Dissolution Patterns Research Articles

Lattice Boltzmann study of dissolution in porous media: Comparison of VOP with VOF-curved boundary coupling

In dissolution processes, during geometry evolution, the two-phase interface is continuously changed. For tracking the position of the interface, the method of combining VOF with curved boundary schemes was presented in this research. Since the combination would be rather complex, in addition to the original approach, a simplified approximate approach was also introduced. Further, for simulating heterogeneous dissolution in lattice Boltzmann (LB) framework, the volume of pixel (VOP) method has been widely used by researchers. Since the area of the solid-fluid interface is not captured by this method, the study of its validity was also presented in this research. For this purpose, simulations of calcium carbonate dissolution by hydrochloric acid in simple and complex porous media were performed. In addition to the VOP method, the volume of fluid (VOF) method in which the solid-fluid interface is tracked, was also applied for all cases of simulations. The results revealed that from a dissolution perspective, the performance of the VOF-curved boundary combination is essentially similar to that of the VOF-bounce back combination. However, the algorithm of bounce back was found to be more than 10% computationally efficient. Also, comparison of the outcomes from VOP and VOF indicated that VOP intrinsically overestimates the surface area of reactions by about 25–35%, which fictitiously leads to 20–30% higher reaction rates and nearly 20–30% less dissolution times. Hence, it was deduced that the temporal outcomes of VOP may not be valid. However, from a non-temporal perspective, the dissolution patterns of both methods were found to be essentially similar.

Establishment of Level a In Vitro-In Vivo Correlation (IVIVC) via Extended DoE-IVIVC Model: A Donepezil Case Study.

This study aimed to establish an extended design of experiment (DoE)-in vitro in vivo correlation (IVIVC) model that defines the relationship between formulation composition, in vitro dissolution, and in vivo pharmacokinetics. Fourteen sustained-release (SR) tablets of a model drug, donepezil, were designed by applying a mixture design of DoE and prepared by the wet granulation method. The in vitro dissolution patterns of donepezil SR tablets were described by Michaelis–Menten kinetics. The mathematical relationship describing the effects of SR tablet compositions on the in vitro dissolution parameter, i.e., the in vitro maximum rate of release (Vmax), was derived. The predictability of the derived DoE model was validated by an additional five SR tablets with a mean prediction error (PE%) of less than 3.50% for in vitro Vmax. The pharmacokinetics of three types of donepezil SR and the immediate-release (IR) tablets was assessed in Beagle dogs following oral administration (n = 3, each). Based on the plasma concentration-time profile, a population pharmacokinetic model was developed, and the in vivo dissolution of SR tablets, represented by in vivo Vmax, was estimated. By correlating the in vitro and in vivo Vmax, level A IVIVC was established. Finally, the extended DoE-IVIVC model was developed by integrating the DoE equation and IVIVC into the population pharmacokinetic model. The extended DoE-IVIVC model allowed one to predict the maximum plasma concentration (Cmax) and the area under the plasma concentration-time curve (AUC) of donepezil SR tablets with PE% less than 10.30% and 5.19%, respectively, by their formulation composition as an input. The present extended DoE-IVIVC model may provide a valuable tool to predict the effect of formulation changes on in vivo pharmacokinetic behavior, leading to the more efficient development of SR formulations. The application of the present modeling approaches to develop other forms of drug formulation may be of interest for future studies.

