Precipitation Of Calcium Sulfate Research Articles

Geochemical Characteristics and Genesis of Brine Chemical Composition in Cambrian Carbonate-Dominated Succession in the Northeastern Region of Chongqing, Southwestern China

Deeply situated brine is abundant in rare metal minerals, possessing significant economic worth. To the authors’ knowledge, brine present within the Cambrian carbonate-dominated succession in the northeastern region of Chongqing, Southwestern China, has not been previously reported. In this investigation, brine samples were collected from an abandoned brine well, designated as Tianyi Well, for the purpose of analyzing the hydrochemical characteristics and geochemical evolution of the brine. Halide concentrations, associated ions, and their ionic ratios within the sampled brine were analyzed. The brine originating from the deep Cambrian aquifer was characterized by high salinity levels, with an average TDS value of 242 ± 11 g/L, and was dominated by a Na-Cl facies. The studied brine underwent a moderate degree of seawater evaporation, occurring between the saturation levels of gypsum and halite, accompanied by some halite dissolution. Compared to modern seawater evaporation, the depletion of Mg2+, HCO3−, and SO42− concentrations, along with the enrichment of Ca2+, Li+, K+, and Sr2+, is likely primarily attributed to water–rock interactions. These interactions include dolomitization, combination of halite dissolution, upwelling of lithium- and potassium-bearing groundwater, calcium sulfate precipitation, biological sulfate reduction (BSR), and the common ion effect within the brine system. This research offers valuable insights into the genesis of the brine within the Cambrian carbonate succession and provides theoretical backing for the development of brine resources in the future.

Methallylsulfonate Polymeric Antiscalants for Application in Thermal Desalination Processes.

Nine copolymers of acrylic acid and sodium methallyl sulfonate were tested as scale inhibitors in thermal desalination. The nine antiscalants covered molar masses between 2000 and 9500 g.mol-1 and concentrations of sulfonated monomer ranging between 10 and 30 mole percent. A pressure measurement and control (P-MAC) unit and a high-temperature pressurized vessel were used to measure the effectiveness of the scale inhibitors in seawater, concentrated seawater, and model solutions at 125 °C. The effectiveness of the novel copolymers was comparable to commercial antiscalant at times up to 15 min and improved at longer times. Molar mass was a more important determinant of effectiveness than degree of sulfonation, with the greatest mitigation of calcium sulfate precipitation observed for antiscalants of molar mass 2000 to 2500 g.mol-1 regardless of sulfonate content. Antiscalants of molar mass 4500 to 5000 g.mol-1 showed a higher threshold effect than antiscalants of molar mass 7000 to 9500 g.mol-1, with a 30% sulfonated polymer of molar mass 4500 g.mol-1 performing appreciably better than other polymers of a similar molar mass.

Calcium sulfate scale: A review of state-of-the-art

Precipitation of sparingly soluble salts is a common issue in water treatment processes and oil and gas production systems. Precipitation occurs when incompatible brine mix and operating parameters like pressure, pH, temperature, etc. change. This may lead to decreased production output, equipment failure, and a decline in process efficacy. Of the many different types of scales known to exist, the calcium sulfate scale is particularly challenging to deal with as CaSO4 is practically insoluble in hydrochloric acid, which is used to remove other types of scales, such as carbonate scales. Although numerous initiatives to inhibit calcium sulfate precipitation have been documented in the literature, it remains largely an issue. This review offers a critical analysis of the state-of-the-art regarding calcium sulfate scaling, exposing its mechanisms of precipitation, inhibition, and methods for mitigating its effects. The limits of current technologies and further research areas are also highlighted in this review.

