Dissolution kinetics and mechanisms at dolomite-water interfaces: effects of electrolyte specific ionic strength

New insights into the dolomitization and dissolution mechanisms of dolomite-calcite (104)/(110) crystal boundary: An implication to geologic carbon sequestration process

Geologic carbon sequestration (GCS) is a promising strategy to reduce the harm of CO2 due to the rapidly increased fossil fuel combustion. Dolomitization and dissolution processes of deeply buried carbonate reservoirs significantly impact the potential of GCS. However, previous investigations mainly focus on the macroscopic batch experiments, the mechanisms at atomic level are still unclear especially for crystal boundary, but urgently required. Herein, the GCS potential and the effects of boundary dissolution on calcite and dolomite were investigated based on both analytical and simulation methods such as molecular dynamics simulation (MDS) and density functional theory (DFT) calculations, to deeply unveil the mechanisms of dolomitization and formation of intergranular secondary pores from the atomic perspective. The morphology results indicated that the dissolution of calcite and dolomite in carbonic acid solution started via the edges and corners. In addition, the simulated results showed that the carbon sequestration potential presented an order in dolomite (PMg50%) > PMg40% > PMg30% > PMg20% > PMg10% > calcite by dolomitization due to the reduced bulk volume but increased lattice stress. Furthermore, both electrons transfer and diffusion coefficients results suggested that the (104)/(110) boundary was preferentially dissolved as compared to the (104) and (110) planes, indicating that crystal boundary was beneficial to the formation of pores for the oil and gas storage, but harmful to the stability of long-term GCS. Therefore, this study, for the first time, provides new insights into uncovering the mechanisms of the GCS process in depth, from an atomic level focusing on the crystal boundary, thereby promoting the understand of the long-term evolution for both calcite and dolomite in deep reservoirs.

Interface dissolution kinetics and porosity formation of calcite and dolomite (110) and (104) planes: An implication to the stability of geologic carbon sequestration

Geologic carbon sequestration (GCS) via injecting CO2 into deep carbonate reservoirs (mainly calcite and dolomite) is a promising strategy to reduce CO2 level. However, the dissolution or precipitation of calcite/dolomite planes on minerals/solution interface during long-term GCS process develops intergranular porosity and thus affects the permeability and stability of reservoirs. To investigate this process, both calcite and dolomite were dissolved in acetic and carbonic acids. A diffusion-controlled process was identified, with greater diffusion rates in acetic acid than that in carbonic acid. Quantified planes activity of both minerals follows (110) > (116) > (101) > (113) > (018) > (104) through density functional theory. Accomplished with preferential dissolution of calcite (110) planes in carbonic acid, calcite crystals precipitated with (104) planes at 423.15 K, under which, more calcite crystals were observed on dolomite surface, producing Ca-deplete surface. Molecular dynamic calculations showed higher dissolution rates of calcite/dolomite (110) planes than (104). In addition, the dissolution coefficients of Ca2+ were approximately triple of that Mg2+. Therefore, this study reveals the interface dissolution mechanisms of calcite and dolomite, especially on (110) and (104) planes at an atomic level, for the first time, providing better understanding for the stability of long-term GCS process.

Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics Simulation Study

Due to the importance of the dolomite mineral in carbonate reservoirs, the wettability characteristics of dolomite surfaces were studied with both experiments and molecular dynamics simulations. Contact angle measurements confirm that the dolomite surface can be rendered oil-wet by carboxylates (acidic components of crude oil) and that the cationic surfactant can reverse the oil-wetness more effectively than the anionic surfactant used in this study. The oil-wetness of an aged dolomite chip was reduced when treated with MgSO₄ solution at 80 °C, while CaCl₂, MgCl₂, and Na₂SO₄ solutions did not produce any significant wettability alteration. The effects of surfactants and divalent ions, Ca²⁺, Mg²⁺, and SO₄^2- (also referred to as Smart Water ions), were simulated with two model dolomite surfaces containing point defects and step vacancies, respectively. The results indicate that the cationic surfactant can weaken the attraction between the oil phase and the carboxylates, while the anionic surfactant tends to maintain the oil-wetness of the dolomite surface by replacing the carboxylates through competitive adsorption. All Ca²⁺, Mg²⁺, and SO₄^2- ions can act as potential determining ions, and the detachment of carboxylates is due to the repulsion from SO₄^2- ions drawn close to the surface in the presence of adsorbed Mg²⁺.