Effect of temperature on sandstone acidizing using non-Newtonian fluids

Rock formation is subjected to acid stimulation to increase oil reservoirs productivity. Acidizing of sandstone rocks is a common method to remove damage and recover the oil flow to the wellbore. A matrix acid stimulation will always require additives depending on mud acid (HF/HCl) components, which cause non-Newtonian fluid behavior. In this study, the 3-D mathematical model of sandstone matrix acidizing in radial flow has been presented by a modified two-parameter model to consider non-Newtonian fluids and temperature effects. To this end, the Carreau-Yasuda equation has been employed to predict dissolution patterns created by non-Newtonian reactive fluids in sandstone formations. Meanwhile, the energy equation has been solved in the porous media to estimate the temperature in each time step, which affects the dissolution patterns in matrix acidizing. Three main phenomena are involved in dissolution patterns: surface reaction, diffusion and convection. Acidizing simulation indicates the acid-rock reaction constant in the formation network is temperature sensitive, and increases exponentially by increasing the reservoir temperature. In this work, the effects of various heterogeneity magnitude and porosity in the porous media, injection rate, reaction rate constant, power-law index (n) and temperature are considered by the various presented dissolution patterns of sandstone acidizing. The results show that, to reach a certain penetration length, the pore volume (PV) of injected acid decreases sharply with reservoir temperature, but after the critical temperature, it decreases slowly. The model discretized the Darcy, mass and energy balance equations by finite difference method and pore evaluation by Karman-Cozney correlations, and then employed the Tri-Diagonal matrix algorithm (TDMA) method to achieve the solution of the model and analysis of dissolution patterns in various acidizing regimes and rheological properties. All mathematical estimations including Darcy correlations, mass and energy balance and pore evaluation have been solved in FORTRAN program by parallel processing to accelerate 3-D calculations.

Experimental Study on the Effect and Mechanism of Chemical Stimulation on Deep High-Temperature Granite

Chemical stimulation has been increasingly applied to improve the performance of geothermal reservoirs since early 1980s. The potential for the successful application of this technique to high-temperature reservoirs hosted in granitic rocks is still uncertain, and one of the keys to assess this potential is to investigate experimentally the geochemical reactivity induced via chemical stimulation on relevant rock specimens. On this premise, we combined high-temperature and high-pressure dynamic simulation and static corrosion experiments to explore the effect of different chemical stimulants on the permeability of granite samples from the Baimiao formation (Hebei Province, China). Experimental results show that NaOH-dominated alkaline stimulants cause only weak dissolution patterns on primary feldspar and quartz, and they do not sensitively affect the original amount of chlorite. The overall effect is a negligible enhancement of the original permeability of all the granite specimens analyzed. Conversely, a large increase in permeability is observed when an acid mixture of 10wt% HCl + 1.5wt% HF is used as a stimulant, with an observed maximum magnifying effect of about 27 times, due to the effective dissolution of feldspar and chlorite. Though quartz appears not to be affected by dissolution, a relatively large amount of secondary neo-formed amorphous silica is also documented.

Incipient karst generation in jointed layered carbonates: Insights from three-dimensional hydro-chemical simulations

We develop a hydro-chemical model to simulate incipient karst generation in three-dimensional (3D) jointed layered carbonates. The discrete fracture networks consist of fractures that are represented by two-dimensional (2D) planes. The numerical model couples the important physical processes of fluid flow, reactive transport, and dissolutional growth of fracture aperture. We represent the typical layered joint systems by the synthetic fracture networks composed of two jointed layers separated by a horizontal bedding plane. By considering a wide range of initial flow rates and aperture distributions of the joints and bedding plane, the combined effects of the penetration length and the flow localization on karst evolution are systematically investigated. The simulation results confirm that different flow regimes (i.e. joint-dominated, transitional, and bedding plane-dominated) may induce distinct features of incipient karst morphologies observed in nature (i.e. pipe-like, stripe-like and sheet-like), which is attributed to the flow reorganization and the scale alteration of effective flow region. We further found the diversity of morphologies also depends on the initial penetration length, which plays a primary role in controlling dissolution patterns in joint-dominated systems. In addition, introducing the second joint set results in distinct 3D flow structure and dissolutional behavior comparing to single joint system. Our findings highlight the importance of 3D hydro-chemical modeling for realistic engineering applications as the structural complexity of 3D fractured rocks may be underrepresented by the previous studies using simplified conduit networks.

