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Fall 2017

A Better Baseline for Carbon Sequestration

Scientists investigate wettability, a key parameter in modeling carbon sequestration

Robert Charles Choens II

The micromechanical model that best describes the observed behavior from experiments. This model can be combined with reservoir-scale simulations to predict the behavior of injected carbon dioxide. Reprinted with permission from J Botto, et al. (see reference). Copyright 2017 American Chemical Society.

As atmospheric carbon dioxide levels continue to rise, society is increasingly interested in moving towards a more sustainable future, finding new clean technologies and ways to make traditional technologies more environmentally friendly. Technologies such as renewable power and hybrid cars are helping to reduce carbon dioxide emissions, but fossil fuels continue to be an abundant resource for power generation, and non-fossil fuel carbon dioxide sources such as fertilizer and cement plants are potent emitters with few green alternatives. One potential way to mitigate carbon dioxide emissions from these sources would be to prevent carbon dioxide from reaching the atmosphere in the first place through geologic carbon sequestration. Carbon dioxide would be stripped out of exhaust gas and stored underground in depleted petroleum reservoirs and deep saline aquifers for geologic time scales.

Wettability, or the tendency of a fluid to spread out on a surface, is one of the most important parameters in simulating the behavior of injected carbon dioxide in underground reservoirs. Wetting phases spread out onto a surface, and non-wetting phases bead on a surface. When carbon dioxide is injected into underground reservoirs, it can either cling to rock surfaces or bead up and flow into pore spaces, depending on the wettability. The injection pressure required for carbon dioxide to invade the pore spaces of a reservoir is known as the capillary pressure and is directly controlled by the wettability. Knowing the wettability of carbon dioxide in a reservoir allows scientists to predict the carbon dioxide injection pressure, distribution and potential storage volumes during carbon sequestration.

To gain a better understanding of wettability — one of the key building blocks for modeling carbon sequestration — researchers at the Center for Geologic Storage of CO2 (GSCO2), an Energy Frontier Research Center, have combined physical experiments, visual observations and mathematical models on a representative material as well as a natural material from the Mount Simon formation, a pilot carbon sequestration reservoir in Illinois. Previous studies have made wettability measurements on analog materials, but this is the first study to make measurements on an actual carbon sequestration reservoir rock and correlate these to rock structural and mineralogical properties.

Wettability measurements were made in a specially designed pressure vessel with an optical viewing cell to image individual drops of carbon dioxide interacting with simulated brines and rock surfaces. Experiments on the individual minerals that make up Mount Simon sandstone showed that wetting decreases as carbon dioxide pressure increases until it reaches a constant state for the range of conditions expected for geologic sequestration. Wettability measurements on the actual Mount Simon samples are closest to the results for a mix of illite and hematite, suggesting that these minerals, mainly present as thin coatings on the surface of other minerals, are controlling the wettability behavior of the sample.

To upscale these experiments to nature, researchers at the GSCO2 compared results to predictions from different theoretical models. The team examined samples from experiments with an optical microscope, environmental scanning electron microscope and laser-based surface profilometer to characterize and quantify the surface morphology. The measured microscale roughness was used as a basis for the different models. The model that best fit the experimental data was based on trapped brine pockets on the surface. This suggests that the filling of surface pits with native brines is the primary control on wettability, followed by the surface mineralogy of illite and hematite grain coatings.

This research goes a long way towards helping understand the behavior of carbon sequestration on vastly different scales. Experiments verify and validate models based on microscopic mechanisms, and these models can be used to simulate the behavior of injected carbon dioxide on a reservoir scale. These models let scientists predict the behavior and potential success of geologic carbon sequestration: how much pressure will be needed to inject carbon dioxide, how much carbon dioxide can be stored, where will the injected carbon dioxide flow into the reservoir, and how likely is it that the injected carbon dioxide will remain in the reservoir.

Acknowledgments

This work was supported as part of the Center for Geologic Storage of CO2, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Botto J, SJ Fuchs, BW Fouke, AF Clarens, JT Freiburg, PM Berger, and CJ Werth. 2017. “Effects of Mineral Surface Properties on Supercritical CO2 Wettability in a Siliciclastic Reservoir.” Energy and Fuels 31(5):5275-5285. DOI: 10.1021/acs.energyfuels.6b03336

About the author(s):

  • Robert Charles Choens II is a postdoctoral researcher at Sandia National Laboratories and a member of the Center for Frontiers of Subsurface Energy Security (CFSES). His research involves experimental investigations into coupled chemical mechanical effects of injecting carbon dioxide into rocks to predict short-term response and long-term stability of geologic carbon sequestration.

Digging into the Details of Geologic Carbon Sequestration

Microscopic surface roughness and coatings affect carbon dioxide's behavior

This model best describes what the scientists observed from the experiments. Image courtesy of Rose Perry, Pacific Northwest National Laboratory

Producing cement, fertilizers, and certain chemicals create carbon dioxide. If we could strip the carbon dioxide from exhaust and store it in underground formations before it ever reaches the atmosphere, this could reduce the industry’s pollution footprint. The challenge? Knowing how the carbon dioxide behaves once underground. Using samples from the Mount Simon, a pilot carbon dioxide reservoir in the Midwest, scientists found that the microscopic pits on reservoir rock surfaces filled with brine controlled the carbon dioxide’s wettability, or the tendency of a fluid to spread or bead on a surface. Researchers also found that minerals coating rock surfaces, while being a small part of the total rock, exerted a strong control over wettability. Knowing wettability allows researchers to predict the behavior of injected carbon dioxide at large scales and likely success of geologic carbon sequestration projects. Scientists at the University of Texas at Austin performed the work, as part of a collaborative effort within the Center for Geologic Storage of CO2 (GSCO2) led by the University of Illinois at Urbana-Champaign.

More Information

Botto J, SJ Fuchs, BW Fouke, AF Clarens, JT Freiburg, PM Berger, and CJ Werth. 2017. “Effects of Mineral Surface Properties on Supercritical CO2 Wettability in a Siliciclastic Reservoir.” Energy and Fuels 31(5):5275-5285. DOI: 10.1021/acs.energyfuels.6b03336

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.