Precipitation Of Calcium Sulfate Research Articles

Development of poly(aspartic acid-co-malic acid) composites for calcium carbonate and sulphate scale inhibition

Polyaspartic acid (PSI) is suitable for the inhibition of inorganic scale deposition. To enhance its scale inhibition efficiency, PSI was modified by reacting aspartic acid with malic acid (MA) using thermal polycondensation polymerization. This reaction resulted in poly(aspartic acid-co-malic acid) (PSI-co-MA) dual polymer. The structural, chemical and thermal properties of the dual polymers were analysed by using scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry and gel permeation chromatography. The effectiveness of six different molar ratios of PSI-co-MA dual polymer for calcium carbonate and calcium sulphate scale inhibition at laboratory scale batch experiments was evaluated with synthetic brine solution at selected doses of polymer at 65–70°C by the static scale test method. The performance of PSI-co-MA dual polymer for the inhibition of calcium carbonate and calcium sulphate precipitation was compared with that of a PSI single polymer. The PSI-co-MA exhibited excellent ability to control inorganic minerals, with approximately 85.36% calcium carbonate inhibition and 100% calcium sulphate inhibition at a level of 10 mg/L PSI-co-MA, respectively. Therefore, it may be reasonably concluded that PSI-co-MA is a highly effective scale inhibitor for cooling water treatment applications.

Carboxylic acids: effective inhibitors for calcium sulfate precipitation?

Results are reported here of an investigation into the effects of three carboxylic acid additives (tartaric, maleic and citric acids) on the precipitation of calcium sulfate phases. Precipitation reactions were followed at pH 7 in the pure CaSO4system and in experiments with 0–20 ppm carboxylic acids added using in situ UV-VIS spectrophotometry (turbidity). The solid products were characterized in terms of their mineralogical composition, using X-ray diffraction, during and at the end of each reaction, and in terms of their morphological features, by scanning electron microscopy. All additives increased the time needed for turbidity to develop (induction time, start of precipitation) and the comparison between additive and additive-free experiments showed that, at equivalent concentrations, citric acid performed far better than the other two carboxylic acids. In all cases bassanite precipitated first and with time it transformed to gypsum. The addition of citrate stabilized bassanite and changed the final gypsum habit from typical needle-like crystals in the pure CaSO4system to plates in the citrate-additive experiments.

Confinement Increases the Lifetimes of Hydroxyapatite Precursors

The mineral component of bone is a carbonated, nonstoichiometric hydroxyapatite (calcium phosphate) that forms in nanometer confinement within collagen fibrils, the principal organic constituent of bone. We here employ a model system to study the effects of confinement on hydroxyapatite precipitation from solution under physiological conditions. In common with earlier studies of calcium carbonate and calcium sulfate precipitation, we find that confinement significantly prolongs the lifetime of metastable phases, here amorphous calcium phosphate (ACP) and octacalcium phosphate (OCP). The effect occurs at surprisingly large separations of up to 1 μm, and at 0.2 μm the lifetime of ACP is extended by at least an order of magnitude. The soluble additive poly(aspartic acid), which in bulk stabilizes ACP, appears to act synergistically with confinement to give a greatly enhanced stability of ACP. The reason for the extended lifetime appears to be different from that found with CaCO3 and CaSO4, and underscores both the variety of mechanisms whereby confinement affects the growth and transformation of solid phases, and the necessity to study a wide range of crystalline systems to build a full understanding of confinement effects. We suggest that in the case of ACP and OCP the extended lifetime of these metastable phases is chiefly due to a slower transport of ions between a dissolving metastable phase, and the more stable, growing phase. These results highlight the potential importance of confinement on biomineralization processes.

