Science of Carbon Storage in Deep Saline Formations

Process Coupling across Time and Spatial Scales

Pania Newell, Anastasia Ilgen

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780128127537
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Science of Carbon Storage in Deep Saline Formations: Process Coupling across Time and Spatial Scales summarizes state-of-the-art research, emphasizing how the coupling of physical and chemical processes as subsurface systems re-equilibrate during and after the injection of CO2. In addition, it addresses, in an easy-to-follow way, the lack of knowledge in understanding the coupled processes related to fluid flow, geomechanics and geochemistry over time and spatial scales. The book uniquely highlights process coupling and process interplay across time and spatial scales that are relevant to geological carbon storage.

• Includes the underlying scientific research, as well as the risks associated with geological carbon storage
• Covers the topic of geological carbon storage from various disciplines, addressing the multi-scale and multi-physics aspects of geological carbon storage
• Organized by discipline for ease of navigation

Chapter 1

Overview of Geological Carbon Storage (GCS)

Pania Newell¹ and Anastasia G. Ilgen², ¹Department of Mechanical Engineering, The University of Utah, Salt Lake City, UT, United States, ²Geochemistry Department, Sandia National Laboratories, Albuquerque, NM, United States

Abstract

Geological carbon storage (GCS) is a promising technology for mitigating increasing concentrations of carbon dioxide (CO₂) in the atmosphere. The injection of supercritical CO₂ into geological formations perturbs the physical and chemical state of the subsurface. The reservoir rock, as well as the overlying caprock, can experience changes in the pore fluid pressure, thermal state, chemical reactivity and stress distribution. These changes can cause mechanical deformation of the rock mass, opening/closure of preexisting fractures or/and initiation of new fractures, which can influence the integrity of the overall geological carbon storage (GCS) systems over thousands of years, required for successful carbon storage.

GCS sites are inherently unified systems; however, given the scientific framework, these systems are usually divided based on the physics and temporal/spatial scales during scientific investigations. For many applications, decoupling the physics by treating the adjacent system as a boundary condition works well. Unfortunately, in the case of water and gas flow in porous media, because of the complexity of geological subsurface systems, the decoupling approach does not accurately capture the behavior of the larger relevant system.

The coupled processes include various combinations of thermal (T), hydrological (H), chemical (C), mechanical (M), and biological (B) effects. These coupled processes are time- and length-scale- dependent, and can manifest in one- or two-way coupled behavior. There is an undeniable need for understanding the coupling of processes during GCS, and how these coupled phenomena can result in emergent behaviors arising from the interplay of physics and chemistry, including self - focusing of flow, porosity collapse, and changes in fracture networks. In this chapter, the first section addresses the subsurface system response to the injection of CO₂, examined at field and laboratory scales, as well as in model systems, addressed from a perspective of single disciplines. The second section reviews coupling between processes during GCS observed either in the field or anticipated based on laboratory results.

Keywords

Geological carbon storage; coupled processes

Acknowledgments

This material was prepared by PMN and AGI with support from the Center for Frontiers of Subsurface Energy Security (CFSES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0001114, awarded to the University of Texas and Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Introduction

Geological sequestration of carbon dioxide (CO₂) known as geological carbon storage (GCS) is a proposed technology to store CO₂ produced by large point sources into deep, porous, and highly permeable rock formations for permanent storage. These geological formations are required to have certain characteristics. For instance, they should be at over 850 m below the ground surface as well as be overlain by one or multiple impermeable formations (caprock) to prevent upward migration of CO₂. Additionally, the storage formation should have sufficient porosity and permeability to hold large amounts of CO₂. Some examples of the proposed types of geological storage units are:

• • Deep saline formations,
• • Unmineable coal seams,
• • Depleted oil and gas reservoirs, and
• • Basalt formations.

The main focus of this book is on deep saline formations. Deep saline formations or deep brine reservoirs exist worldwide, making them accessible targets for CO₂ storage. These deep geological formations are usually at depths greater than 850 m, which allows storage of CO₂ in a supercritical (sc) state, and therefore in larger volumes. The buoyant scCO₂ plume is expected to persist over centuries if not thousands of years until CO₂ predictably dissolves into the formation brine. Therefore, as noted earlier, it is necessary that the storage formation is overlain by an impermeable layer (caprock) to limit the upward migration of buoyant scCO₂. In general, various geochemical and physical trapping mechanisms would prevent the CO₂ from migrating to top surfaces (IPCC, 2005).

Perturbation of Subsurface During GCS

The injection of supercritical CO₂ into the subsurface perturbs the pressure (and, therefore, the state of stress), chemical, thermal, and biological steady-state or equilibrium conditions. Additional complexity is introduced because of the presence of two distinct and immiscible phases (e.g., scCO₂ and brine), resulting in a two-phase fluid flow in the system. After CO₂ injection, the re-equilibration of the system is nonlinear in space and time, with different processes proceeding along vastly different timescales. For example, some geochemical reactions take place within days, while others, with slow kinetics, require decades or even thousands of years (Ilgen and Cygan, 2016). Similarly, flow within nanoporous caprock may take hundreds of years to advance, while the same distance in sandstone will be traveled by fluids in minutes.