Oxidative dissolution of Sb2O3 mediated by surface Mn redox cycling in oxic aquatic systems

Antimony trioxide (Sb2O3) is one of the primary forms of Sb in the environment, and its dissolution significantly impacts the migration and bioavailability of Sb. However, the dissolution of Sb2O3 coupled with abiotic redox of Mn processes is unclear. Here, we investigated the kinetics of Sb2O3 dissolution in the presence of the ubiquitous Mn(II) by kinetic experiments, spectroscopies, density functional theory calculations and the chemical kinetic modeling. The oxidative dissolution of Sb2O3 was catalyzed by Mn(II) through the in-situ generated amorphous Mn oxides (MnOx) under oxic conditions, during which the generation of Mn(III) is a critical step in Sb(V) release. The released Sb(V) was partially retained on MnOx through bidentate-binuclear (corner-sharing) complexes as revealed by extended X-ray absorption fine structure analysis. The coexistent morphological forms of Sb2O3, i.e., senarmontite and valentinite exhibited distinct dissolution patterns. Valentinite showed higher activity in catalyzing Mn(II) oxidation and faster oxidative dissolution than senarmontite, due to its higher surface energy and lower conduction band minimum of its exposed facets. These abiotic processes can extrapolate to other metal(loid)s (hydr)oxides, further supplying for the comprehensive understanding of the redox transformation of Mn

QbD Consideration for Developing a Double-Layered Tablet into a Single-Layered Tablet with Telmisartan and Amlodipine.

The aim of this study was to develop a single-layered version of commercially available Twynstar® (Telmisartan + Amlodipine) double-layered tablets to improve the dosing convenience. A quality-by-design approach was applied to develop the single-layered version. To evaluate the range and cause of risks for a single-layered tablet in the formulation design research, we used the tools of the risk assessment, initial risk assessment of preliminary hazard analysis and main risk assessment of failure mode and effect analysis to determine the parameters affecting formulation, drug dissolution, and impurities. The critical material attributes were the stabilizer and disintegrant, and the critical process parameters were the wet granulation and tableting process. The optimal range of the design space was determined using the central composite design in the wet granulation and tablet compression processes. The stabilizer, kneading time, and disintegrant of the wet granulation were identified as X values affecting Y values. The compression force and turret speed in the tablet compression were identified as X values affecting Y values. After deciding on the design space with the deduced Y values, the single-layered tablets were formulated, and their dissolution patterns were compared with that of the double-layered tablet. The selected quality-by-design (QbD) approach single-layered tablet formulated using design space were found to be bioequivalent to the Twynstar® double-layered tablets. Hence, the development of single-layered tablets with two API using the QbD approach could improve the medication compliance of patients and could be used as a platform to overcome time-consuming and excessive costs and the technical and commercial limitations related to various multi-layered tablets.

Transitions of Dissolution Patterns in Rough Fractures

AbstractRock fractures are ubiquitous in geological systems and usually provide dominant pathways for fluid flow in fractured reservoirs. When the flowing fluid is reactive, fracture dissolution expands the aperture and forms various dissolution patterns that amplify the pathways. Previous works focused on the dissolution processes in Hele‐Shaw cells (parallel‐plate) and porous media, but the transitions of dissolution patterns in radial rough fractures are not well understood. Here we combine flow‐visualization experiments with theoretical analysis to elucidate the transitions of dissolution patterns under various flow‐rate and reaction‐rate conditions. We observe and quantify three distinct dissolution morphologies as compact, wormhole, and uniform patterns. We show that the critical Péclet numbers, corresponding to the transitions from compact to wormhole and to uniform patterns, increase with the reaction rate. Based on the growth of dissolution channels in the flow and transverse directions, we establish a theoretical model that describes the transitions of these three distinct dissolution patterns. The phase diagram predicted by the model exhibits good agreement with our experimental results and also well captures the pattern transitions reported in the previous studies. This work improves the understanding on how fracture aperture expands in the dissolution processes that lead to various dissolution channels. Our work is also critical for controlling the fluid flow behavior in dissolving natural rock fractures in subsurface flow systems.