Investigation of the Precipitation of Calcium Sulfate in Porous Media: Experimental and Mathematical Modeling

Water injection is used worldwide as an improved oil recovery (IOR) method upstream in the petroleum industry to sweep oil toward production wells or to maintain the appropriate pressure in reservoirs. During the injection of water, differences in the properties of the injection water and the formation water can adversely affect the IOR operations. Such incompatibility leads to supersaturating the water with minerals salts that subsequently precipitate and impair the porous medium. Experiments were performed with three different injection rates to study the effect of the flow rate of the injected brine on damage to the porous medium. In addition, a kinetic mass-transfer model was developed for the transient sweep of the formation water by the injection water within the porous medium. Then, formation damage due to the reduced permeability of the medium was observed and modeled based on the dynamic behavior of scale formation in the cores. The effluent concentrations of magnesium (Mg2+) and calcium (Ca2+) ions were used to obtain the dispersion coefficients and the reaction rate constants for the precipitation of calcium sulfate in the sandstone cores. The formation damage coefficient for each run was calculated by the regression of the experimental and calculated permeability reduction ratios so that the average deviations for this step were less than 3.9%.

The gypsum–anhydrite paradox revisited

Despite much experimentation precipitation of anhydrite from solution in conditions similar to those occurring in sedimentary environments has not yet been reproduced in the laboratory. To resolve this long-standing contradiction we have monitored the precipitation and stability behaviors of calcium sulfate during experiments lasting up to two years. Calcium sulfate was precipitated from solution between 40 and 120°C at three different salinities and the formed solid phase was sampled at different time intervals (from 2min up to 2yr). We found that below 80°C gypsum is the sole primary phase and in the range of 80 to 120°C gypsum and bassanite are the primary phases. The stability of the latter increased with increasing salinity. As expected, we did not observe primary anhydrite precipitation, but over time phase transition occurred and anhydrite eventually appeared at temperatures >80°C. We show that intrinsic thermodynamic and kinetic properties severely constrain the precipitation of anhydrite (compared to gypsum and bassanite), and consequently, a considerable amount of time (e.g. >2yr at 60°C) is needed for anhydrite to form. Even so, at a geological time-scale, anhydrite can be considered as a pseudo-primary phase thus resolving the long-standing paradox of our inability to directly precipitate anhydrite in the laboratory at temperatures below 120°C and the abundant presence of anhydrite in evaporitic environments. Our results also show that at low water activity, bassanite becomes an important phase, which could be relevant to explain its presence on the surface of Mars.

Evaluation of a low-phosphorus terpolymer as calcium scales inhibitor in cooling water

Scale formation, e.g. precipitation of calcium carbonate and calcium sulfate, is a significant problem in cooling water system. For the control of calcium scale and in response to environmental guidelines, the novel low-phosphorus terpolymer was prepared through free radical polymerization reaction of acrylic acid (AA), oxalic acid-allypolyethoxy carboxylate (APEM), and phosphorous acid (H3PO3) in water with redox system of hypophosphorous and ammonium persulfate as initiator. The synthesized AA–APEM–H3PO3 terpolymer was characterized by Fourier transform infrared spectrometer (FT-IR) and 1H NMR. The inhibition property of the low-phosphorus terpolymer towards CaCO3 and CaSO4 in the artificial cooling water was studied through static scale inhibition tests, and the effect on morphology of CaCO3 and CaSO4 was investigated with combination of scanning electron microscopy and X-ray powder diffraction analysis, respectively. FT-IR was also used to study the effect on morphology of CaCO3. It was shown that AA–APEM–H3PO3 exhibited excellent ability to control calcium scale, with approximately 90.16% CaCO3 inhibition and 96.94% CaSO4 inhibition at levels of 8 and 4 mg/L AA–APEM–H3PO3, respectively. AA–APEM–H3PO3 also displayed ability to change the morphologies and crystal structures of CaCO3 and CaSO4 precipitates. The proposed inhibition mechanism suggests the surface complexation and chelation between the functional groups –P(O) (OH)2, –COOH, and Ca2+, with polyethylene glycol segments increasing its solubility in water.