Investigation on the influence of additives on the oriented dissolution of calcite

Similar to the crystal growth process, additives have a strong influence on the dissolution process of crystals. Studies on the dissolution process may shed light on understanding the biomineralization and bioinspired crystallization process. The influence of different kinds of additives including surfactants and polymers on the dissolution process of calcite {104} planes was investigated in detail in this work. The additives can be classified into three kinds according to their influence on the dissolution process of calcite under different concentration windows. The additives show three different kinds of dissolution behaviors with the increase of additive concentrations according to the tomographic variation of the calcite surface after the dissolution process. There are four dissolution modes of calcite while changing the additive concentrations in the solution. Rhombohedral etch pits with [4[combining macron]41] and [481[combining macron]] step edges are formed on the calcite {104} planes after the dissolution process at low additive concentrations (mode I). Calcite micropyramids begin to appear on the calcite surface and the densities of micropyramids increase with the increase of the additive concentrations until they cover the entire calcite surface after the dissolution process at medium additive concentrations (mode II). Instead of micropyramids, large pyramids with [481[combining macron]] and [4[combining macron]41] step edges and a size of about 50 μm form after the dissolution process at high additive concentrations (modes III and IV). We propose that the different anisotropic dissolution behaviors of calcite are strongly related to the concentrations and the adsorption features of the additives on the calcite surface. The additives may act as inhibitors of calcite dissolution, possibly through adsorption on calcite surfaces without preferred adsorption, or adsorption at specific kink sites or step edges. The influence of additives on the oriented dissolution of calcite is generally related to the adsorption density and homogeneity of additives on the calcite substrates.

Anorthite Dissolution under Conditions Relevant to Subsurface CO2 Injection: Effects of Na+, Ca2+, and Al3

Supercritical CO₂ is injected into subsurface environments during geologic CO₂ sequestration and CO₂-enhanced oil recovery. In these processes, the CO₂-induced dissolution of formation rocks, which contain plagioclase, can affect the safety and efficiency of the subsurface operation. In subsurface brines, Na⁺ and Ca²⁺ are naturally abundant, and Al³⁺ concentration increases due to acidification by injected CO₂. However, our current understanding of cation effects on plagioclase dissolution does not provide sufficiently accurate prediction of plagioclase dissolution at such high salinities. This study investigated the effects of up to 4 M Na⁺, 1 M Ca²⁺, and 200 μM Al³⁺ on anorthite (as a representative mineral of Ca-containing plagioclase) dissolution under conditions closely relevant to subsurface CO₂ injection. For the first time, we elucidated the inhibition effects of Al³⁺ on anorthite dissolution in far-from-equilibrium systems, and found that the Al³⁺ effects were enhanced at elevated temperature. Interestingly, Na⁺ inhibited anorthite dissolution as well, and the effects of Na⁺ were 50% stronger at 35 °C than at 60 °C. Ca²⁺ had similar effects to those of Na⁺, and the Ca²⁺ effects did not suppress Na⁺ effects when they coexisted. These findings can contribute to better predicting plagioclase dissolution in geologic formations and will also be helpful in improving designs for subsurface CO₂ injection.

A hot tip: imaging phenomena using in situ multi-stimulus probes at high temperatures

Accurate high temperature characterization of materials remains a critical challenge to the continued advancement of various important energy, nuclear, electronic, and aerospace applications. Future experimental studies must assist these communities to progress past empiricism and derive deliberate, predictable designs of material classes functioning within active, extreme environments. Successful realization of systems ranging from fuel cells and batteries to electromechanical nanogenerators and turbines requires a dynamic understanding of the excitation, surface-mediated, and charge transfer phenomena which occur at heterophase interfaces (i.e. vapor-solid, liquid-solid, solid-solid) and impact overall performance. Advancing these frontiers therefore necessitates in situ (operando) characterization methods capable of resolving, both spatially and functionally, the coherence between these complex, collective excitations, and their respective response dynamics, through studies within the operating regime. This review highlights recent developments in scanning probe microscopy in performing in situ imaging at high elevated temperatures. The influence of and evolution from vacuum-based electron and tunneling microscopy are briefly summarized and discussed. The scope includes the use of high temperature imaging to directly observe critical phase transition, electronic, and electrochemical behavior under dynamic temperature settings, thus providing key physical parameters. Finally, both challenges and directions in combined instrumentation are proposed and discussed towards the end.

Calcite microneedle arrays produced by inorganic ion-assisted anisotropic dissolution of bulk calcite crystal

Besides studies on the mineralization process, research on the demineralization of minerals provides another way to understand the crystallization mechanism of biominerals and fabricate crystals with complicated morphologies. The formation of ordered arrays of c-axis-oriented calcite microneedles with a tri-symmetric structure and lengths of more than 20 μm was realized on a large scale for the first time through anisotropic dissolution of calcite substrates in undersaturated aqueous solution in the presence of ammonium salts. The lengths and the aspect ratios of the calcite microneedles can be tuned by simply changing the concentrations of the ammonium salts and the dissolution time. The shape of the transverse cross sections of the calcite microneedles obtained in the presence of NH4 Cl and NH4 Ac is almost regularly triangular. The tri-symmetric transverse cross-section geometry of the calcite microneedles could be attributed to the tri-symmetric feature of rhombohedral calcite atomic structures, the synergetic interactions between electrostatic interaction of ammonium ions and dangling surface carbonate groups, and the ion incorporation of halide ions.