3-D numerical simulation of sandstone matrix acidizing by non-Newtonian fluid

Matrix acidizing is a common technique to increase oil recovery in sandstone oil reservoirs. In the matrix acidizing process, reactive fluids are injected into the reservoir with the scope of dissolving and dispersing the rocks near the wellbore.In this study, the matrix acidizing of sandstone wells by a hydrochloric acid/hydrofluoric acid/additives mixture is described through mathematical modeling using the two-scale continuum model.The model developed in this study indicates the reactions and transport phenomena at pore and Darcy scales in 3-D radial flow. Due to using several additives during the acid injection process, the fluid phase has non-Newtonian behavior (shear-thinning). The effect of non-Newtonian fluid in porous media was investigated using a modified continuum model due to importance calculations in field scale. The Carreau-Yasuda constitutive equation has been used to modify the continuum model to consider the non-Newtonian fluid effects. In the proposed model, partial differential equations of reaction and diffusion at pore-scale, and simultaneously convection and dispersion phenomena at Darcy scale are solved using numerical methods. The finite volume and tri-diagonal matrix algorithm (TDMA) techniques were employed to gain a precise solution for the model and observe the dissolution pattern in different acidizing regimes and rheological properties. The effect of acid injection rate, dissolution rate constant and different reservoir properties, including permeability, porosity and heterogeneity on dissolution patterns, was investigated by considering non-Newtonian fluid effects, and concluded that acid having high shear-thinning behaviour (i. e., acid having low power-law index) seemed more promising for field operations because of deep penetration. The reservoir heterogeneity was defined by selecting a random local porosity distribution.It should be noted that the presented model can reproduce different types of acid/rock system (carbonate or sandstone reservoirs). Finally, the skin factor would be calculated to study the acidizing performance in the damaged zone. To solve the porosity evolution equation, couple pore and Darcy scales, and update the pore-scale parameters at each time step, the program was developed in FORTRAN by Parallel Processing and consisting of functions and routines. • Simultaneous numerical solution of Darcy, mass and pore equations in 3-dimension. • 3-D modeling of sandstone matrix acidizing by non-Newtonian fluid. • Pore volume to breakthrough and injection rate calculations in various conditions. • Consideration of power-law index (n) and viscosity effects in dissolution patterns. • Consideration of skin factor variation during the matrix acidizing.

Dissolution and sedimentation patterns of typical minerals in artificial reservoirs under different environments

Injection of circulating fluids (usually water) is the predominant means of heat extraction in enhanced geothermal systems (EGS), but the additional injection of circulating fluids can cause a water-rock reaction with the dry-thermal rock mass, resulting in the generation of a large number of secondary minerals. These secondary minerals cause fracture closure in artificial reservoirs and severely impede the sustainable use of geothermal systems. So, this paper combines the laboratory test with PHREEQC hydrogeochemical simulation. Firstly, the changes and mechanisms of mineral and aqueous chemical fractions in the artificial reservoir fractures were analysed after using groundwater from the sandstone aquifer at the FORGE site, as a heat transfer fluid, in the FORGE project of USA. Secondly, The patterns of dissolution and precipitation of minerals commonly found in artificial fractures in four different environments were then simulated with the aid of Phreeqc after completion of the transconversion. Finally, the effect of different unblocking fluids on the unblocking of these typical minerals is evaluated. The results show that groundwater as a heat transfer fluid causes dissolution of K-feldspar, albite, illite and partial decomposition of kaolinite and proto-silicate, and is also accompanied by rapid production of anorthite, decomposition of dolomite, sodization of Ca-montmorillonite and dolomitization of black mica. Mud acid will be more effective in unblocking fissure closures caused by anorthite, chlorite and kaolinite. NaOH will be more effective in unblocking fissure closures caused by illite and Ca-montmorillonite. Groundwater is not suitable as a blocking remover and can produce secondary mineral blockages(calcite, aragonite)easily. The effect of ultrapure water is relatively stable, it can be used as a heat exchange fluid or blocking remover, but when used as a blocking remover, it is mainly a physical-mechanical action process.

Dissolution-After-Precipitation (DAP): a simple microfluidic approach for studying carbonate rock dissolution and multiphase reactive transport mechanisms.