Calcium carbonate and calcium sulfate precipitation, crystallization and dissolution: Evidence for the activated steps and the mechanisms from the enthalpy and entropy of activation values

Calculation of the thermodynamic parameters Eact, ΔH≠, ΔS≠ and ΔG≠ for the precipitation, crystallization and dissolution processes of the salts CaCO3 and CaSO4, leads to very important conclusions about the activated steps and the mechanisms. The ΔG≠ values are almost the same for all the processes at the same temperature, suggesting that the electrostatic forces between the ions Ca2+ and CO32− (CaCO3) and Ca2+ and SO42− (CaSO4) are the most important factors governing the above processes. The values of ΔH≠ and ΔS≠ differ but the values of ΔG≠ which refer to the overall transformations are the same and are independent of the various steps that take place as well as the mechanisms (associative, dissociative). Dehydration and aquation of the ions are revealed by the values of ΔH≠ and ΔS≠. The precipitation of the two salts may take place both by a dissociative mechanism (ΔS≠>0) or by an associative mechanism (ΔS≠<0). For processes taking place without the need for diffusion of the ions, (Ca2+, CO32−, SO42−), the free energy of activation is about 85kJmol−1, whereas when diffusion of the ions is necessary, an additional amount of ~20kJmol−1 is required. This amount is the activation energy for the diffusion. In the case of CaCO3, a wide range of values is found for ΔH≠ from −66.00 to 162.00kJmol−1, and for ΔS≠ from −501.00 to +248.00JK−1mol−1 while the ΔG≠ values cover only a small range from 75 to 90kJmol−1. Values of 120, 131 and 132kJmol−1 are reported for cases where retardation is caused due to the presence of foreign compounds. In the case of CaSO4, a wide range of values is found for ΔH≠ from 6.00 to 122.00kJmol−1 and for ΔS≠ from −342.00 to +117.00JK−1mol−1 while the ΔG≠ values fall in the narrow range from 80.00 to 89.00kJmol−1. A value of 126kJmol−1 is reported for cases where retardation is caused due to the presence of foreign compounds. The Eact values vary between −63 and 164kJmol−1 for CaCO3 and between 8 and 184kJmol−1 for CaSO4, demonstrating once again that the ΔG≠ value is more realistic, being almost the same for similar processes. The various small differences for the values of ΔG≠ arise from the different ionic strengths due to the concentration and charge of the foreign ions affecting the rate constants and thus the activation parameters. The pH also has an effect, as does the nature of the solvent. The very large and very small absolute values of ΔH≠ and ΔS≠ suggest composite reactions. Composite reactions in the precipitation, crystallization and dissolution processes are the ones where dehydration takes place (positive values of ΔH≠ and ΔS≠) followed by association of the ions (negative values of ΔH≠ and ΔS≠). The algebraic sum of the relevant ΔH≠ and ΔS≠ values gives the total value of ΔH≠ and ΔS≠. Activation energy values reported in the literature for certain cases compared to the values reported in the presence of adducts do not indicate retardation. On the contrary, they suggest acceleration of the reactions. The use of ΔG≠ instead of the Eact values is more realistic in showing the large retardation effect. The presence of salt, for example NaCl, causes retardation of the crystallization process.

Evaluating the Damage Caused by Calcium Sulfate Scale Precipitation During Low- and High-Salinity-Water Injection

