ISEN Report

Multi-scales models for assessing the basic processes that regulate CO2 storage

PI: Giuseppe Buscarnera

Co-PIs: Aaron I. Packman and Andrew D. Jacobson

Background and Objectives

The geological sequestration of carbon dioxide (CO2) is a well-known source of permanent changes to the fluid chemistry and to the physical properties of the host rocks. The coupling between hydrologic, chemical and deformation processes related with CO2 injection is indeed a major cause of distributed damage, and hence of potential leakage of the injected fluid. Since the success of this technology depends on predicting the performance of CO2 trapping across timescales of thousands of years, coupled models incorporating geochemistry of carbon storage, underground flow and solid mechanics are primary tools for assessing the long-term fate of the implemented procedures. This project has focused on the formulation of interdisciplinary modeling frameworks able to combine flow/deformation analyses and geochemical processes. The goal has been to (i) incorporate mineral dissolution/precipitation occurring at the pore scale in fluid transport models and (ii) use statistical homogenization to infer macroscopic features of chemically altered continua. Hence, to understand the mechanisms that regulate the mechanics of solids subjected to the flow of a reactive fluid, we have used data extracted from 3D imaging technologies and multi-scale modeling techniques.

Results

Coupled stress-strain, fluid-retention model for multi-phasic crushable solids

Stress-strain models for geomaterials are based on a macroscopic description of the medium. In other words, heterogeneous solids with complex microstructure are approximated as equivalent continua. While this strategy has computational advantages, it also produces macroscopic formulations that must be calibrated against numerous macro-scale experiments. The coupling between environmental agents (e.g., variable moisture content and fluid chemistry), complicates the picture, as it difficult to consider all combinations among the relevant controls. In this project we have used an alternative strategy, in which the scheme of effective continuum is enhanced by state variables associated with the key microstructural attributes affecting all the phenomena at stake (i.e., both the flow of a reactive fluid and the elastic energy storage in compressed heterogeneous media). Given their importance in the geosciences, the study has tackled granular materials at high pressures (i.e., deeply buried), i.e. solids in which the evolution of the microstructure is associated with alterations of the grain size distribution. In nature, in fact, high-pressure compaction, dissolution and faulting cause permanent alterations of grain size attributes (e.g., the average grain size). As a result, crushing affects all the mechanical properties of rocks and sediments, causing massive changes in surface area and reaction-transport properties.

To study these phenomena, we have used a multi-scale formulation based on physical hypotheses at the grain-scale. More specifically, we have used particle mechanics to characterize how the grains interact with each via contact force transfer. In addition, we have considered simple pore-scale considerations to describe the capillary interactions at the solid fluid interfaces. The combination of these hypotheses has enabled us to describe the simultaneous evolution of mechanical and hydrologic variables, as well as variations of particle grading associated with comminution (Fig. 1).

Figure 1. Schematic description of particle breakage in granular matter. Effects on stress-strain response and quantitative prediction of the degree of comminution.

During the project, our conceptual model has been validated through coordinated one-dimensional compression tests, measurements of water retention properties and quantitative image analysis for assessing the extent of grain breakage (Fig. 2). The latter analysis was based on the use of a high-resolution microscope to map both coarse and fine particles resulting from comminution.

Figure 2. a) Schematic description of the pressure-plate apparatus used to measure the water retention curve; b) Example of pre- and post-crushing retention curves; c) Schematic description of image-based grain size distribution analysis.

Conceptual model for chemically-weathered porous media

Similar to changes in degree of water-saturation, an alteration of the chemistry of the pore fluid has detrimental consequences on the mechanical response. These effects are enhanced by the supply and/or removal of mass during reactive-transport, a process that produces a permanent alteration of the pore-space topology and of the solid microstructure. To include such effects in the above mentioned formulation, we have introduced chemical state variables able to describe the clogging of the pore space as a result of cement precipitation. In particular, to define the topology of these alterations, we have performed reactive-flow experiments in small columns of packed calcite, quantifying the evolving geometry of the precipitant phase via X-ray tomography at the Advanced Photon Source (APS) located at the Argonne National Laboratory.

