From Carbon Sequestration Initiative

Contents 1 IS³ Overview 1.1 Understanding Reactions in the Caprock and Near the Borehole 1.2 Science Challenges in Sequestration Geochemistry 2 IS³ Projects 2.1 In Situ Nuclear Magnetic Resonance Investigations of Trapping Mechanisms in CO2 Storage 2.2 In Situ Imaging of Mineral- Supercritical CO2 Reactions with Atomic Force Microscopy 2.3 A Real-Time Optical Spectroscopy Platform for Investigating Molecular Mineral Transformations for CO2 Storage 2.4 In Situ High-Pressure X-Ray Diffraction Investigations of Cap Rock Mineral Reactions 2.5 In Situ Molecular-Scale Investigations of Reactions between Supercritical CO2 and Minerals Relevant to Geological Carbon Storage

IS³ Overview

IS³ is a group of one-of-a-kind instruments that will allow researchers to probe geochemical reactions under supercritical pressures and temperatures. Information provided by IS³ feeds directly into GS³ for simulating the performance of a future injection site.

Instrumentation at EMSL (the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy user facility at PNNL)is being modified for high pressure and temperature sequestration experiments. These experiments will focus on understanding the supercritical geochemistry of the caprock as well as injection and monitoring boreholes.

Understanding Reactions in the Caprock and Near the Borehole

Integrity of the caprock and injection and monitoring boreholes will determine the long-term permanence of CO₂ storage in deep geologic reservoirs. Therefore, understanding fracture opening or self-sealing in the caprock and reactions near injection and monitoring boreholes are central to this experimental area.

It has been found that CO₂ dissolved in groundwater in deep saline aquifers has minimal reaction with the mineral matrix of the reservoir. It has also been found that pumping supercritical CO₂ into a reservoir has a drying effect around the borehole, where the supercritical CO₂ is acting as a separate phase not dissolved in water.

Current simulators represent reactions where CO₂ is first dissolved in water, so thermodynamic and kinetic parameters are valid as long as water is the solvating fluid. Research at PNNL has shown that when CO₂ is the solvating fluid and contains small amounts of water, this solution is highly reactive with minerals. In addition, CO₂ trapping processes over the short-term (e.g., capillary trapping) and long-term (e.g., mineralization) depend on how the chemistry of the fluid-mineral interfaces evolves over time. The rate and extent of reaction at fluid-mineral interfaces around the injection borehole and at the caprock are important scientific unknowns that must be resolved.

Science Challenges in Sequestration Geochemistry

Virtually no thermodynamic or kinetics data exist for wet CO₂-mineral reactions, that is, when supercritical CO₂ is the solvating fluid instead of water. The instrumentation in IS³ will address the following science challenges:

• role of water activity in mineral transformations; particularly determining water activity thresholds
• mechanistic characterization of reactions under high pressure and temperature
• relevant time scales for mineral transformations with respect to fluid flow through fractures
• method for predicting the conditions for fluid transmission through fractures, including fracture opening/self-sealing
• representation of water-wet CO₂ reactions in simulators.

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IS³ Projects

In Situ Nuclear Magnetic Resonance Investigations of Trapping Mechanisms in CO₂ Storage

PIs: David Hoyt, Jian Hu, Ja Kwak, Jesse Sears

Geologic sequestration is a viable method for reducing the emission of greenhouse gases from fossil-based energy production; however, our ability to accurately predict the long-term storage and disposition of these greenhouse gases will be enhanced by science. More information is needed on the molecular based mechanisms that govern subsurface trapping of CO₂, which requires an understanding of the geochemical processes associated with CO₂ sequestration in complex and heterogeneous subsurface mineral assemblages comprising porous rock formations, and in the equally complex fluids that may reside in and flow through those formations.

Unique NMR techniques available in EMSL are advancing our understanding of geochemical processes associated with the precipitation and dissolution of CO₂ at molecular level, especially under relevant pressures and temperatures found during sequestration. We are studying the interfacial structure, kinetics and mechanisms of common primary and secondary mineral phase dissolution in CO₂/water mixtures and carbonate nucleation and mineralization reactions as a function of composition, temperature, and pressure. Molecular interaction between the CO₂ molecules and the surface moieties/functional groups of selected geological media, including mineral interfacial reactions with CO₂, or mixed solvent CO₂-water solutions in porous media, and the interaction of CO₂ with H₂O molecules at the relevant geological temperature (from -20 to 80°C) and pressure (from less than 1.0 to 80 atmospheres) are also being determined.

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In Situ Imaging of Mineral- Supercritical CO₂ Reactions with Atomic Force Microscopy

PIs: A Scott Lea, James Amonette, Steven Higgins

Storage of anthropogenic carbon dioxide (CO₂) in deep underground saline aquifers is a viable method of slowing the increase of CO₂ in the earth’s atmosphere while continuing to allow the use of fossil-based fuels for energy. In these aquifers, the CO₂ will exist in the supercritical state. Consequently, numerous bulk laboratory studies are under way to measure the thermodynamic and kinetic properties of the reactions that would occur for various mineral systems in the presence of hydrous, supercritical CO₂. Yet, the molecular-scale, site-specific reactions that take place at these mineral-fluid interfaces are virtually unknown. These fundamental reactions ultimately sum to produce the interfacial phenomena that are observed in bulk studies and described macroscopically by equilibrium thermodynamics or kinetic rate laws. Attempts to understand and control the chemistry at these interfaces, which is necessary to assess the practicality of subsurface carbon sequestration, should be based on explicit and quantitative understanding of these fundamental interactions. To address this, we are building a hyperbaric atomic force microscope that is capable of imaging mineral-H₂O-supercritical CO₂ systems in situ and in real-time on a molecular-scale. This instrument is based on the hyperbaric, hyperthermal AFM that has been developed in recent years, but will be able to handle the pressures necessary to operate with supercritical CO₂. Calcite will be used as a test specimen to facilitate the development of the system. Once its functionality is established, the kinetics of MgO and wollastonite carbonation, which are involved in “mineral trapping” processes that are expected to occur in some underground storage sites, will be investigated.