Summary Scale deposition is a serious oilfield problem, when two incompatible waters interact chemically and precipitate minerals. Typical examples are seawater with a high concentration of sulfate ions and formation water with high concentrations of calcium, barium, and strontium ions. Mixing these waters may cause precipitation of calcium, barium, and/or strontium sulfate. The removal of the entire sulfate content from seawater might be a costly process and requires highly advanced techniques (nanofiltration). This study was conducted to investigate the damage caused by deposition of calcium sulfate precipitation and to describe the damage by use of the material-balance method, and then a new technique is proposed to prevent the damage caused by calcium sulfate scale. Coreflooding experiments were performed to assess the damage caused by calcium sulfate and a computed-tomography (CT)scan was used to locate the damage inside the core. Chelating agents, such as hydroxyl ethylene diamine triacetic acid (HEDTA), ethylene diamine tetra acetic acid (EDTA), and hydroxy ethyl imino diacetic acid (HEIDA), were used to prevent scale deposition in the Berea sandstone cores. The pressure drop across the core caused by scale precipitation will be predicted analytically. The results of the experimental data showed reduction of permeability by 20% from its initial value after seawater injection, caused by calcium sulfate precipitation. The results of the new analytical model showed that in approximately 1 month, the injection will stop or the injection pressure will exceed the fracture pressure of the formation. High-salinity-water injection caused severe formation damage, and the injectivity declined faster compared with the low-salinity-water injection. The material-balance calculations showed a good match between the experimental, field, and predicted data. The developed model of the pressure drop caused by calcium sulfate precipitation was used to predict the pressure drop across the core, and its result was in a good match with the experimental results. The new method was effective in preventing and removing sulfate precipitation.

Rigorous modeling of gypsum solubility in Na–Ca–Mg–Fe–Al–H–Cl–H2O system at elevated temperatures

Precipitation and scaling of calcium sulfate have been known as major problems facing process industries and oilfield operations. Most scale prediction models are based on aqueous thermodynamics and solubility behavior of salts in aqueous electrolyte solutions. There is yet a huge interest in developing reliable, simple, and accurate solubility prediction models. In this study, a comprehensive model based on least-squares support vector machine (LS-SVM) is presented, which is mainly devoted to calcium sulfate dihydrate (or gypsum) solubility in aqueous solutions of mixed electrolytes covering wide temperature ranges. In this respect, an aggregate of 880 experimental data were gathered from the open literature in order to construct and evaluate the reliability of presented model. Solubility values predicted by LS-SVM model are in well accordance with the observed values yielding a squared correlation coefficient (R 2) of 0.994. Sensitivity of the model for some important parameters is also checked to ascertain whether the learning process has succeeded. At the end, outlier diagnosis was performed using the method of leverage value statistics to find and eliminate the falsely recorded measurements from assembled dataset. Results obtained from this study indicate that LS-SVM model can successfully be applied in predicting accurate solubility of calcium sulfate dihydrate in Na---Ca---Mg---Fe---Al---H---Cl---H2O system over temperatures ranging from 283.15 to 371.15 K.

Performance of an environmentally friendly anti-scalant in CaSO4 scale inhibition

The precipitation of calcium sulfate (CaSO4) scale on heat transfer surfaces widely occurs in numerous industrial processes. For the control of calcium sulfate scale, a novel environmentally friendly type of scale inhibitor Acrylic acid–oxalic acid–allypolyethoxy carboxylate (AA-APEM9) was synthesized and characterized, and the anti-scale property of the AA-APEM9 copolymer towards calcium sulfate in the artificial cooling water was studied through static scale inhibition tests. It was shown that AA-APEM9 exhibited excellent ability to control calcium sulfate scale, with approximately 97.1% calcium sulfate inhibition at a level of 4 mg/L AA-APEM9. Scanning electron microscopy and X-ray powder diffraction studies have been performed to identify the change in crystal surfaces by the addition of the copolymer. Inhibition mechanism is proposed that PAA can recognize and react with positively charged calcium ions on the surface of inorganic minerals and water-compatible PEG segments increase its solubility in water.

