Greenhouse Gases and Clay Minerals

Environmental interactions are instrumental to sustainability of life on our planet. Evolution of life adapts to the environment and encompasses a wide range of phenomena, from the emergence of major lineages to mass extinctions. This book invites you to learn the science of carbon management, with particular focus on carbon capture and storage in geological formations. Rather than jumping to any conclusions, we will review the current state of knowledge and discuss the risks, available options, and likely scenarios. In the early chapters, students and avid readers who are passionate about the global challenges, can learn the basics about greenhouse gases and their role in the Earth’s radiative balance, about natural carbon cycles and the challenges of controlled carbon management (capture, storage a.k.a. sequestration, and utilization) including the role of fossil fuels and common geological materials. More advanced chapters are reserved for recent research developments in understanding the nature of interactions between greenhouse gases and ubiquitous geomaterials, such as clay. Motivation for this exciting effort is to elucidate the state of understanding in the science of clay—CO₂ interactions, to bridge between traditional and modern geoscience perspectives, and to introduce consistent terminology that will facilitate communication between different generations and areas of scientific and technological inquiry.

The chapter begins with a comprehensive review of the representative greenhouse gases and their role in the Earth’s radiative balance. Eight greenhouse gases (CO₂, CH₄, N₂O, HFC-23, HFC-134a, PFC, SF_6, and NF₃) and their contributions to radiative heating of the atmosphere are analyzed, and the mechanisms associated with global warming potential are discussed. To illustrate the reported evidence of changes in the radiative balance attributed to greenhouse gases, the correlation between the global temperature rise and the increase in atmospheric concentration of CO₂, the most important greenhouse gas, is presented. The primary anthropogenic CO₂ emission sources and the amount of CO₂ emissions by region, and the disturbance of anthropogenic CO₂ emission to global carbon cycle are discussed. At the conclusion of this chapter, a brief review of global actions to mitigate anthropogenic CO₂ emissions is presented.

The term clay is used interchangeably for the particles and the minerals, the latter commonly referred to as clay minerals to distinguish them from the clay particle size. Clay sediments are typically deposited in quiet-water environments, settling out as fine-grained mud, which may then be buried and lithified into shale. The quiet-water depositional environments are favorable for deposition of organic material as well, which over geologic time, may result in the shale becoming a source rock for petroleum and natural gas. Hydrocarbons have traditionally been produced from porous and permeable reservoir rock, where they had migrated from source rock and become concentrated in geologic traps. The recent development of “unconventional resources” like shale gas and tight oil has allowed hydrocarbons to be produced directly from the source rock. Conventional natural gas and oil reservoirs that have been depleted of hydrocarbons provide a viable option for secure carbon storage because there is a known trap and seal. CO₂ can potentially be used for enhanced recovery of the hydrocarbons and for pressure management in shale, to minimize the loss of permeability that comes from increased net stress during drawdown. As a side note, natural clay-rich geomaterials can be used in agriculture, industrial processes, and for clay liners in chemical and radioactive waste disposal sites. The purpose of addressing geomaterials here is to give the readers an idea of the enormous breadth of each subject and point them toward other resources for additional information.

When discussing clays, it is important to understand what they are, their origins, their chemical and physical properties, and their crystal structures. Clay minerals, a subgroup of the phyllosilicates, are a major and important type of mineral in the Earth’s crust. Phyllosilicates exist over a crystal-size range from traditional clay-size range to very large crystals. However, in terms of their impact on everyday life, small grain-sized geomaterials are of most importance. Chemical weathering of primary minerals is one of the processes of principal interest when fine-grained crystalline hydrous aluminosilicates are considered. There are a variety of detailed approaches for the orderly classification of clay minerals, which is outside the scope of this work; for simplicity, the two principal layer types and three clay mineral families—kaolin, smectite, and chlorite—are discussed here. Importance of pillared and lateritic (a source of rare-earth elements) clays is also highlighted. While there is no comprehensive nomenclature for mixed-layer clays, statistical treatment of the binary mixed systems can be simplified by assuming three standard sequences: random, ordered, and segregated. The primary analytical tool used to sort out all these structural variations is powder X-ray diffraction. Though the structure of clay will inform many of the physical properties and allows for differentiation of the various clay minerals, substitution into the individual clay layers can alter the base physical properties and provides variation within the clay families. For more precision in clay mineral identification, additional analytical information is required (as further discussed in this book).

Has anyone wondered why clay is the most ubiquitous geomaterial in earth’s crust but we still are in need of developing more sophisticated methods and techniques to properly characterize it? The main reason is: it is not well defined; in fact, the variations in local clay structure and composition are virtually infinite. Geological origin descriptions provide an important foundation for clay models needed for interpretation of the experimental data collected on heterogeneous samples. Chemical analysis is the most essential step in mineral analysis; it usually follows structural analysis, in order to identify the major crystalline phases and impurities. Non-destructive techniques that are complementary to crystallography are electron microscopy and NMR spectroscopy for structure determination and study of dynamics. Some of the important methods in clay mineral identification are determination of coherent scattering domain size from XRD, Bertaut-Warren-Averbach analysis, counting layers on TEM lattice-fringe images, Pt-shadowing, and calculation of the average number of fundamental particles per MacEwan crystallite. A combination of the X-ray and neutron diffraction can be used for advanced model refinement, by utilizing a technique devised by Rietveld. Synchrotron radiation can be advantageous to laboratory sources. Several other advanced techniques are described in this chapter as well. Advances (including in situ analysis) in experimental methods go hand-in-hand with advances in conceptual understanding of the experimental observations.