Scale inhibition mechanism of calcium sulfate by gum Arabic: Experimental testing, quantum chemical calculations, and molecular dynamics simulations

In this paper, the scale inhibition mechanism of gum arabic (GA) on calcium sulfate fouling is theoretically and experimentally investigated. A combination of experiments, quantum chemical calculations, and molecular dynamics simulations are carried out. The results demonstrated that GA has the potential to inhibit the precipitation process of calcium sulfate fouling significantly. As the concentration of GA increases, the scale inhibition efficiency also increases. Moreover, with a concentration of 6.8 g/L of calcium sulfate in the solution, it is shown that the scale inhibition efficiency of 1 g/L GA is 97.5 %. Additionally, it is noted that the variation in the solution pH does not significantly affect the GA scale inhibition effect. It was highlighted that carboxyl groups, hydroxyl groups, and ether bond oxygens in the GA molecule are the functional groups that exhibit scale inhibition. These groups serve as the low electronegativity regions with strong interactions with calcium ions. In addition, GA can be adsorbed on the active growth sites on the gypsum surfaces to form a dense, self-assembled barrier film. This film aids in inhibiting the calcium sulfate precipitation process. The findings of this study rank the scale inhibition of GA at different crystal surfaces as follows: gypsum (010) > gypsum (-111) > gypsum (120).

Investigating the Effect of Water Softening on Polymer Adsorption onto Carbonates through Single-Phase and Two-Phase Experiments

Summary Polymer retention is considered a major challenge in polymer flooding applications, especially in carbonates. This is due to the prevailing conditions of low permeability (<100 md), high temperature (>85°C), and high salinity (>100,000 ppm) generally found in these formations, which limit the effectiveness of commonly used polymers such as hydrolyzed polyacrylamide (HPAM) and xanthan gum. To address these challenges, a polymer based on acrylamide tertiary butyl sulfonate (ATBS) has been used due to its tolerance to high-temperature and -salinity conditions. However, the high cost of manufacturing these polymers, combined with their anionic properties that promote adsorption onto positively charged carbonate rocks, necessitates the exploration of methods to reduce polymer retention. In this study, we aim to determine the sufficient concentration of hardness ions (Ca2+ and Mg2+) required to significantly reduce the adsorption of this polymer. The study is unique in its focus on mitigating polymer retention in carbonate formations using softened brine, as no prior research has investigated this aspect. Four different brines were investigated with a salinity of 8,000 ppm total dissolved salts (TDS) and varying ionic composition designed mainly by eliminating the hardness-causing ions, Ca2+ and Mg2+. A geochemical study was performed using the PHREEQC software to analyze the interaction between these injected brines and the rock. Furthermore, comprehensive rheological and static adsorption studies were performed at a temperature of 25°C using the potential ATBS-based polymer to evaluate the polymer performance and adsorption in these brines. Later, dynamic adsorption studies were conducted in both single-phase and two-phase conditions to further quantify polymer adsorption. The geochemical study showed an anhydrite saturation index (SI) of less than 0.5 for all the brines used when interacting with the rock, indicating a very low tendency for calcium sulfate precipitation. Furthermore, the rheological studies showed that polymer viscosity significantly increased with reduced hardness, where a polymer solution viscosity of 7.5 cp was obtained in zero hardness brine, nearly 1.5 times higher than the polymer viscosity of the base makeup brine of 8,000 ppm. Moreover, it was observed that, by carefully tuning the concentrations of the divalent cations, the polymer concentration consumption for the required target viscosity was reduced by 40–50%. For the single-phase static adsorption experiments, the polymer solution in softened brines resulted in lower adsorption in the range of 37–62 µg/g-rock as opposed to 102 µg/g-rock for the base makeup brine. On the other hand, the single-phase dynamic adsorption results showed an even lowered polymer adsorption of 33 µg/g-rock for the softened brine compared with 45 µg/g-rock for the base makeup brine. Additionally, the single-phase dynamic adsorption studies showed a remarkable improvement in polymer injectivity using softened brine. The polymer retention in wettability-altered cores was further reduced. The study highlights that water softening improves the performance of polymers, specifically in terms of lowering polymer adsorption. It concludes that a threshold hardness level (Ca2+ and Mg2+) of approximately 100 ppm is sufficient to achieve a significant reduction in polymer adsorption for the tested experimental conditions. In this paper, we show that the softened water increases the polymer viscosity and reduces polymer adsorption, which leads to an overall reduction in polymer consumption. Hence, the softened makeup water has the potential to enhance the application envelope of this potential polymer for polymer flood, especially in the case of carbonate reservoirs.