First Report on Separation of Sulfate Ions from Evaporated Residue of RO Reject Stream Generated from the Leather Industry

Reverse Osmosis (RO), the membrane separation process, is widely being used on the fresh water supply to industrial and domestic sectors with the generation of reject streams containing huge concentrations of cations and anions besides organic impurities. Hence, the reject stream is advised to evaporate in solar pans/multiple effect evaporators to reduce the volume of disposal. The evaporated residue consists of a high concentration of chloride and sulfate ions along with organic impurities. In our previous study, chloride ion was separated as sodium chloride, and in the present investigation, an attempt was made to separate sulfate ions as calcium sulfate from the saturated solution of reject stream generated from the leather industry. The optimum conditions for the precipitation of calcium sulfate were observed as follows: pH 4.0 and temperature 40 °C. The separated calcium sulfate was calcined at 700 °C to destruct the occluded organic impurities. The recovered calcium sulfate was characterized for sur...

Confinement Leads to Control over Calcium Sulfate Polymorph

AbstractAlthough many processes of great biological, technological, and environmental importance, such as the formation of biominerals, the templating of nanostructures, and salt weathering, occur in confinement rather than in bulk solution, little is known about the influence of confinement over the precipitation of inorganic crystals. The effects of confinement on the precipitation of calcium sulfate are investigated using a crossed‐cylinder apparatus which offers confinement that varies continuously from zero to tens of micrometers. While the thermodynamically stable form of calcium sulfate (gypsum) is always observed at large surface separations, a remarkable stabilization of the metastable phases amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate (bassanite, plaster of Paris) is observed even at micrometer‐scale separations. For the first time, the approach is extended to study the combined effects of soluble additives and confinement, which are often present in natural or synthetic systems, and it is shown that this considerably extends the lifetimes of ACS and hemihydrate. These confinement effects are attributed to hindered aggregation of precursor particles at small surface separations, which limits polymorph conversion. While these results have immediate relevance to salt weathering and biomineralization processes, they are also important to the many crystallization and aggregation‐driven processes occurring in small volumes.

Calcium sulfate precipitation studies with scale inhibitors for reverse osmosis desalination

This study is focused on precipitation and inhibition studies of calcium sulfate (gypsum) scale in reverse osmosis (RO) desalination plants. The temperature and concentration range encountered in brackish water RO desalination process has been considered in conducting precipitation and inhibition experiments. Initial screening of a number of additives has been carried out. Formation of gypsum scale has been verified by XRD and SEM analysis. Successful additives were evaluated by measuring induction times using conductivity of solutions and their performance was compared with that of blank supersaturated calcium sulfate solution. The induction time of STD100, SHMP, STD101 was found to be 70, 690 and 1200min respectively at dose level of 10ppm, whereas a blended formulation FM101 (10ppm) did not show any precipitation till 3000min. The SEM micrographs of precipitate collected when FM101 was added as a scale inhibitor with that of a blank solution and other inhibitors indicate that, the formulation FM101 has significantly modified the crystal morphology and is probably responsible for increased induction time.

Reuse of Iraqi Agricultural Drainage Water Using Nanofiltration

Irrigated areas between Euphrates and Tigris rivers in Iraq suffer from salinity buildup in the root zone of corps. Agricultural drain water (ADW) from these areas is collected in a single main drain canal, in an annual flow rate of about 6 billion cubic meter. In the present work, a pilot-scale nanofiltration membranes unit was used to evaluate the feasibility of desalinating ADW from the main drain canal for further reuse. Bench scale experiments were conducted to determine the optimum anti-scale dosage values in the unit. These values were verified in a plate type laboratory scale NF membrane to visually monitor the onset of crystal appearance behavior. A method for calcium sulfate precipitation control is presented and an empirical correlation of anti-scale dosage as a function of concentration factor (CF) was obtained. A pilot-scale unit was used to investigate the performance of NF membrane. High rejection values for both cations and anions indicate that the use of NF membrane in desalinating ADW from the Iraqi main drain canal is promising. The treated drainage water is considered good for irrigation when classified according to Wilcox classification.