Reactive-flow experiments and interpretation via X-ray tomography

The experiments were performed using small columns of 4.32mm in diameter and 3cm in length. The columns were packed via wet deposition with 400µm calcite particles pre-treated with ultrasonic bath to remove possible fines. The columns were then subjected to the flow of a supersaturated reactive pore solution consisting of CaCl2 and NaHCO3, that were stored in separate reservoirs, pumped to the packed column at the same flow rate and mixed through a T-valve at the inlet. Constant flow rate, concentration, and reaction time, were imposed to promote the production of precipitants and the alteration of the pore structure. Samples of pore solution were collected during the experiments at both the inlet and the outlet, with the purpose to measure pH, calcium concentration and alkalinity. At the end of the experiments, the samples were analyzed via X-ray micro-tomography to generate a complete 3D mapping of the reaction products (Fig. 3).

Figure 3. 3D rendering of precipitation reaction products mapped via X-ray tomography: a) packed calcite column prior to the imposition of reactive-flow; b) packed calcite column after the experiment.

Implications for carbon sequestration

• Multi-scale strategies can be used to derive enriched continuum models with microstructural descriptors. The ability to capture the evolution of these attributes during loading, wetting and chemical forcing enables these models to cope with a number of coupled phenomena, such as the injection of fluids underground and the extraction of fossil fuels.

• Compression experiments have shown that the evolution of the average particle size has major effects on the water-retention properties of a granular medium. Since in highly comminuted zones such as faults the average grain size is considerably smaller than that of the parent rock, this property can have crucial consequences on both permeability and fluid retention mechanisms of faulted reservoir rocks.

• Colum experiments have revealed that the topology of the pore structure changes considerably during reactive transport. The key control factors have been studied to design experiments during which the rate of such pore structure changes can be controlled. The key component for the success of the experiments that emerge from our preliminary studies are the packing procedure, the chemical composition of the pore fluid and the injection method.

Project Participants

ISEN funding has provided partial support for a postdoctoral collaborator, Dr. Arghya Das, who has collaborated to the development of a multi-scale mechanical model for the analysis of hydrologic and chemical processes in granular media. In addition, the project has supported the experimental activities of three PhD students, Yang Bai, Minwei Xie and Yida Zhang, providing them with the opportunity to develop expertise in the set-up of complex reactive-flow and deformation tests, as well as in the collection and interpretation of large datasets retrieved from X-ray tomography.

Publications resulting from this ISEN project

The results of this ISEN project have originated the two publications cited here below. The support of ISEN has been acknowledged in both publications:

Zhang, Y. D., & Buscarnera, G. (2014). Grainsize dependence of clastic yielding in unsaturated granular soils. Granular Matter, 1-15

Zhang, Y. D., & Buscarnera, G. (2014). Model predictions of hydro-mechanical coupling in unsaturated crushable soils. Proceedings of the 6th International Conference on Unsaturated Soils (UNSAT 2014), Sydney, Australia, 2-4 July, 2014.

External proposals submitted as a result of this ISEN project

Title: CAREER: Mechanics of Geomaterials Exposed to Multi-Physical Perturbations: Innovating Science, Training and Education through Fundamental Principles PI: Giuseppe Buscarnera Sponsoring Agency: NSF –Division of Civil, Mechanical and Manufacturing Innovation Budget: $400,000 Status: Funded Title: Chemo-mechanics of granular geomaterials subjected to reactive-transport: small-scale testing, microstructural imaging and multi-scale constitutive modeling PI: Giuseppe Buscarnera Co-PIs: Aaron Packman and Andrew Jacobson Sponsoring Agency: DOE – Division of Chemical Sciences, Geosciences, & Biosciences Budget: $347,048 Status: Pending