Dissolution-induced Changes Research Articles

Pore merging and flow focusing: Comparative study of undissolved and karstified limestone based on microtomography

We develop methods for qualitative and quantitative assessment of pore geometry transformation within a rock as a result of karstification. We then apply these tools to characterize dissolution-induced changes in limestone samples collected from a quarry in Smerdyna (Poland), where intense epikarst development is observed, consisting of a large number of solution pipes. Partially dissolved samples collected in the immediate vicinity of the pipes were compared with undissolved samples collected 1.5 meters away. For both types of samples 26 micron resolution grayscale X-ray scans were performed. The irregular geometry of the pore space, the vast majority of which forms a single connected component, can be conveniently characterized by a local thickness function that corresponds to the diameter of the largest sphere that fits within the pore space and contains a given point. We compare the local thickness distributions of the undissolved and dissolved samples as well as a numerically generated uniformly dissolved sample. Such a comparison allows us to quantify the extent of homogeneity of the natural karstification process. The above analysis is complemented by the calculation of connectivity of the pore space as well as their flow characteristics. All of the results consistently indicate an important role of pore merging and inhomogeneous dissolution in the process of natural dissolution for the analyzed samples.

Predictions of permeability, surface area and average dissolution rate during reactive transport in multi-mineral rocks

The interactions between minerals and reactive fluids have great impact on the pore structure and hydrologic properties of rocks. A 3D pore-scale reactive transport model is applied to predict multi-mineral reactions in porous rocks. Eight mineral phases are distinguished in 2D slices of a reservoir sandstone by using Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN) imaging technique. The 2D image is then registered into the corresponding cross-section of the 3D rock image obtained from micro-computed tomography (CT) scanning. Reactive transport with chemical dissolution is simulated directly on the 3D image of the sandstone. In the simulations, each mineral is assigned with a specific intrinsic reaction rate constant. The dissolution-induced changes in the pore structure are measured. The pore size distribution of the rock is analysed to investigate the evolution of the pore space. The changes of surface area and permeability are compared with single mineral simulations to illustrate the importance of mineralogical heterogeneity on reactive transport. The results show that the 3D pore-scale mineralogical heterogeneity causes overestimation of the permeability variations during reactive transport. The average effective dissolution rates for the rock are also measured. The simulations with single mineral assumptions present an overall reaction rate which is 10 times higher than the predictions with multi-mineral dissolution at the start of reactions. Relationships between average reaction rate and time are proposed to characterise the evolution of dissolution rate. A correction is also suggested to reduce the errors in predictions of reaction rates with single mineral assumptions.

Lattice Boltzmann simulation of dissolution-induced changes in permeability and porosity in 3D CO2 reactive transport

Dissolution and precipitation of rock matrix are one of the most important processes of geological CO2 sequestration in reservoirs. They change connections of pore channels and properties of matrix, such as bulk density, microporosity and hydraulic conductivity. This study builds on a recently developed multi-layer model to account for dynamic changes of microporous matrix that can accurately predict variations in hydraulic properties and reaction rates due to dynamic changes in matrix porosity and pore connectivity. We apply the model to simulate the dissolution and precipitation processes of rock matrix in heterogeneous porous media to quantify (1) the effect of the reaction rate on dissolution and matrix porosity, (2) the effect of microporous matrix diffusion on the overall effective diffusion and (3) the effect of heterogeneity on hydraulic conductivity. The results show the CO2 storage influenced by factors including the matrix porosity change, reaction front movement, velocity and initial properties. We also simulated dissolution-induced permeability enhancement as well as effects of initial porosity heterogeneity. The matrix with very low permeability, which can be unresolved on X-ray CT, do contribute to flow patterns and dispersion. The concentration of reactant H+ increases along the main fracture paths where the flow velocity increases. The product Ca++ shows the inversed distribution pattern against the H+ concentration. This demonstrates the capability of this model to investigate the complex CO2 reactive transport in real 3D heterogeneous porous media.

Increased diatom dissolution in Prydz Bay, East Antarctica linked to inception of the Prydz Bay gyre