Effect of spacer on mass transfer coefficient and calcium sulfate scaling phenomena in reverse osmosis desalination

Inorganic material (scale) precipitation on reverse osmosis (RO) membranes is a serious issue in plants because performance recovery via cleaning is challenging, and the replacement expenses are enormous. This study investigated the calcium sulfate scaling phenomena on membranes using a flat membrane cell with a spacer to model a spiral wound-type RO element. Subsequently, the mass transfer coefficient of the flat membrane cell with the spacer and the convergent value of permeate flux due to solute precipitation were determined. The dimensionless equation for the mass transfer coefficient of the cell with the spacer could be expressed using the dimensionless equation for the mass transfer coefficient of a flat membrane cell without a spacer. The behavior in permeate flux due to the precipitation of calcium sulfate on the RO membrane of the cell with the spacer was analyzed based on the scaling-based critical flux and scaling index, which discriminate scale precipitation. Finally, scale precipitation experiments were conducted by varying spacers and the operating conditions of pressure, solute concentration, temperature, and Reynolds number; the permeate flux was confirmed to converge to the scaling-based critical flux, and the scaling index could discriminate the operating condition area of scale precipitation.

Homopolymers and copolymers with iminodiacetic acid chelating units for scale inhibition

Novel homopolymer and copolymer scale inhibitors with iminodiacetic acid chelating units were synthesized for inhibiting calcium carbonate and calcium sulfate precipitation. Static scale inhibition tests of SI-1 demonstrated excellent performance, with inhibition performance of 83 % at a concentration of 30 mg/L for CaCO3 and >99 % at a concentration of 8 mg/L for CaSO4. These novel inhibitors surpassed the performance of a commercially available industrial scale inhibitor, 2-phosphonobutane-1,2,4-tricarboxylic acid. The homopolymer significantly altered CaCO3 and CaSO4 crystal structures and reduced crystal size at low concentrations. The primary mechanism of inhibition appears to be a two-fold effect: strong and specific complexation between the iminodiacetic acid chelating units and free Ca2+ ions, coupled with the adsorption of the polymers on the crystal surfaces. These promising scale inhibitors offer a potential replacement for phosphorus-containing compounds currently used in industrial applications.

Fluorescent-tagged antiscalants as a tool of homogeneous membrane scaling mitigation and inhibition mechanism investigation in electrodialysis processing of solutions prone to calcium sulfate precipitation

Fluorescent-tagged antiscalants as a tool of homogeneous membrane scaling mitigation and inhibition mechanism investigation in electrodialysis processing of solutions prone to calcium sulfate precipitation

Study on Mechanical Properties of Sulfate Saline Soil Improved by CLI-Type Polymer Active Agent

Large amounts of soluble salts in a soil enhance the soil sensitivity to changes in its properties induced by changes in environmental conditions, such as easy dissolution in water and easy occurrences of salt heaving in low-temperature environments, which make the soil volume swell rapidly, leading to a series of engineering disasters. Moreover, the growth and development of surface vegetation will be inhibited due to excessive salinity, resulting in a gradual decline in the ecological functionality of the area. A polymer active agent (CLI) was selected for the ecological improvement of sulfuric acid saline soils. Triaxial compression tests and a test on the soluble salt content of the treated soil were carried out to investigate the effects of polymer active agent content and maintenance time on the mechanical properties and soluble salt content of sulfate saline soils. The results showed that the addition of CLI can improve the soil strength by increasing the cohesion of the specimen, and the improvement increases significantly with the content of CLI and the curing age. Meanwhile, the calcium ions in CLI can react with sulfate ions in sulfate-salted soils to produce calcium sulfate precipitation to alleviate soil salinization. The scanning electron microscopy (SEM) images indicated that an appropriate content of CLI (about 8%) can strengthen the soil structure through an excellent chelating ability, enhancing the strength of the soil.

