The most recent U.N. climate report describes carbon dioxide removal and storage as "unavoidable" for dealing with emissions in the next few decades from hard-to-decarbonize industries such as cement and steel manufacturing. This image shows workers outside a cement factory. (Image: via Getty Images)

As emissions and temperatures continue to rise, there’s a growing scientific concern that countries will need to actively remove and manage carbon dioxide (CO2) for the world to avoid warming beyond the threshold of 1.5 °C above pre-industrial levels.

One method studied often for keeping removed carbon out of the atmosphere long-term involves injecting CO2 into rock formations deep underground. However, some kinks still need to be worked out.

“Injection of carbon dioxide in storage formations can lead to complex geochemical reactions, some of which may cause dramatic structural changes in the rock that are hard to predict,” said Ilenia Battiato, the study’s Primary Investigator and an Assistant Professor of Energy Resources Engineering at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth).

For years scientists have simulated fluid flow, reactions, and rock mechanics to try to predict how CO2 injections, or other fluids, will affect a rock formation.

However, existing models don’t reliably predict the full consequences of geochemical reactions, which often produce tighter seals but can also lead to cracks and wormholes, which may allow buried carbon dioxide to affect drinking water or escape to the atmosphere.

“These reactions are ubiquitous. We need to understand them because they control the effectiveness of the seal,” Battiato said.

The new solution uses a microfluidics device — often called a “lab-on-a-chip” but in this instance it’s a “rock-on-a-chip,” as the technology involves embedding a tiny sliver of shale rock into a microfluidic cell.

To demonstrate the device, the researchers used eight rock samples taken from the Marcellus shale in West Virginia and the Wolfcamp shale in Texas. They cut and polished the slivers of rock to bits about the size of a few grains of sand, with each containing varying amounts and arrangements of reactive carbonate minerals. The samples were then placed into a sealed-in-glass polymer chamber with two tiny inlets left open for acid-solution injections. High-speed cameras and microscopes allowed step-by-tiny-step viewings of how chemical reactions caused individual mineral grains in the samples to dissolve and rearrange.

“If you can see it, you can describe it better,” said Co-Author Anthony R. Kovscek, the Keleen and Carlton Beal Professor at Stanford Earth and a Senior Fellow at Stanford’s Precourt Institute for Energy. “These observations have a direct connection with our ability to assess and optimize designs for safety.”

“Nothing of this sort exists for really looking at how the grain shapes are changing,” he added.

“Researchers need to incorporate this knowledge in their models to make good predictions about what’s going to happen once you inject CO2, to make sure it stays there and doesn’t do strange things,” Battiato said.

The team plans to use the same platform to study geochemical reactions triggered by injections of wastewater from oil production, desalination plants, or industry, as well as hydrogen.

Here is an interview with Kovscek.

Tech Briefs: What's the next step in your research?

Kovscek: We plan to proceed in a number of complementary directions. First, we are planning to quantitatively translate our findings and observations from the millimeter scale to the laboratory scale (decimeter) and beyond (meters, km). We are particularly interested in assessing to what extent our direct observations of the evolution of rock porosity and surface areas during acidification will improve our ability to better predict the interactions between acidic fluids and the host rock at much larger scales. The main objective is to use this platform to experimentally determine parameters in large-scale numerical models, used for design and optimization of underground storage systems. Currently, such parameters are based on strongly idealized systems, which can greatly differ from real rocks. Better characterization will improve the design of such systems (e.g., site selection, operation conditions).

Second, we are planning to generalize this experimental platform to investigate the interactions between rocks and different fluids relevant to energy storage, such as hydrogen.

Finally, we plan to expand the experimental capabilities to study samples under mechanical stress. As the rock undergoes structural changes due to reactions with one or multiple fluids, it is also subjected to overburden pressure due to the rock layers above it. This can cause pore collapse and other important structural changes that can affect storage capacity.

Tech Briefs: When will this technology be readily available?

Kovscek: The good news is that this technology is available today. We can already test rocks for their likelihood of reaction, the types of grains dissolved, and the minerals deposited. What remains is to add more of the complexity as in the answer to the first question.

Tech Briefs: Will it catch on? What are the pros? Cons?

Kovscek: Microfluidics, sometimes referred to as a lab-on-a-chip, is a widespread approach throughout various industries. There is no reason to believe that it won’t catch on here for testing of rock samples. Our platform offers unique capabilities for observing rock changes during reactions in real time. The equipment involved is a little expensive (it requires high-end optical and non-optical imaging) and requires specialized expertise. We believe that initially high costs and specialization will lead to deployment in academic settings followed by wider adoption as industry seeks to make decisions in a timelier fashion in the field.

Tech Briefs: How will this impact underground storage?

Kovscek: Better characterization and fundamental understanding leads to better decision-making and models. Better models lead to better predictions, and, as a result, better and more optimized designs. This could lead to improved site selection and better management of field operations. More specifically, the highly visible nature of results, as well as its high-throughput capabilities mean that more samples can be tested and analyzed in comparison to the past.

Tech Briefs: Are you working on other such technologies?

Kovscek: Yes. The reactivity of rocks is a general problem in CO2 storage, hydrogen storage, and any water injection where the injected water and the in-situ water don’t have the same composition. This is part of our effort to understand how storage systems evolve as non-equilibrated fluids are introduced. We are planning to expand such experimental capabilities to include the impact of mechanical deformations during reaction, as well as similar setups to study rock-hydrogen interactions for subsurface H2 storage. We also plan to develop computational models that can appropriately capture and model at large (reservoir) scales the physical-chemical processes observed. The combination of experimental characterization and advanced modeling capabilities can be directly used in assessment, design, and optimization of storage systems.