Stanford Earth Researchers Develop New Methods for Removing Carbon from Atmosphere

Of the roughly two dozen medical CT scanners spread across Stanford’s main campus and medical centers, two can be found in basement labs of the Green Earth Sciences Buildings.

The two scanners are being utilized for some decidedly off-label uses in research headed by Anthony Kovscek, a professor of energy resources engineering at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). The machines in this case have aided researchers extract oil and gas more successfully, and are currently revealing techniques of storing carbon dioxide (CO₂) deep underground while the world continues to depend upon fossil fuels for transportation and energy.

The motorized tables that would typically slide patients in and out of the scanners’ openings rather than support machines made from an overwhelming array of connected pressure vessels, gauges and nozzles shrouded beneath tubing and wires. The larger of the two scanners is around the length of a person and looks like it was devised by a mad scientist to run a time-traveling DeLorean. “I’ll still sometimes say ‘Whoa’ when I see it,” said Muhammad Almajid, a graduate student in Kovscek’s lab.

The ad hoc devices, which Almajid and his colleagues meticulously craft and assemble by hand, are engineered to force pressurized oil, gas, or water through slim cylinders of apparently solid rock, which the scanners then examine with X-rays.

We try to visualize things that people say you can’t visualize, it’s happened more than once, where someone will say, ‘Well, you just can’t do it.’ And I point to our results and say, ‘Well, I beg to differ.’

Anthony Kovscek, who is the Keleen and Carlton Beal Professor of Petroleum Engineering at Stanford.

The lab experiments are meant to recreate, in small, the movement of a range of substances through massive rock formations and to deliver real-world validation of computer simulations of the same methods. This type of experimentation and simulation has aided the United States transition towards energy security by enabling the country to tap huge reserves of formerly unreachable oil and natural gas, such as shale oil.

But what is learned from that bulk of tubes and gauges could also help alleviate the effects of burning 100 million barrels of oil daily, which is projected to continue for a minimum of 50 years while the world moves to renewable energy sources. Results like the ones from Kovscek’s lab are currently guiding new approaches of sequestering the powerful greenhouse gas carbon dioxide, which is discharged from burning fossil fuels, deep within rocks, for eternities, while avoiding leaks and other negative concerns – a plan many experts say is going to be essential in order to avoid the dangers of climate change.

“Greater efficiency in oil recovery and conversion of the energy system to renewables has to happen. But there is a multi-decade period where we really have to make sure that CO₂ emissions are under control while hydrocarbons are still being consumed at high rates,” said Hamdi Tchelepi, a professor of energy resources engineering who frequently collaborates with Kovscek. “The only way to achieve that is through a serious, large-scale deployment of carbon sequestration. It’s not optional. In the eyes of the scientific community, this has to happen.”

Trapping carbon underground

A few of the plans for efficiently extracting natural gas and oil from reservoirs that came out of research like Kovscek’s are also a key step towards carbon sequestration.

Particularly pertinent is a practice called enhanced oil recovery (EOR) by gas injection, which requires pumping pressurized gases into current oil fields to displace or decrease the viscosity of crude oil, rendering it easier to extract. Kovscek says that even the best performing fields still leave almost 50% of oil in the ground. For unconventional resources such as shale rocks, which are a lot harder to extract oil from, the recovery rate can be as low as 5%.

“That means if we don’t do anything further on a well after the initial recovery, we’re going to leave as much as 95 percent of the resource in the ground,” Kovscek said. “That’s a huge waste of all the energy that it took to drill the well and the water injection and hydraulic fracturing that may have been required to operate it.”

CO₂, it turns out, blends really well with crude oil, making it, in petroleum engineering jargon, an exceptional “EOR fluid.” Therefore, for years now, and for its own purposes, the fossil fuel industry has been tuning the process of injecting massive amounts of CO₂ deep underground.

The challenge facing Kovscek and his colleagues is keeping that gas trapped underground and out of the atmosphere for lengthy time periods without unforeseen consequences.