Optimization of salinity and composition of injected low salinity water into sandstone reservoirs with minimum scale deposition

In this study, a mechanistic and comprehensive examination of the impact of the scale formation situation of different diluted seawater levels was conducted to investigate the influence of important factors on the performance and efficiency of low salinity water. To clarify the effective participating mechanisms, scale precipitation by compatibility test, field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX) analysis, zeta potentials as surface charge, ion concentration changes, contact angle, pH, CO2 concentration, electrical conductivity, and ionic strength were analyzed. The results showed that increasing the dilution time to the optimal level (10 times-diluted seawater (SW#10D)) could effectively reduce the amount of severe precipitation of calcium carbonate (CaCO3) and calcium sulfate (CaSO4) scales. However, the reduction in CaCO3 scale precipitation (due to mixing different time diluted seawater with formation brine) and its effect on the wettability alteration (due to the change in surface charge of OLSW/oil and sandstone/OLSW) had higher impacts. The zeta potential results have shown that OLSW with optimum salinity, dilution, and ionic composition compared to different low salinity water compositions could change the surface charge of OLSW/oil/rock (− 16.7 mV) and OLSW/rock (− 10.5 mV) interfaces toward an extra negatively charged. FESEM and contact angle findings confirmed zeta potential results, i.e. OLSW was able to make sandstone surface more negative with diluting seawater and wettability changes from oil-wet toward water-wet. As a result, SW#10D was characterized by minimum scaling tendency and scale deposition (60 mg/l), maximum surface charge of OLSW/oil/rock (− 16.7 mV), and the potential of incremental oil recovery due to wettability alteration toward more water-wetness (the oil/rock contact angle ~ 50.13°) compared with other diluted seawater levels.

The Role of Sulfate in Cation Exchange Reactions: Applications to Clay–Brine Interactions on Mars

Phyllosilicates on Mars record a complex history of aqueous activity, including at Gale crater and Meridiani Planum, where stratigraphic differences in clay mineralogy have been recorded in outcrops that also contain calcium sulfate minerals. Thus, characterizing associations between phyllosilicates and calcium sulfates may provide constraints that are useful for constraining the geochemical environments that formed these outcrops. Previous studies have documented calcium sulfate precipitation as a result of clay–salt–atmospheric H2O interactions, but the compositions of brines throughout Mars’ history would have depended on the volume of water available on the Martian surface. Variations in brine composition influence the type and extent of reactions between the brines and the minerals that they come in contact with. To better understand how clay–brine interactions affected near-surface mineral assemblages on Mars, we performed two sets of experiments. The first set of experiments examined the effect of differing total brine concentrations and the second set explored variations in Na+ and SO4 2− concentrations independently. The results of this study show that gypsum readily forms due to cation exchange between montmorillonite and Na2SO4 brines of any concentration, but only near-saturated MgSO4 brines produced gypsum, and these also produced higher quantities of epsomite. Additionally, we found that the amount of gypsum produced from clay–Na2SO4 brine reactions is more strongly influenced by SO4 2− than Na+ or Cl− concentrations. Understanding how rapidly gypsum forms as a product of clay–brine interactions, as well as the influence of SO4 2− on cation exchange, will aid interpretations of sediments and environments that are observed on Mars.

Chemical inhibition of combined gypsum and iron oxides membrane fouling during reverse osmosis desalination process: Prevention and regeneration of membranes

This paper explores the performance of carboxylic acids as environmentally friendly chemical inhibitors and cleaning agents during combined membrane fouling by gypsum and iron oxides. The efficiency of citric acid and two other carboxylic acids was evaluated during combined fouling trials by measuring normalized fluxes drops, using an RO pilot unit. The effect of acid concentration, pH variation and gypsum supersaturation were evaluated. SEM, EDX, X-Ray diffraction and FTIR were used to identify the composition and morphology of fouled membranes. The results indicated that at low pH, citric acid serves to restore the flux drop. This acid serves to orient the crystals towards the hemihydrate form with a flat, thin structure. By increasing the pH, the acid retains its effectiveness and serves to delay the Fe2+ oxidation ions and the precipitation of calcium sulfate for more than 3 h. By comparing them to the citric acid, ascorbic and tartaric acid showed a similar effect at acidic pH. In both cases, normalized fluxes were recovered with the use of 100 ppm of each acid. However, at alkaline pH, citric shows a better performance. The regeneration of RO membranes fouled by iron oxides and gypsum appears to be more effective with citric acid than the other two acids.