We propose a simple microfluidic approach: Dissolution-After-Precipitation (DAP), to investigate regimes of carbonate rock dissolution and multiphase reactive transport. In this method, a carbonate porous medium is created in a glass microchannel via calcium carbonate precipitation, after which an acid is injected into the channel to dissolve the precipitated porous medium. Utilizing the DAP method, for the first time we realized all five classical single-phase carbonate rock dissolution regimes (uniform, compact, conical, wormhole, ramified wormholes) in a microfluidic chip. The results are validated against the established theoretical dissolution diagram, which shows good agreement. Detailed analysis of these single-phase dissolutions suggests that the heterogeneity of the porous medium may significantly impact how the dissolution patterns evolve over time. Furthermore, DAP is utilized to investigate multiphase dissolution. As examples we tested the cases of an oleic phase (tetradecane) and a gaseous phase (CO2). Results show that the presence of a nonaqueous phase in pore spaces induces the formation of wormholes despite weak advection, and these wormholes ultimately become pathways for nonaqueous phase transport. However, the transport of tetradecane in the wormhole is very slow, causing acid breakthrough into neighboring regions. This mechanism enhances lateral connectivity between wormholes and may lead to a wormhole network. In contrast, CO2 moves rapidly and continuously seeks to enter a widening wormhole from a narrower wormhole or the porous regions, generating phenomena such as ganglia redistribution and counterflow (flow of gas opposite to acid flow). Extensive independent experiments are conducted to verify the reproducibility of the observed phenomena/mechanisms and further analyze them. Real-time monitoring of fluid pressure drop during dissolution is implemented to complement microscopy image analysis. Our method can be implemented repeatedly on the same chip, which offers a convenient and inexpensive option to study pore-scale reactive transport mechanisms.

Suitability of electrolytes and anodic dissolution behavior of 17-4 PH stainless steel

17-4 PH SS alloy finds widespread applications in turbine blades, power plants, marine and chemical industries. However, some of its mechanical properties such as high tensile and fatigue strength, good toughness, high hardness and formability make its machinability poor. Thus, non-conventional machining processes such as electrochemical micromachining (ECMM), which removes the material by chemical dissolution, has better potential to fabricate micro features of high surface quality. However, the ECMM behavior of 17-4 PH SS alloy has not been exhaustively explored yet. It is very important to understand the regular dissolution patterns to improve machining stability and machined surface quality. Therefore, the present work aims at studying the suitability of different electrolytes in ECMM process for the fabrication of micro features with better dimensional accuracy and surface quality on 17-4 PH SS alloy. The electrolytes used in the experimentation are an aqueous solution of NaBr, NaCl, NaNO3and a compound aqueous solution of NaCl + NaNO3. The performance characteristics such as radial overcut, dimple depth and some aspects of surface integrity have been investigated. Experimental results showed that better dimensional accuracy and surface quality have been achieved with NaCl + NaNO3electrolyte owing to the combined properties of passive and non-passive electrolytes.

Dissolution Depth and Surface Morphological Alterations in Ultrathin Glass Ceramic Etched with Different Hydrofluoric Acid-etching Protocols.

To evaluate the effect of different HF-etching protocols on the dissolution depth and micromorphology of the etched and adjacent surfaces of ultrathin glass-ceramic specimens. One hundred twenty specimens (6 x 6 x 0.3 mm) of leucite-reinforced glass-ceramic (LEU, IPS Empress, Ivoclar Vivadent) and lithium-disilicate-reinforced glass-ceramic (LD, IPS e.max, Ivoclar Vivadent) were prepared. Specimens were divided into 5 groups (n = 12) according to etching protocol: G1: control, untreated; G2: 5% hydrofluoric acid (HF) etching for 20 s (HF5%20s); G3: HF5%60s; G4: HF10%20s; and G5: HF10%60s. To analyze the dissolution depth, specimens were sectioned into two similarly sized halves using a chisel to create an internal surface (IS). Specimens were analyzed with scanning electron microscopy (SEM) on the following surfaces: HF application surface (AS), lateral surface (LS), internal surface (IS), and the surface opposite to the AS (OS). Dissolution patterns were identified. Data were submitted to one-way ANOVA and Bonferroni's test (α = 0.05). Dissolution depth data were submitted to Kruskal-Wallis and Mann-Whitney U-tests (α = 0.05). The prevalence of different dissolution patterns was analyzed using SEM. HF gel applied on the AS also affected the adjacent surfaces of all specimens. Different dissolution patterns were observed, which were dependent of HF-etching protocol and proportion of the glass phase in the ceramic. These patterns were categorized into four types for LEU (I-IV) and three for LD (I-III) according to the micropore size. The greater the micropore size, the more pronounced the etching pattern (p < 0.001). Higher HF times and concentrations showed prevalence of more severe etching patterns. HF10%60s produced greater dissolution depth in both materials when compared with other HF-etching groups (p < 0.05). Hydrofluoric acid etching not only affects the surface upon which it is applied, but internal, lateral and even opposite edges of glass ceramic. Different dissolution patterns and depths can be formed which are dependent of hydrofluoric acid concentration, application time, and proportion of the glass phase in the ceramic.