Water vapor, carbon dioxide, and methane have extensively been studied because of their significant impact on energy security and environmental sustainability. Interaction of water with clay minerals strongly depends on the exchangeable cations in the interlayer. Interlayer H₂O forms a coordination shell around cation through temperature-dependent interaction between the cation and water oxygen—three distinct mechanisms have been identified. Various forms of CO₂ and its interaction with geomaterials include supercritical fluid, gas dissolved into brine, and (bi-) carbonate species. Due to the heterogeneity of geological formations, the interactions between geomaterials and injected fluids (and gas-in-place) are very complex and may result in dramatic changes in the rock formations. Mineral precipitation and dissolution reactions under low-water conditions have not received much attention so far, although water-bearing CO₂ can mediate important geochemical reactions; which is also true for water-saturated samples exposed to dry CO₂. In swelling smectites, the term “nano-confinement” was introduced to characterize the initial trapping of CO₂ molecules in the interlayer, with subsequent conversion to carbonates. The nano-confined CO₂ is distinguished by the red-shift in asymmetric-stretch vibration, which depends on the hydration state as confirmed by exposure to elevated temperatures. The presence of CO₂ and H₂O has a considerable effect on CH₄ sorption on clays. The idea of utilizing competitive sorption of CO₂ and CH₄ on shales in depleted reservoirs for enhanced gas recovery and concomitant carbon storage has been gaining momentum and for a good reason as discussed in this chapter.

The best-known characteristic of clay is a dramatic change in its morphological and geomechanical properties: from hard, dense, and brittle upon drying or firing to soft, pliable, and swelling upon exposure to water. Chemical properties of the 1:1 and 2:1 clay minerals are significantly different, which is mainly related to the bonds between individual layers. The interlayer environment is determined by the chemical nature of clay layers, the layer charge, interlayer cations, and water molecules forming hydration shells around the cations and H-bonding with clay surfaces. Mechanisms of water sorption and cluster organization are electrochemical in nature and fundamental to the swelling process. Some researchers also observed irreversible CO₂-induced swelling with smectite in 1–2 W hydration state, but the others reported only shrinking attributed to drying effects of high-pressure CO₂, for the cation-exchanged smectite with partly filled second hydration layer. The current interpretation of swelling phenomena evolves rapidly, following advances in experimental techniques and Monte Carlo and MD simulations of the structured fluid behavior. MD simulations show that the interlayer molecules do not organize themselves in a strictly tilted or strictly parallel to the surface configuration, which may result in fairly steep but gradual rather than stepwise increase in the basal spacing as the interlayer is filled with the solvent molecules. The magnitude of swelling hysteresis varies with the hydration energy of the interlayer cations and is generally more pronounced for vermiculite than montmorillonite.

This chapter is focused on reviewing molecular dynamics and Monte Carlo simulations of greenhouse gases’ interactions with swelling clay minerals. This chapter unfolds with the results of simulations on stepwise expansion of interlayer in hydrated montmorillonite. Next, an overview of the simulation data on carbon dioxide intercalation in clays is given with respect to structural changes, transport properties, thermodynamics, spectroscopic characteristics, sorption behavior at the basal clay surfaces, and surface wettability changes in CO₂-brine-mineral systems. Effects of the chemical nature of interlayer ions, as well as charge density and its distribution within clay layers on carbon dioxide/water intercalation and interaction with clay surfaces, are discussed. Then, results of methane interaction with hydrated swelling clays are presented. The discussion is centered around the formation of gas hydrate phase in the interlayer under suitable pressure and temperature conditions. Dynamic nature of hydrate cages encapsulating methane molecules is considered together with a mechanism of their formation in interlayer. A shift of the equilibrium pressure and temperature conditions in comparison with bulk phase is attributed to distortion of hydrate lattice in clay and to finite pore space. Finally, intercalation of the carbon dioxide/methane molecules in interlayer is reviewed through competitive adsorption of the binary mixture on clay surfaces.

The unique structure and behavior of swelling clay minerals, as observed in the laboratory and in the environment, present a challenge in understanding of the molecular details associated with these minerals. The chapter introduces the essence of classical methods involving empirically derived potential energy expressions that allow simulation of periodic cells representing bulk and interfacial clay mineral systems. The classical models provide the simulation and analysis of many thousands to more than a million atoms for evaluating structures, adsorption, diffusion, intercalation, physical, and other properties. Quantum chemical calculations, including molecular orbital methods and density functional theory, optimize the configuration of electrons about atoms from first principles, but require significant computational cost to examine many of the important topics in clay mineralogy. Molecular simulation methods such as energy minimization, molecular dynamics, Monte Carlo techniques, vibrational analysis, thermodynamics calculations, transition state analysis, and a variety of related computational methods are utilized to improve our understanding of clay minerals, and to better interpret traditional characterization and spectroscopic methods. An example showing the use of molecular simulation for clay minerals is presented for the process of montmorillonite’s swelling as a function of interlayer water.