Oil companies are mostly interested in what’s called the active injection period because their goal is to get a return on investment within one or two decades, for sequestration, you need to inject CO₂ for a couple of decades and then turn the valves off. And the physics of what happens after you stop injecting is more complicated than the physics used for oil recovery. You have to truly go a step beyond, because the time scales are so long.

Hamdi Tchelepi, a professor of energy resources engineering

Understanding the dangers

A key focus of Tchelepi’s research is using 4D computer simulations to envisage how sequestered CO₂ will interact with faults deep within the Earth across centuries and eternities. “In some cases, we’ve modeled 3,000 to 4,000 years into the future,” Tchelepi said. “The ideal target for sequestration would be a large, reasonably homogenous rock basin that doesn’t have any fractures. But nature isn’t always so kind to us.”

The hazards related to CO₂ interacting wrongfully with faults are numerous. One risk is that the very pressurized gas might push an already stressed fault towards its breaking point, producing earthquakes. Another risk is that a current crack is expanded by the CO₂, creating a pathway for other, more toxic gases to leak to the surface or pollute aquifers used for  agriculture and drinking. “Carbon dioxide and water is Pellegrino [sparkling water],” Tchelepi said, “but it’s not pure Pellegrino, because CO₂ usually contains harmful impurities that get generated when coal is burned but that aren’t fully removed before injection.”

Trying to lessen the risk of CO₂ seepage is a center of Lou Durlofsky’s research. Durlofsky, who is a professor of energy resources engineering, is adapting methods from oil and gas applications to mimic the impacts of placing CO₂ injection wells in various locations and changing the rates at which injection occurs. With robust computer simulations, Durlofsky’s team can play virtual “what-if” games that allow it to establish well locations and injection sequences that push sequestered CO₂ toward a preferred fate.

The best-possible scenario for trapped CO₂ is for it to react with rock and form long-lasting minerals, but this process needs extremely extended timescales to occur. However, there are two other advantageous outcomes, which can occur much faster: dissolution, whereby the CO₂ becomes dissolved in salt water, and residual trapping, which is when CO₂ becomes broken up into minute bubbles that resist leakage.

“Because the CO₂ is now in the form of discrete blobs, it can’t flow as easily and is effectively trapped in place,” explained Durlofsky, who is the Otto N. Miller Professor in Earth Sciences at Stanford. “By varying where and how CO₂ gets injected in the formation, we can increase the likelihood that one or the other of these outcomes will occur.”

‘Green oil’

Despite the dangers of leakage, Tchelepi said the results from Kovscek’s scanner experiments and his own team’s computer simulations show that safe, long-term carbon sequestration is close at hand — and he thinks the method should be deployed immediately, even though it is still in its early stages. “It’s far from being perfected, but we know more than enough, in my opinion, to start using it,” Tchelepi said. “Clearly, there will be issues and problems, but the only way to deal with them is to put them under the control of science and engineering, to monitor them and to spend the resources to learn from the mistakes. The risk of waiting for perfection is too big. We know enough.”

One concept that Kovscek and Tchelepi’s labs are studying is integrating EOR and carbon sequestration to develop what they call as “green oil.” “If you can take all of the CO₂ that is generated from burning the oil or natural gas that’s extracted in the future from a reservoir, inject it back into the reservoir and store it securely, you would have net-zero carbon emissions,” Kovscek said. “Sequestration is expensive. If we can recover something valuable in the process, it can be used to pay for the sequestration.”

While some might see an inconsistency in helping maximize the extraction of oil and natural gas on the one hand, and working to sequester the CO₂ resulting from the burning of those same fuels on the other, Tchelepi views the two goals as balancing.

You have to be realistic that we will use 100 million barrels of oil a day for the next 50 years, should we do that in a messy, uncontrolled way? Or should we do it with the best possible engineering, maximize the recovery and optimize it by coupling it with sequestration? I’m working on the second option.

Hamdi Tchelepi, a professor of energy resources engineering