Rheological effects on the acidizing process in carbonate reservoirs

In this work, we present a new formulation to study the flow of a reactive non-Newtonian fluid in porous media. The mathematical model consists of a two-scale continuum model that couples information from the pore scale to Darcy one. In the place of the traditional Darcy Law, we consider a complete linear momentum equation, based on the Continuum Theory of Mixtures. The upscaling is made by an interaction source term that corresponds to the flow resistance caused by the matrix inside the pores, which considers the non-newtonian nature of the fluid. Motivated by the often applied self diverting acids we analyze the rheological dependence on the local level of pH. Simulations are performed using the finite volume method with the OpenFOAM software. We investigate the influence of the Damköhler number and the power-law index over dissolution patterns and Pore Volume to Breakthrough (PVbt) curves. The results are compared with the corresponding Newtonian cases. The effect of both shear rate and pH on viscosity are also studied. We verify a significant dependency of the dissolution patterns and also in the optimum injection rate on the rheology of the fluid. More shear-thinning fluids lead to lower values of PVbt. Although the divergent acids were really able to find different paths for the breakthrough, higher times were needed for this achievement.

Numerical Study of Reactive Flow in Fractured Carbonate Rock

Acidizing technology is an effective reformation method of oil and gas reservoirs. It can also remove the reservoir pollution near wellbore zones and enhance the fluid transmissibility. The optimal injection rate of acid is one of the key factors to reduce cost and improve the effect of acidizing. Therefore, the key issue is to find the optimal injection rate during acid corrosion in fractured carbonate rock. In this work, a novel reactive flow mathematical model based on two-scale model and discrete fracture model is established for fractured carbonate reservoirs. The matrix and fracture are described by a two-scale model and a discrete fracture model, respectively. Firstly, the two-scale model for matrix is combined with the discrete fracture model. Then, an efficient numerical scheme based on the finite element method is implemented to solve the corresponding dimensionless equations. Finally, several important aspects, such as the influence of the injection rate of acid on the dissolution patterns, the influence of fracture aperture and fracture orientations on the dissolution structure, the breakthrough volume of injected acid, and the dynamic change of fracture aperture during acidizing, are analyzed. The numerical simulation results show that there is an optimal injection rate in fractured carbonate rock. However, the fractures do not have an impact on the optimal acid injection rate, they only have an impact on the dissolution structure.

Dissolution Hotspots in Fractures

AbstractThe injection of reactive fluid into fractured reservoirs is relevant to many subsurface processes. During injection, fracture dissolution forms various patterns. Despite its importance, the evolution of geometries in radial fractures remains unexplored experimentally. Here, by flow‐visualization experiments, we characterize dissolution patterns and show that the patterns shift from compact to wormhole to uniform as flow rate increases. For the first time, we observe dissolution hotspots with maximum local dissolution rate in radial fractures, occurring at a distance from the inlet. We elucidate the key role of gravity for the occurrence of hotspots. The locations of hotspots correspond to the state that the effects of transport in the horizontal and vertical directions are comparable. We establish a theoretical model for predicting locations of hotspots in good agreement with experiments. This work demonstrates transitions of dissolution patterns in fractures and reveals the underlying mechanism for dissolution hotspots previously unidentified.