Effectiveness of Calcium Sulfate Scale Inhibitors in Spent Hydrochloric Acid/Seawater System

Previous results have reported the precipitation of calcium sulfate during matrix acid treatments when seawater was used to prepare HCl-based acid. Precipitations of calcium sulfate causes severe damage on the permeability of carbonate reservoirs and negatively impacts the performance of acid treatments. Typically, scale inhibitors are applied in the field to prevent the formation of calcium sulfate scale, and the object of this study is to evaluate the effectiveness of different types of scale inhibitors to inhibit the formation of calcium sulfate scale during the acid stimulation process. Scale inhibition efficiency was determined in batch tests under different temperatures (77 to 210°F) and different degrees of spent acid conditions. Sulfate ions were analyzed in the samples of the batch tests to detect the precipitations of calcium sulfate. Results showed that application of scale inhibitors can successfully mitigate calcium sulfate scale formation up to 210°F. At higher temperatures, the rate of calcium sulfate precipitation increased and the effectiveness of all types of scale inhibitors decreased greatly. Most of the scale inhibitors were not effective in acidic conditions and some of the scale inhibitors even precipitated out of the solution in the presence of high concentration of calcium ions. Methylene phoshponic acid scale inhibitor was the most effective one under various experimental conditions. The findings in this study provided information for better calcium sulfate scale control under acid stimulation conditions.

Modified pectin-based polymers as green antiscalants for calcium sulfate scale inhibition

In water processing industry, the precipitation of calcium and magnesium as carbonate, sulfate or phosphate and their adhesion to equipment surfaces is the main problem of mineral scaling. Many chemicals and antiscalants have been widely reported to mitigate the mineral scaling problems, but the efficiency is low on both economic and environmental grounds. In view of that, hydrolyzed pectin-based graft copolymers were synthesized and further functionalized by polymer analogous reactions for use as green antiscalant. The effect of functional graft copolymers on the crystal formation of calcium sulfate was investigated. From there it was observed that the ability to prevent the precipitation of calcium sulfate from homogeneous condition was almost independent of temperature and pH. The morphology and crystal structure of the precipitates were investigated using imaging and X-ray diffractometer (XRD). The result shows that the reported functional graft copolymers changed the morphology and shape of the crystal structures of CaSO4 precipitates from tubular to spherical.

Performance of a non-phosphorus antiscalant on inhibition of calcium-sulfate precipitation

The aim of this study is to report on the performance of a novel non-phosphorus antiscalant, acrylic acid (AA)-allylpolyethoxy carboxylate (APEC), being developed for calcium-sulfate scale inhibition in industrial water systems. The performance of AA-APEC on calcium-sulfate scale inhibition was compared with that of the two commercial inhibitors, polyamino polyether methylene phosphonates (PAPEMP) and polyacrylic acid (PAA), containing the same polyethylene glycol segments or carboxyl functional groups as AA-APEC. The study indicated that AA-APEC could act as a highly effective calcium sulfate inhibitor, having strong ability to inhibit the precipitation of calcium sulfate at a dosage of 2 mg L(-1), showing approximately 83.6% inhibition. The results also showed that AA-APEC dosage, the solution pH, inhibiting temperature, concentration of Ca(2+), and SO(4)(2-) all play important roles in inhibiting calcium-sulfate precipitation. The precipitation thermodynamics and kinetics at different temperatures were also discussed. X-ray diffractometer (XRD) and scanning electron microscope (SEM) analysis showed that AA-APEC strongly affected the texture and the morphology of the deposited calcium sulfate. Calcium sulfate has been inhibited through stabilization by adsorption onto crystal growth sites of nascent crystals altering their morphology.