Phase changes in cementitious materials exposed to saline solutions

This article summarizes long-term degradation mechanisms of cementitious materials in contact with fresh and saline solutions based on a review of experimental observations in field and laboratory studies. In addition, a simplified thermodynamic modelling approach was used to calculate the effect of the solution composition, ranging from river water to sea water, on the intensity of leaching and the kind and quantity of phases formed at the interface with the environment. This study shows that leaching is the main underlying degradation mechanism for all investigated exposure solutions. The presence of carbonates and sulphates in the solution increases leaching and decalcification of Ca-rich phases leading to the precipitation of calcium carbonate and calcium sulfate, while the presence of chloride has little influence on the intensity of leaching. Carbonates in the interacting solution can suppress ettringite formation. If present, magnesium precipitates as brucite, M-S-H or hydrotalcite-like phase.

Oxidative Decomposition of Silver Telluride (Ag2Te) Using Hypochlorite in Different Acid Environments

ABSTRACT Silver telluride represents one of the most important refractory silver mineral phases and requires the development of alternative processing routes. This work presents a detailed analysis of the effect of sodium and calcium hypochlorite species, HNO3, HCl, H2SO4 and pH on the kinetics and efficiency of silver telluride dissolution. The understanding of the reaction mechanism was complemented with the use of thermodynamic diagrams and characterization of the solid residues by the X-ray diffraction and SEM-EDS techniques. The results revealed that the presence of Ca(ClO)2 and H2SO4 promotes the secondary and massive precipitation of calcium sulfate. A comparison of the oxidative systems containing Ca(ClO)2 and NaClO showed that the solutions containing NaClO are more efficient and selective. The decrease in pH favors the tellurium dissolution due to an increase in the in-situ concentration of chlorine oxidizing species; the silver is transformed into silver chloride solid species on the surface of unreacted silver telluride. Thermodynamic and experimental studies revealed that silver chloride layer and the continuous consumption of hypochlorous acid promote the passivation of the system. The characterization of the solid residues evidenced that the type of acid contained in the systems modifies the morphology of the silver chloride layer. Finally, the surface characterization showed that passivation due to the formation of compact layer of silver chloride can be modified using a hypochlorite system containing nitric acid; this is due to a synergistic effect of nitrate and hypochlorite species on the kinetics of silver telluride dissolution. This effect is also discussed from a mechanistic viewpoint.

Analysis of Calcium Sulfate Scaling Phenomena on Reverse Osmosis Membranes by Scaling-Based Flux Model.

In this study, the behavior of permeate flux decline due to scale precipitation of calcium sulfate on reverse osmosis membranes was investigated. The proposed scaling-based flux model is able to explain that permeate fluxes attributed to three mechanisms of scale precipitation—cake formation, surface blockage, and mixed crystallization—converge to the same newly defined scaling-based critical flux. In addition, a scaling index is defined, which determines whether scale precipitates on the membrane. The experimental results were analyzed based on this index. The mass-transfer coefficients of flat membrane cells used in the experiments were measured and, although the coefficients differed, they could be summarized in the same form as the Leveque equation. Considering the results of the scale precipitation experiments, where the operating conditions of pressure, solute concentration, temperature, and Reynolds number were varied, the convergent values of the permeate fluxes are explained by the scaling-based critical fluxes and the scale precipitation zones by the scaling indexes.