Influence of pore space heterogeneity on mineral dissolution and permeability evolution investigated using lattice Boltzmann method

The influence of pore space heterogeneity on mineral dissolution and permeability evolution in porous media was investigated using a numerical approach. Artificial porous media were generated by the linear Boolean model, and pore heterogeneity was evaluated using the Euler–Poincaré characteristic (i.e., Euler number). We applied the lattice Boltzmann method with dual particle distribution functions to simulate mineral dissolution under the combined effect of fluid flow and a diffusion process. Simulations were conducted to investigate dissolution patterns for a wide range of Péclet (Pe) and Damköhler (Da) numbers and various pore geometries. Six dissolution regimes were observed, and two types of transition phenomena between these dissolution regimes could be characterized. At high Pe and Da, the dissolution patterns strongly depended on the pore heterogeneity. In addition, four types of porosity–permeability relationship were observed. These relationships were influenced by the pore heterogeneity at high Da numbers.

Numerical Studies and Analyses on the Acidizing Process in Vug Carbonate Rocks

A vug porosity system, in addition to a matrix, is also the target rocks for the acidizing. In this work, the acidizing process in two typical core-scale separate-vug porosity systems is studied in detail. Numerous cases are conducted to discuss a parametric study on the acidizing process and hydraulic behavior. Results indicate that the presence of vug reduces the pore volume of acid solution consumed to achieve a breakthrough, which is consistent with experimental observations. Increasing the vug diameter and porosity decreases pore volume to breakthrough both for a vugular carbonate rock and isolated vug carbonate rock. In comparison, the acid mass does not change a lot. Typical dissolution patterns can also be observed in the acidizing process when a vug exists. Compared to matrix dissolution patterns, the presence of vug induces wormhole to pass through the vug region.

In vitrodissolution of bioactive glass S53P4 microspheres

AbstractStatic and dynamic in vitro dissolution studies showed large differences for various size‐fractions of non‐porous, flame‐sprayed commercial microspheres (45–500 µm) of bioactive glass S53P4. The smaller the spheres, the more their composition deviated from the nominal glass. The dissolution studies were carried out in simulated body fluid and tris(hydroxymethyl) aminomethane buffer for seven days. The ion concentrations in solutions were analyzed using inductively coupled plasma optical emission spectrometry, and the pH was measured as a function of time. Also, changes in the sphere size distribution and mass losses were determined. The calcium phosphate and the silica‐rich layers at the sphere surfaces were investigated using scanning electron microscopy after several immersion times. The smallest (45–90 µm) spheres appeared almost inert. In contrast, typical silica‐rich and calcium phosphate layers were identified at the largest spheres after three days of static and dynamic dissolutions. During the past years, bioactive glass microspheres have been added to paste‐like injectable bone grafting materials, putties to enhance their molding properties. The obtained results provide a better understanding of the dissolution patterns of bioactive glass microspheres.

Palaeoceanographic significance of aragonitic pteropods in the Indian Ocean – a review

The Quaternary pteropod records published in the late 1990s from the Indian Ocean provided vital information on past oceanographic and climatic changes in this region. This paper reviews the published work on pteropod preservation and abundance in the Indian Ocean to evaluate the usefulness of this microfossil group in palaeoceanographic reconstructions. Their aragonitic shell chemistry makes them reliable indicators of ocean acidification, paleaoceanography and paleoclimate. An increased interest in studying this faunal group has arisen from recent findings on the vulnerability of aragonite shell preservation to human impacted water column chemistry changes. The compiled pteropod studies show that most of the pteropod databases contain information on abundance and distribution. However, the biogeochemical parameters and processes, such as aragonite production in the water column, their settling fluxes and the shell dissolution patterns are less explored. Despite the wide geographical range of pteropods and their significance in the marine food web, the ecology and taxonomy of the holoplanktonic gastropods in the Indian Ocean are poorly known. Though these pteropods are recovered in plankton tows (standing crop) and sediment traps (settling fluxes), the surface sediments in large parts of the northern Indian Ocean are completely devoid of them. Thus, assessing their fate in the water column and the sediment–water interface and their response to prevailing biogeochemical conditions are worth pursuing. Genomic studies and isotopic and elemental chemistry of living and fossil pteropods could help assess their response to ecological changes. To date, such modern techniques have been little utilized in studying Indian Ocean pteropods. A knowledge of their shell chemistry is crucial in the context of increased input of anthropogenic CO2 and the susceptibility of pteropod preservation to changing climate.