 reacts to form bicarbonate," Mayer says. "In the future, this will allow us to assess how much injected CO2 has reacted - and hence how much carbon has been trapped in the reservoir brines."

Ray Knudsen, project director at the Petroleum Technology Research Centre in Regina for the Weyburn-Midale CO2 storage project, says the tracking method could provide a useful tool to help identify not only where the carbon dioxide is moving within the reservoir, but also how much of it is being permanently trapped.

Mayer says the method now must be further refined and applied to a larger part of the oilfield.

Continued monitoring, coupled with advanced seismic, geochemical modelling and other technologies, will help the research team predict the fate of CO2 stored in reservoirs for 100 to 1,000 years - necessary to show that long-term CO2 sequestration can be done safely.

(Mark Lowey is managing editor of EnviroLine, a business publication for Western Canada's environmental industry, and is also the communications director for the Institute for Sustainable Energy, Environment and Economy at the University of Calgary.)