Competition policy newsletter Volume 1, No 2, Summer 1994

When reactive fluids flow through a dissolving porous medium, conductive channels form, leading to fluid breakthrough. This phenomenon is caused by the reactive infiltration instability and is important in geologic carbon storage where the dissolution of CO₂ in flowing water increases fluid acidity. Using numerical simulations with high resolution digital models of North Sea chalk, we show that the breakthrough porosity is an important indicator of dissolution pattern. Dissolution patterns reflect the balance between the demand and supply of cumulative surface. The demand is determined by the reactive fluid composition while the supply relies on the flow field and the rock’s microstructure. We tested three model scenarios and found that aqueous CO₂ dissolves porous media homogeneously, leading to large breakthrough porosity. In contrast, solutions without CO₂ develop elongated convective channels known as wormholes, with low breakthrough porosity. These different patterns are explained by the different apparent solubility of calcite in free drift systems. Our results indicate that CO₂ increases the reactive subvolume of porous media and reduces the amount of solid residual before reactive fluid can be fully channelized. Consequently, dissolved CO₂ may enhance contaminant mobilization near injection wellbores, undermine the mechanical sustainability of formation rocks and increase the likelihood of buoyance driven leakage through carbonate rich caprocks

Particle Diffusion in Complex Nanoscale Pore Networks

We studied the diffusion of particles in the highly irregular pore networks of chalk, a very fine-grained rock, by combining three-dimensional X-ray imaging and dissipative particle dynamics (DPD) simulations. X-ray imaging data were collected at 25 nm voxel dimension for two chalk samples with very different porosities (4% and 26%). The three-dimensional pore systems derived from the tomograms were imported into DPD simulations and filled with spherical particles of variable diameter and with an optional attractive interaction to the pore surfaces. We found that diffusion significantly decreased to as much as 60% when particle size increased from 1% to 35% of the average pore diameter. When particles were attracted to the pore surfaces, even very small particles, diffusion was drastically inhibited, by as much as a factor of 100. Thus, the size of particles and their interaction with the pore surface strongly influence particle mobility and must be taken into account for predicting permeability in nanoporous rocks from primary petrophysical parameters such as surface area, porosity, and tortuosity

Direct Observation of Coupled Geochemical and Geomechanical Impacts on Chalk Microstructure Evolution under Elevated CO₂ Pressure

Dissolution in natural porous media by injected CO₂ can undermine the mechanical stability of the formation before carbon mineralization can take place. The geomechanical impact of geologic carbon storage therefore affects the structural integrity of the formation. Here, using <i>in situ</i> X-ray imaging, we show the coupled geochemical and geomechanical processes in natural chalk in the presence of aqueous CO₂. We first measured the chalk dissolution rate in a closed, free drift system and obtained a phenomenological correlation between the rate and evolving aqueous calcium concentration. We then used this rate correlation in a segregated flow model to estimate the visual pattern of chalk microstructure dissolution. The model predicted a homogeneous pattern, which resulted from an increase in the reactive subvolume. This prediction was validated using <i>in situ</i> X-ray tomography. The imaging technique further revealed three typical mechanical impacts during microstructure disintegration in an imposed flow field: material compaction, fracturing, and grain relocation. These impacts differ but are strongly coupled with CO₂-induced geochemical reactions and provide different types of feedback to the dissolution front migration. These observations led us to conclude that the presence of dissolved CO₂ makes the migration of reactive fluid less sensitive to perturbations in the coupled geochemical and geomechanical processes

Water Mobility in Chalk: A Quasielastic Neutron Scattering Study

Water mobility through porous rock has a role to play in many systems, such as contaminant remediation, CO₂ storage, and oil recovery. We used inelastic and quasielastic neutron scattering to describe water dynamics in two different chalk samples that have similar pore volume (ranging from tens of micrometers to a few nanometers) but different water uptake properties. We observed distinct water populations, where the analysis of the quasielastic data shows that after the hydration process most of the water behaves as bulk water. However, the lack of quasielastic signal, together with the observation of a translational mode at 10 meV, imply that in chalk samples that take up less water confinement occurs mostly in the pore volume that is accessible with nitrogen adsorption measurements

Incorporation of Monovalent Cations in Sulfate Green Rust

Green rust is a naturally occurring layered mixed-valent ferrous–ferric hydroxide, which can react with a range of redox-active compounds. Sulfate-bearing green rust is generally thought to have interlayers composed of sulfate and water. Here, we provide evidence that the interlayers also contain monovalent cations, using X-ray photoelectron spectroscopy and synchrotron X-ray scattering. For material synthesized with Na⁺, K⁺, Rb⁺, or Cs⁺, interlayer thickness derived from basal plane spacings correlates with the radius of the monovalent cation. In addition, sequential washing of the materials with water showed that Na⁺ and K⁺ were structurally fixed in the interlayer, whereas Rb⁺ and Cs⁺ could be removed, resulting in a decrease in the basal layer spacing. The incorporation of cations in the interlayer opens up new possibilities for the use of sulfate green rust for exchange reactions with both anions and cations: e.g., radioactive Cs