Diatom assemblages and abundance in marine sediment cores are utilised as proxies of paleoceanographic conditions. However, diatoms are altered by taphonomic processes, which cause diatom accumulation rates to vary while simultaneously altering assemblages by removing some species from the sedimentary record. Furthermore, these taphonomic processes vary with space and time. Additionally, significant insights into the marine carbon and silicon cycles can be gained by assessing variability in diatom dissolution. Sediment core NBP0101-JPC24 from Prydz Bay, East Antarctica, provides a detailed Holocene record of diatom community shifts in an ecologically significant section of the Antarctic coast. The sedimentary and paleontologic record in JPC24 records the inception of the Prydz Bay gyre during deglaciation, as well as an increase in sea ice and in wind-related turbulence over the last 7000 years. A record of diatom preservation for JPC24 has been established by examining dissolution-induced changes in the morphology of Fragilariopsis curta, a sea-ice-related diatom common in JPC24. Diatom dissolution in NBP0101-JPC24 does not significantly correlate with the relative abundance of any diatom species or ecological species group. However, dissolution is low during the deglacial, prior to the inception of the Prydz Bay gyre and resultant polynya, and increases as the gyre increases in size and strength to present conditions. Circulation in the upper water column associated with the modern persistent gyre causes diatoms and other silt-sized particles to remain in suspension. Prior studies have shown that diatoms exposed to caustic marine water continue to dissolve until they are sequestered in sediments. Once sequestered in the sediments, diatoms will stop dissolving when a chemical equilibrium has been achieved between silicic acid in solution and solid opal. Diatom dissolution, therefore, is seen to be controlled by physical oceanographic factors rather than diatom ecology.

Diatom species abundance and morphologically-based dissolution proxies in coastal Southern Ocean assemblages

Taphonomic processes alter diatom assemblages in sediments, thus potentially negatively impacting paleoclimate records at various rates across space, time, and taxa. However, quantitative taphonomic data is rarely included in diatom-based paleoenvironmental reconstructions and no objective standard exists for comparing diatom dissolution in sediments recovered from marine depositional settings, including the Southern Ocean’s opal belt. Furthermore, identifying changes to diatom dissolution through time can provide insight into the efficiency of both upper water column nutrient recycling and the biological pump. This is significant in that reactive metal proxies (e.g. Al, Ti) in the sediments only account for post-depositional dissolution, not the water column where the majority of dissolution occurs.In order to assess the range of variability of responses to dissolution in a typical Southern Ocean diatom community and provide a quantitative guideline for assessing taphonomic variability in diatoms recovered from core material, a sediment trap sample was subjected to controlled, serial dissolution. By evaluating dissolution-induced changes to diatom species’ relative abundance, three preservational categories of diatoms have been identified: gracile, intermediate, and robust. The relative abundances of these categories can be used to establish a preservation grade for diatom assemblages.However, changes to the relative abundances of diatom species in sediment samples may reflect taphonomic or ecological factors. In order to address this complication, relative abundance changes have been tied to dissolution-induced morphological change to the areolae of Fragilariopsis curta, a significant sea-ice indicator in Southern Ocean sediments. This correlation allows differentiation between gracile species loss to dissolution versus ecological factors or sediment winnowing. These results mirror a similar morphological dissolution index from a parallel study utilizing Fragilariopsis kerguelensis, suggesting that results are applicable to a broad spectrum of diatoms typically preserved in the sediments.

Pore-scale study of dissolution-induced changes in permeability and porosity of porous media

Summary We apply a reactive transport lattice Boltzmann model developed in previous studies to study the dissolution-induced changes in permeability and porosity of two porous media at the pore scale. The permeability–porosity relationship is explored for a wide range of Peclet and Damkohler numbers. It is found that this relationship depends not only on different dissolution regimes characterized by Pe and Da, but also on the specific porous medium structure. The permeability–porosity relationship for the more geometrically complex porous medium shows much more complexity than that for the simple fractured medium. While a very small Da sets an upper bound for the permeability–porosity relationship for the simple medium, a combination of a high Da and Pe results in wormholing, and the fastest permeability increase for the complex medium. At a moderate Pe but large Da, a transition from transport-limited dissolution regime to wormholing is also observed for the complex medium.

Permeability evolution due to dissolution and precipitation of carbonates using reactive transport modeling in pore networks

[1] A reactive transport model was developed to simulate reaction of carbonates within a pore network for the high-pressure CO2-acidified conditions relevant to geological carbon sequestration. The pore network was based on a synthetic oolithic dolostone. Simulation results produced insights that can inform continuum-scale models regarding reaction-induced changes in permeability and porosity. As expected, permeability increased extensively with dissolution caused by high concentrations of carbonic acid, but neither pH nor calcite saturation state alone was a good predictor of the effects, as may sometimes be the case. Complex temporal evolutions of interstitial brine chemistry and network structure led to the counterintuitive finding that a far-from-equilibrium solution produced less permeability change than a nearer-to-equilibrium solution at the same pH. This was explained by the pH buffering that increased carbonate ion concentration and inhibited further reaction. Simulations of different flow conditions produced a nonunique set of permeability-porosity relationships. Diffusive-dominated systems caused dissolution to be localized near the inlet, leading to substantial porosity change but relatively small permeability change. For the same extent of porosity change caused from advective transport, the domain changed uniformly, leading to a large permeability change. Regarding precipitation, permeability changes happen much slower compared to dissolution-induced changes and small amounts of precipitation, even if located only near the inlet, can lead to large changes in permeability. Exponent values for a power law that relates changes in permeability and porosity ranged from 2 to 10, but a value of 6 held constant when conditions led to uniform changes throughout the domain.