Perturbation of stress and geomechanical response plays a prominent role in both short- and long-term performance of GCS; however it plays the most crucial role in the physical trapping during CO₂ injection (Rutqvist, 2012). Physical trapping prevents the upward migration of CO₂ through one or multiple layers of impermeable caprock above the storage formation. Physical trapping can also be provided through capillary forces in the porous rock formation. However, because of the combined effect of these mechanisms (e.g., structural and capillarity) over time, the dominant mechanism can change as time progresses (Wu et al., 2014).

Immediately following the injection of CO₂, the structural trapping mechanism plays the leading role in retaining CO₂ within the storage formation. In the presence of faults, they can act either as barriers or preferential leakage pathways, depending on the permeability of the fault, which may impact the structural trapping mechanism of GCS (Wu et al., 2014). Injection of CO₂ into the rock formation can also change the pore pressure and the stress state of the geological formation which may trigger seismic events (reservoir, basement, and caprock). As noted earlier, the geomechanical events (e.g., fracturing) are caused by changes in the stress field as a result of CO₂ injection (e.g., change in the pore pressure). This coupling between deformation and change in the pore pressure can be expressed as:

${\sigma \prime }_{ij}={\sigma }_{ij}-\alpha {\delta }_{ij}{P}_p$ (1.1)

(1.1)

where ${\sigma }_{ij}$

and ${\sigma \prime }_{ij}$

are the components of the total and effective stress tensor, $\alpha$

is the Biot’s coefficient, ${\delta }_{ij}$

is the Kronecker delta, and $P_p$

is the pore pressure (scalar).

Fig. 1.1 shows possible geomechanical events during GCS. Ground–surface movement and microseismic events are geomechanical responses which have been observed at various CO₂ storage sites. As a result of change in the pore pressure during GCS, pre-existing fractures and faults may be re-activated or new fractures may form within the reservoir, caprock, or overburden, which could lead to new leakage pathways.

Figure 1.1 Schematic of geomechanical processes associated with GCS in deep saline formations. Source: From Rutqvist, J., 2012. The geomechanics of CO₂ storage in deep sedimentary formations. Geotech. Geol. Eng. 30 (3), 525–551.

The effectiveness of the GSC does not only depend on physical trapping, but also geochemical trapping mechanisms. Chemical reactions are triggered by the injection of CO₂ because of perturbation of the existing state of the aquifer. The chemical reactions of scCO₂ with rock minerals are very complex due to the dependency on rock type and porosity, rock compositions, available reactive surface area, etc. (Silva et al., 2015). Carbonate-rich (carbonate-cemented) rock assemblages are most vulnerable to chemical attack by CO₂, because of the fast dissolution rate of carbonate minerals, compared to other mineral types. Other grain cement materials (e.g., quartz cement) are less reactive and exhibit less alteration of both chemical and mechanical properties on the timescales examined in the field and laboratory. Initially dry scCO₂ becomes partially wet as it moves through the formation and interacts with formation brine. This humid scCO₂ can form water films on the water-wetting mineral surfaces, with reactivity of these films deviating from the reactions observed in the systems where activity of water is not limited. Chapter 4, Experimental Studies of Reactivity and Transformations of Rocks and Minerals in Water-Bearing Supercritical CO₂, by Loring et al. (this volume) provides more details on this subject.

Natural analog sites show that some CO₂ is sequestered as carbonate minerals over geologic time periods. Dissolution of CO₂ into parent brine, or solubility trapping, is necessary for further carbonation reactions (mineral trapping). Mineral trapping refers to the formation of carbonates from the parent mineral assemblage. To date, to estimate the CO₂ storage capacity, the fo...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• List of Contributors
• About the Editors
• Preface
• Acknowledgments
• Chapter 1. Overview of Geological Carbon Storage (GCS)
• Chapter 2. CO2 Enhanced Oil Recovery Experience and its Messages for CO2 Storage
• Chapter 3. Field Observations of Geochemical Response to CO2 Injection at the Reservoir Scale
• Chapter 4. Experimental Studies of Reactivity and Transformations of Rocks and Minerals in Water-Bearing Supercritical CO2
• Chapter 5. Reactive Transport Modeling of Geological Carbon Storage Associated With CO2 and Brine Leakage
• Chapter 6. Multiphase Flow Associated With Geological CO2 Storage
• Chapter 7. Laboratory Studies to Understand the Controls on Flow and Transport for CO2 Storage
• Chapter 8. Numerical Modeling of Fluid Flow During Geologic Carbon Storage
• Chapter 9. Field and Laboratory Studies of Geomechanical Response to the Injection of CO2
• Chapter 10. Numerical Geomechanics Studies of Geological Carbon Storage (GCS)
• Chapter 11. Thermal Processes During Geological Carbon Storage: Field Observations, Laboratory and Theoretical Studies
• Chapter 12. Field Observations, Experimental Studies, and Thermodynamic Modeling of CO2 Effects on Microbial Populations
• Chapter 13. Hydraulic-Chemical Coupling Associated With Injection and Storage of CO2 Into Subsurface
• Chapter 14. Fracture Specific Stiffness: The Critical Link Between the Scaling Behavior of Hydro-Mechanical Coupling in Fractures and Seismic Monitoring
• Chapter 15. Coupled Chemical-Mechanical Processes Associated With the Injection of CO2 into Subsurface
• Chapter 16. Hydrologic, Mechanical, Thermal, and Chemical Process Coupling Triggered by the Injection of CO2
• Chapter 17. Carbon Geological Storage: Coupled Processes, Engineering and Monitoring
• Chapter 18. Closing Remarks: Future Research Needs for Geological Carbon Storage
• Index