Acidizing-Induced Damage in Sandstone Injector Wells: Laboratory Testing and a Case History

Summary Throughout a well's lifetime, formation damage can occur during the activities of drilling, completion, injection, or well-stimulation treatments. Typically, remedial treatments to restore the well performance involve injection of reactive fluids capable of removing such damage. Therefore, understanding damage mechanism and type is critical for fluid selection and effective treatment design. Without this knowledge, the conducted stimulation treatment could cause a more-severe form of formation damage. This report discusses the improper use of mud acid [at 9 wt% hydrochloric acid (HCl)/1 wt% hydrofluoric acid (HF)] in restoring the injectivity of Well N-510. The subject well was stimulated with two acid-stimulation treatments in an attempt to improve the poor results of a previous cleanout job, conducted to remove mud filter cake. These treatments were designed to remove the damage that has been limiting the well injectivity. However, it was found that these acidizing treatments created a new formation damage that resulted in the severe decline of well injectivity. Integration of chemical-analysis techniques performed on return fluids and coreflood experiments was used to assess the effectiveness of all conducted treatments. This report demonstrates the techniques used to identify the source and type of formation-damage mechanism that occurred during each treatment. On the basis of these studies, it was found that the poor results of the cleanout job were caused by precipitation of calcium sulfate. This precipitation was a result of the mixing between spent cleanout acid, having a high amount of calcium, and the high-sulfate-content water. Most of this precipitation occurred in the wellbore vicinity during the preceding stages of the well flowback. Calcium sulfate precipitation had a negative impact on the performance of the conducted acid-stimulation treatments. In the presence of this precipitation, the two successive mud-acid-stimulation treatments created another form of damage (i.e., in-situ fluoride-based scale). Initially, the fresh injected mud acid dissolved most of the calcium sulfate scale, and as a result, it contained a high amount of dissolved calcium ions. However, upon the spending of injected mud acid in the formation, calcium fluoride precipitated as a result of the increase of solution pH value. The interactions between different acid systems and the constituents of the downhole environment, resulting in the precipitation of calcium sulfate and calcium fluoride, are discussed. In addition, this report provides recommended modifications for future stimulation treatments, conducted under similar conditions, so as to prevent the formation of these scales.

A new precipitation pathway for calcium sulfate dihydrate (gypsum) via amorphous and hemihydrate intermediates

This work investigates the early stages of precipitation of calcium sulfate from aqueous solution at room temperature and shows for the first time that amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate are sequentially precipitated prior to calcium sulfate dihydrate (gypsum).

Phase Stability and Inhibition of Calcium Sulfate in the System NaCl/Monoethylene Glycol/H2O

Summary Calcium sulfate is one of the major mineral scales in oil and gas production. Hemihydrate (CaSO4·0.5H2O) and anhydrite (CaSO4) are the predominant sulfate scales formed at high temperature, while gypsum (CaSO4·2H2O) scale may form at low temperatures (&amp;lt;~45°C). However, it has been shown in this study that anhydrite can form at low temperature in the presence of excess amounts of monoethylene glycol (MEG), and this may occur during offshore production with long tie-backs. The prediction and prevention of calcium sulfate scales requires knowledge of the phase behavior of the three major phases of calcium sulfate. The phase behavior of different calcium sulfate phases is related to the supersaturation state, temperature, and fugacity of water. In this study, the effect of a common hydrate inhibitor, MEG, on calcium sulfate solubility and phase behavior was investigated. This study was run with NaCl/CaSO4/MEG/H2O solutions at 0–6 molality (M) NaCl and 0–95 wt% MEG at 4–70°C. Three approaches were taken to determine the kinetics of calcium sulfate phase transition at various temperatures, ionic strengths, and MEG concentrations: (1) dissolution of gypsum, (2) dissolution of anhydrite, and (3) nucleation and precipitation of calcium sulfate by mixing calcium- and sulfate-containing solutions. The effect of scale inhibitors on phase transition was also evaluated. Phase transition of gypsum to anhydrite was observed in the presence of high concentrations of NaCl and MEG, regardless of the experimental approach. The transition boundary of temperature and concentrations of NaCl and MEG can be estimated from solubility of calcium sulfate and the fugacity of water. The inhibition mechanism of hexamethylene diamine tetra (methylene phosphonic acid) (HDTMP), one of the most effective inhibitors for calcium sulfate scale, was also tested by investigating the kinetics of precipitation and inhibition of calcium sulfate.