Minimizing calcium lactate precipitation via the addition of gluconate ions for matrix acidizing with lactic acid

Organic acids are commonly used to replace hydrochloric acid (HCl) in high reservoir temperature applications, as they are less corrosive and weaker than HCl. However, organic acids have shown some problems due to acid reaction product solubility. One such organic acid, lactic acid, produces calcium lactate when it reacts with calcite, which has a low solubility in water. However, reaction product solubility can be improved by up to five times when gluconate ions coexist with lactate and calcium ions. The objective of this research is to evaluate lactic and gluconic acid mixtures in term of dissolving calcite, reaction product, corrosion, wettability and generating dominant wormhole.Lactic and gluconic acids were mixed together using deionized water and seawater to conduct calcite solubility tests. Corrosion tests, between 4 and 12 h, were also run under reservoir conditions. A formation response test (FRT) apparatus was used to run different coreflood tests using different combinations of injection rates, total acid concentration, and temperatures. These tests were accompanied with analytical analysis utilizing ICP and IC to measure calcium, iron and sulfate ions in solution.The results showed that mixing lactic and gluconic acids at a 1:1 M ratio provided the optimal results as no precipitation occurred at total acid strengths of 10 wt% and up to 27 wt%. Seawater usage caused calcium sulfate precipitation; therefore, three scale inhibitors were evaluated to determine mitigation rates. Acid-calcite dissolving results were satisfactory when limestone was exposed to a 1:1 and 2:1 M ratio of crushed core-to-acid ratios as at least 50% of the crushed core was dissolved. However, the two-acid mixture showed a corrosion rate that was higher than the acceptable rates and a trace of iron lactate precipitation occurred at 200 and 300 °F. Five gpt from a sulfur-based corrosion inhibitor was enough to mitigate the corrosion rate to allow for 8 h of testing. Coreflood tests showed that the mixture penetrated limestone core with minimal acid pore volume without any face dissolution or salt precipitation on the core faces.This research presents a set of diverse experimental data to confirm that lactic acid accompanied by gluconic acid can penetrate carbonate formation without any by-product precipitation. The two organic acids are less corrosive and less hazardous which can provide a safe operation environment and can decrease replacement and maintenance costs.

Geochemical investigation of electrical conductivity and electrical double layer based wettability alteration during engineered water injection in carbonates

The injection of engineered water to increase the oil recovery from carbonates is increasingly becoming popular due to its reduced environmental impact and low cost of operation. However, the related variation in electric properties of rock and fluid with this technique is still ambiguous and needs thorough investigation. This study explores the variation in electrical conductivity, ion mobility, electrical double layer thickness, and the related oil recovery with the change in water composition from a geochemical perspective. In this study, we implemented an improved wettability alteration model based on the variation in electrical conductivity with a Matlab-IPhreeqc coupled simulator, to model the electrical conductivity, ion mobility, and electrical double layer (EDL) thickness. The variation in concentration of the ionic species obtained from the geochemical model is used to determine the electrical conductivity. This electrical conductivity-based wettability modification is dynamically simulated in the transport model. The model is validated with experimental coreflood data conducted on carbonates by simulating the electrical conductivity measurements reported in the literature. From the findings, it is evident that the formation temperature, sulfate concentration, and dilution of injected seawater has a noticeable effect on electrical conductivity during engineered water injection. It is important to mention that the EDL thickness is the main parameter affected by the change in electrical conductivity. In consequence, it is suggested to inject high-temperature water in carbonate reservoirs because it will increase ion mobility. This increase in ion mobility will enhance the EDL thickness and water film will become stabilized. Moreover, seawater dilution decreases electrical conductivity while spiking of sulfate concentration increases the activity of sulfate ions. However, the concentration of sulfate ions must be controlled as a wettability alteration agent, as it can cause the formation and precipitation of calcium sulfate. Furthermore, the variation in electrical conductivity and EDL thickness caused by the injection of seawater and diluted seawater increased the recovery of oil by approximately 16–21% in the selected case study.

