Hannah Menke, Ph.D.

Significance: Reactions at the fluid-rock interface play a large role in many of the processes associated with fluid flow in porous media, including diagenesis, contaminant transport, and well acidization. The focus of this paper is on reaction in carbon capture and storage (CCS), where a major concern is long-term storage security. The injected carbon dioxide, CO₂, will dissolve in the host brine, forming an acidic solution. This acid in turn may react with the host rock, causing dissolution, particularly in carbonates, and−over longer time-scales− precipitation of carbonate. Dissolution may compromise the integrity of geologic seals, allowing CO₂ to escape to the surface, while precipitation leads to long-term storage security.

Abstract: Quantifying CO₂ transport and average effective reaction rates in the subsurface is essential to assess the risks associated with underground carbon capture and storage. We use X-ray microtomography to investigate dynamic pore structure evolution in situ at temperatures and pressures representative of underground reservoirs and aquifers. A 4-mm diameter Ketton carbonate core is injected with CO₂-saturated brine at 50 °C and 10 MPa while tomographic images are taken at 15 min intervals with a 3.8 μm spatial resolution over a period of 2.5 hrs. An approximate doubling of porosity with only a 3.6% increase in surface area to volume ratio is measured from the images. Pore-scale direct simulation and network modeling on the images quantify an order of magnitude increase in permeability and an appreciable alteration of the velocity field. We study the uniform reaction regime, with dissolution throughout the core. However, at the pore scale, we see variations in the degree of dissolution with an overall reaction rate which is approximately 14 times lower than estimated from batch measurements. This work implies that in heterogeneous rocks, pore-scale transport of reactants limits dissolution and can reduce the average effective reaction rate by an order of magnitude.

Significance: A major concern of carbon capture and storage (CCS) is long-term storage security. Carbon dioxide, CO₂, injected into the subsurface will dissolve in the host brine and form carbonic acid. Carbonate host rocks have the potential to react with and be dissolved by CO₂ acidified brine. Dissolution of the host rock may instigate unpredictable fluid flow and can weaken carbonate cements and damage injection wells. A complete understanding of dissolution in the brine-rock system is therefore important to predict the distribution and the rate of fluid movement and the amount and impact of dissolution in the subsurface. However, the nature and the rate of dissolution in carbonates are dependent on both the properties of the brine and the host rock. Previous experiments have observed that field-scale reaction rates are typically orders of magnitude lower than experimental batch reactor measurements. However, it is not possible to assess the most significant factors without directly observing the evolution of the pore space during reaction. Thus, dynamic pore-scale experiments are required to provide both insights into the interplay between transport and reaction, and to validate predictive models.

Abstract: We investigate the impact of initial pore structure and velocity field heterogeneity on the dynamics of fluid/solid reaction at high Péclet numbers (fast flow) and low Damköhler number (relatively slow reaction rates). The Diamond Lightsource Pink Beam was used to image dissolution of Estaillades and Portland limestones in the pres- ence of CO2-saturated brine at reservoir conditions (10 MPa and 50 °C representing ~1 km aquifer depth) at two flow rates for a period of 2 h. Each sample was scanned between 51 and 94 times at 4.76-μm resolution and the dynamic changes in porosity, permeability, and reaction rate were examined using image analysis and flow modelling.
We find that the porosity can either increase uniformly through time along the length of the samples, or may ex- hibit a spatially and temporally varying increase that is attributed to channel formation, a process that is distinct from wormholing, depending on initial pore structure and flow conditions. The dissolution regime was structure- dependent: Estaillades with a higher porosity showed more uniform dissolution, while the lower porosity Portland experienced channel formation. The effective reaction rates were up to two orders of magnitude lower than those measured on a flat substrate with no transport limitations, indicating that the transport of reactant and product is severely hampered away from fast flow channels.

Abstract: We investigate the impact of initial pore structure and velocity field heterogeneity on the dynamics of fluid/solid reaction at high Péclet numbers (fast flow) and low Damköhler number (relatively slow reaction rates). The Diamond Lightsource Pink Beam was used to image dissolution of Estaillades and Portland limestones in the pres- ence of CO2-saturated brine at reservoir conditions (10 MPa and 50 °C representing ~1 km aquifer depth) at two flow rates for a period of 2 h. Each sample was scanned between 51 and 94 times at 4.76-μm resolution and the dynamic changes in porosity, permeability, and reaction rate were examined using image analysis and flow modelling.
We find that the porosity can either increase uniformly through time along the length of the samples, or may ex- hibit a spatially and temporally varying increase that is attributed to channel formation, a process that is distinct from wormholing, depending on initial pore structure and flow conditions. The dissolution regime was structure- dependent: Estaillades with a higher porosity showed more uniform dissolution, while the lower porosity Portland experienced channel formation. The effective reaction rates were up to two orders of magnitude lower than those measured on a flat substrate with no transport limitations, indicating that the transport of reactant and product is severely hampered away from fast flow channels.

Abstract: We have experimentally investigated the impact of heterogeneity on the dissolution of two limestones, char-acterised by distinct degrees of flow heterogeneity at both the pore and core scales. The two rocks were reactedwith reservoir-condition CO2-saturated brine at both scales and scanned dynamically during dissolution. First,1 cm long 4 mm diameter cores were scanned during reactive flow with a 4 μm voxel size between 10 and 71times using 4D X-ray micro-tomography (μ-CT) over the course of 90 min. Second, 3.8 cm diameter, 8 cm longcores were reacted at the same conditions inside a reservoir-condition flow apparatus and imaged using amedical-grade X-ray computed tomography scanner (XCT). Each sample was imaged ~13 times over the courseof 90 min at a 250 × 250 × 500 μm resolution. These larger cores were then scanned inside a μ-CT at a 27 μmvoxel size to assess the alteration pore-space heterogeneity after reaction. Both rock types exhibited channelwidening at the mm scale and progressive high porosity pathway dissolution at the cm scale. In the more het-erogeneous rock, dissolution was more focussed and progressed along the direction of flow. Additionally, thedissolution pathways contained a distinct microstructure captured with the μ-CT that was not visible at theresolution of the XCT, where the reactive fluid had not completely dissolved the internal pore-structure. This microstructure was further analyzed by performing a direct simulation of the flow field and streamline tracing on the image voxels.
We found that at the larger scales the interplay between flow and reaction significantly affects flow in theunreacted regions of the core. When flow is focussed in large reacted channels, this focusing is carried through tothe unreacted parts of the rock where flow continues to be confined to preferential pathways after passing thereaction front. This focussing effect is greater with increasing pore space heterogeneity indicating that the re-presentative elementary volume (REV) for dissolution is far greater than the dissolution front itself. This study ofscale dependence using in situ 4D tomography provides insight into the mechanisms that control local reactionrates at the mm and cm scales. Furthermore, this work suggests that under these conditions at larger scales it islikely to be structural heterogeneity that dominates the pattern of dissolution and therefore the evolution of highpermeability pathways.