Fractures: Small Volume, Big Implications on CO2 Storage
Although encompassing a small percentage of the subsurface volume, fractures frequently act as the primary conduits for fluid flow, solute transport, and geochemical reactions.1 Originating from strain localization in rock, fracture networks are critical to industrial-scale carbon mineralization, a developing technology aimed at mitigating the impacts of climate change by injecting anthropogenic CO2 into underground rock masses. The Center on Geo-processes in Mineral Carbon Storage (GMCS) seeks to understand the coupled geochemical and geomechanical reactions occurring during the injection and subsequent mineralization of CO2 in fractured mafic and ultramafic rocks. These rock types, including well-known basalt and peridotite, are favored for CO2 sequestration due to their high reactivity and ability to rapidly store CO2 in minerals, effectively “locking” the CO2 underground. Indeed, small, pilot-scale operations in basalts (e.g., CarbFix in Iceland and Wallula Basalt Project in Washington) have demonstrated effective and rapid (2–3 years) CO2 storage through mineralization.2,3 Notably, these rocks can exhibit low permeability, which underscores the importance of fractures for the passage and distribution of CO2 within the rock mass. One key unanswered question is how much carbon can be stored in the vicinity of an injection well, before the fracture network becomes clogged thereby impeding carbon mineralization. The evolution of fractures and their role in permanent CO2 storage must be understood to advance carbon mineralization to long-term, large-scale operations.
One drawback of subsurface CO2 storage is that it is impossible to directly observe—to “see”—the processes once CO2 has been pumped into a rock mass. While field tests, including hydrologic testing, fluid sampling, and core drilling, can give estimates of the amount of CO2 mineralized, it can be challenging to understand what geochemical reactions are taking place, where the mineralization is occurring spatially within the rock mass, and how the mineralization is affecting the reservoir integrity. Laboratory experiments and computer simulations provide valuable tools for understanding these interconnected questions.
An Energy Frontier Research Center (EFRC) is established to promote team science, following a multi-disciplinary and multi-institutional approach to tackle pressing scientific challenges. The GMCS EFRC has harnessed this approach by coupling experimental and numerical studies to understand the thermal, hydrological, mechanical, and chemical processes occurring during carbon mineralization at the pore (mm) scale, fracture-porous medium (m) scale, and fracture network (km) scale. In a recent 3D numerical modeling study, GMCS researchers created the first high-resolution pore-scale model that included coupled dissolution and precipitation in a fractured rock mass with evolving fracture geometry4. The authors designed their study to investigate the dissolution/precipitation patterns and evolution of fracture geometry by varying Peclet (Pe) and Damkholer (Da) numbers, two dimensionless parameters that describe the relation between the rate of advection and molecular diffusion, and chemical reaction (dissolution and precipitation) and molecular diffusion, respectively. In this model, a barium fluid was injected into a fracture composed of pure CaCO3, resulting in the dissolution of the CaCO3 and subsequent precipitation of barium carbonate (BaCO3). By systematically varying the parameters in both a smooth fracture and a rough fracture, they determined that the interplay between diffusion, advection, and reaction plays a vital role in the evolution of a fracture and its capacity to transport and store CO2.
An example of this can be seen in Figure 1a–b, which depicts how the fracture geometry and the relation between the dissolving and precipitating mineral varies as a function of the relative values of Da(dissolution) and Da(precipitation), at a constant Pe number. When the dissolution rate is low relative to precipitation, the boundary between the dissolved and precipitated minerals on the fracture surface is uniform (1a). In contrast, when both the rate of precipitation and dissolution are high, the topography of the mineral surface is highly variable (1b), a fascinating result the authors are still investigating. Furthermore, Kang et al. (2023) discovered that in a low advection, high precipitation flow regime (low Pe number), the precipitation of BaCO3 is concentrated at the inlet of the channel (1c), while mineralization concentrates downstream when advection dominates (1d). These findings demonstrate that under specific flow regimes, mineralization can lead to clogging of the flow channel, limiting access of the CO2 charge to the rest of the fracture.
Complementary to this model are preliminary experimental findings derived from Asem et al. (2023), who investigated the evolution of fracture permeability and topology during CO2 mineralization. Their experiment involved injecting a fluid supersaturated with respect to calcite (CaCO3) through a fractured rock to stimulate the precipitation of calcite. Based on crack gage measurements, it was observed that the extent of mineralization led to the opening of the fracture but a reduction in permeability. Whether this mineralization could eventually shut down fluid passage through the fracture, with the result of clogging, will be investigated in future experiments. An additional test was conducted to understand where mineralization will nucleate within a fracture, i.e., whether it forms on the walls of the fractured rock (heterogeneous nucleation) or suspended in the fluid (homogeneous nucleation) – a phenomenon that remains ambiguous in natural fractured systems. The authors created an artificial fracture (interface) consisting of two smooth plexiglass surfaces. They measured the upward displacement of the top plexiglass surface over time due to calcite precipitation. It was found that the mineralization within the fracture formed preferential flow passages and resulted in a decrease in permeability, even when the nucleation occurred in the solution suspension. By observing how a fracture is affected by carbonate mineralization, these and future experiments will provide important parameters for modeling studies.
Experiments are essential in providing key parameters for model execution. In turn, these models extend laboratory scales by accurately simulating chemical and physical events—spatially and temporally—and permit a more rigorous evaluation of real-world systems. Thus, it is imperative for these methods to be closely interlinked. Experimental and modeling studies are underway at GMCS aimed at understanding the complex geochemical and geomechanical processes that enable rapid CO2 storage in fractured mafic and ultramafic rocks. Through these collective efforts, GMCS aims to clarify the role of fractures in CO2 transport and storage during industrial mineralization. Ultimately, the overarching goal of the center is to guide efforts in the establishment of mineral carbon storage operations that will permanently lock CO2 in the subsurface.
(1) McGrail, B. P.; Schaef, H. T.; Spane, F. A.; Horner, J. A.; Owen, A. T.; Cliff, J. B.; Qafoku, O.; Thompson, C. J.; Sullivan, E. C. Wallula Basalt Pilot Demonstration Project: Post-Injection Results and Conclusions. Energy Procedia 2017, 114, 5783–5790. https://doi.org/10.1016/j.egypro.2017.03.1716.
(2) Snæbjörnsdóttir, S. Ó.; Sigfússon, B.; Marieni, C.; Goldberg, D.; Gislason, S. R.; Oelkers, E. H. Carbon Dioxide Storage through Mineral Carbonation. Nat. Rev. Earth Environ. 2020, 1 (2), 90–102. https://doi.org/10.1038/s43017-019-0011-8.
(3) Kang, Q.; Liu, M.; Carey, J. W.; Viswanathan, H. S. 3D Pore-Scale Modeling of Mineral Dissolution and Precipitation in Fractured Rocks. In All Days; ARMA: Atlanta, Georgia, USA, 2023; p ARMA-2023-0737. https://doi.org/10.56952/ARMA-2023-0737.
Research supported as part of the Center on Geo-process in Mineral Carbon Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0023429.
(4) Asem, P.; Detournay, E.; Guzina, B.; Labuz, J. F. Precipitation of Calcite in a Rock Fracture. In All Days; ARMA: Atlanta, Georgia, USA, 2023; p ARMA-2023-0799. https://doi.org/10.56952/ARMA-2023-0799.
This work was supported as part of the Center on GeoProcesses in Mineral Carbon Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, at the University of Minnesota under award # DE-SC0023429.