Geochemical modeling of CO2 injection and gypsum precipitation at the Ketzin CO2 storage site

Gypsum crystals are found at the well perforation of observation well Ktzi 202 of the test site for CO2 storage at Ketzin, Germany. XRD analysis confirms pure gypsum. Fluid samples before and after CO2 injection are analyzed. Geochemical modeling is conducted to identify the mechanisms that lead to gypsum formation. The modeling is carried out with PHREEQC and Pitzer database due to the high salinity of up to 5 mol per kg water. Due to their significantly higher reactivity compared to other minerals like silicates, calcite, dolomite, magnesite, gypsum, anhydrite, and halite are considered as primary mineral phases for matching the observed brine compositions in our simulations. Calcite, dolomite, and gypsum are close to saturation before and after CO2 injection. Dolomite shows the highest reactivity and mainly contributes to buffering the brine pH that initially decreased due to CO2 injection. The contribution of calcite to the pH-buffering is only minor. Gypsum and anhydrite are no geochemically active minerals before injection. After CO2 injection, gypsum precipitation may occur by two mechanisms: (i) dissociation of CO2 decreases activity of water and, therefore, increases the saturation of all minerals and (ii) dolomite dissolution due to pH-buffering releases Ca2+ ions into solution and shifts the mass action to gypsum. Gypsum precipitation decreases with increasing temperature but increases with increasing partial CO2 pressure. Our calculations show that calcium sulfate precipitation increases by a factor of 5 to a depth of 2000 m when Ketzin pressure and temperature are extrapolated. In general, gypsum precipitation constitutes a potential clogging hazard during CO2 storage and could negatively impact safe site operation. In the presented Ketzin example, this threat is only minor since the total amount of gypsum precipitation is relatively small.

Effect of sulfate on CO2 binding efficiency of recycled alkaline materials

CO2 mineralization using alkaline waste materials and by-products is a promising technology to reduce CO2 emissions. This work investigates the impact of alkali sulfates on the carbonation process. Detailed investigations of the carbonation reaction show that the sulfates are important phases impacting the mineralization. Alkalis alone accelerate the kinetics and change the morphology of the precipitated phases. Sulfates additionally reduce the amount of CO2 mineralized since a part of calcium precipitates as calcium sulfate. The mutual interaction between alkalis and sulfate ions provides a tool to control the mineralization kinetics and especially the extent of carbonation by preventing calcium sulfate precipitation at surplus of alkalis. Furthermore, sulfate-containing materials, which so far have not been considered as suitable for CO2 mineralization (e.g. gypsum), can now be considered for CO2 sequestration. A modification of the Steinour formula, used for calculations of the carbonation potential, is proposed to account for the observed effects.

CALCIUM SULFATE SCALE IN THE PETROLEUM INDUSTRY

Oilfield scale is defined as the precipitation of hard, adherent deposits of inorganic solid originating from aqueous media. This constitutes sulfate and carbonate of the alkaline earth metals calcium, barium and strontium and complex salts of iron. Generally, the process of the scale deposition occurs when the product solubility of a compound considered is exceeded. The formation of scale, such as calcium sulfate, has long recognised as one of the serious problems in oil and gas production leading to reduced production rates as flow becomes restricted. Calcium sulfate scale found in the oilfield is in the form of gypsum (CaSO4, 2H2,0) which is the most stable form at temperatures of 40 °C or less at atmosphere pressure. Above this temperature, anhydrite (CaSO4,) may be found, although hemihydrate (CaSO. 1/2H,O) may form under certain conditions. The reaction for precipitation of calcium sulfate is as follows: Ca+2 (aq) + so4-2 (aq) = CaSO4 (solid) The solubility of calcium sulfate in distilled water is 2080 mg/l at 25 "C. Calcium sulfate scale arises from several causes, such as temperature, dissolved salts, pressure, and time. The main points of this paper are focused on nomenclature, chemical structure, the occurrence of calcium sulfate scale, example of calcium sulfate scale in the petroleum industry, and calculation of calcium sulfate solubility in brine.