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A Real-Time Optical Spectroscopy Platform for Investigating Molecular Mineral Transformations for CO₂ Storage

PIs: Zheming Wang, Alan Joly, Christopher Thompson

Carbon capture and sequestration from coal and gas-burning power plants are currently viewed as some of the most promising technologies for mitigating greenhouse gas emissions. This strategy involves injection of supercritical CO₂ (scCO2) into deep geological formations, such as depleted oil and gas reservoirs and deep saline aquifers. The ultimate fate of the stored CO₂ is determined by the interactions between scCO₂, various minerals in the rock formations, and the saline solutions. Unfortunately, little is known about the physical and chemical processes that occur with scCO₂ and water at various solid-liquid and liquid-liquid interfaces. The available thermodynamic database includes mostly phenomena in the aqueous phase or at a limited scale in organic solvents.

This project is developing an integrated scCO₂-optical spectroscopy platform with a modularized design for studying the molecular mineral transformation processes involved in geologic sequestration at relevant temperatures and pressures. These processes include mineral dissolution/nucleation/precipitation, hydration/dehydration, and sorption/desorption in scCO₂. In situ experimental studies will use a suite of optical spectroscopic techniques including ultraviolet-visible spectroscopy, Fourier transform infrared resonance (FTIR), Raman, laser fluorescence spectroscopy and sum frequency generation. The unique capabilities of this spectroscopy platform will be exploited in the investigation of:

• in situ observation of the transformation kinetics of Mg-chlorite [A_l2Mg₅Si₃O₁₀(OH)₈], wallastonite (CaSiO₃) and forsterite (Mg₂SiO₄) into calcium and magnesium carbonate minerals (e.g., calcite, aragonite and magnesite) by combined FTIR and time-resolved fluorescence spectroscopy at different temperatures and pressures in scCO₂
• FTIR, sum frequency generation, and fluorescence spectroscopic studies of the properties of dissolved water in scCO₂ and at the scCO₂-mineral (wallastonite and forsterite) interface and its correlation with the concentration and hydration status of other metal ions (Na, Ca, Eu) in the solution under variable pressure, temperature and water content.

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In Situ High-Pressure X-Ray Diffraction Investigations of Cap Rock Mineral Reactions

PIs: H Todd Schaef

Deep saline reservoirs with confining layers (cap rocks) are considered primary targets for carbon storage by most countries. Post-injected CO₂ will largely reside as a water rich supercritical buoyant fluid, constrained from vertical migration by a very low permeability cap rock seal or seals, thus allowing time for slow trapping mechanisms (CO₂ dissolution in pore water, and precipitation of carbonate minerals) to sequester CO₂ from the system. Despite this well-accepted conceptual model, the majority of research on flow, transport, and chemical reaction processes has remained in aqueous-dominated geologic systems, which will not represent the vast majority of the rock volume where supercritical CO₂ (scCO₂) is stored.

Geologic systems dominated by layer silicates (sometimes generalized in the literature and collectively referred to as clays) are of inherent interest because of their predominance in shale cap rocks, siliclastic reservoir rocks, and as weathering products in flood basalts. Geochemical processes associated with cap rock systems in contact with scCO₂ are complex and range from simple desiccation to structural transformations and secondary mineral formation. Current high-pressure X-ray diffraction capabilities are being enhanced to examine transformation reactions of minerals including layer silicate minerals and their kinetics when exposed to water solvated in scCO₂ fluids under pressure, temperature, and fluid composition conditions relevant to geologic sequestration of CO₂. This new high-pressure X-ray diffraction reactor will allow measurements in situ of mineralogical changes occurring in minerals at elevated pressure and temperature when contacted by scCO₂ containing variable amounts of H₂O. ____________________________________________________________________________________

In Situ Molecular-Scale Investigations of Reactions between Supercritical CO2 and Minerals Relevant to Geological Carbon Storage

PIs: John Loring, Jian Hu, A Scott Lea, Kevin Rosso, H Todd Schaef, Christopher Thompson, Zheming Wang

Carbon capture and sequestration from coal and gas-burning power plants is currently viewed as one of the most promising technologies for mitigating green house gas emissions. This strategy involves injection of supercritical CO2 (scCO2) into deep geological formations, such as depleted oil and gas reservoirs and deep saline aquifers. The ultimate fate of the stored CO2 is determined by the interactions between scCO2, various minerals in the host and caprock formations, and the saline solutions. Unfortunately, little is known about the physical and chemical processes that occur with scCO2 and water at various possible mineral-fluid interfaces in this system. The available thermodynamic database includes mostly phenomena in the aqueous phase. Moreover, the scientific community generally lacks the necessary experimental infrastructure to address these complex chemical issues at either a process level or a molecular level.

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