Precipitation of Calcium Carbonate in Porous Media in the Presence

Article pubs.acs.org/crystal

Precipitation of Calcium Carbonate in Porous Media in the Presence of n‑Dodecane Sofia Jaho,†,‡ Varvara Sygouni,†,‡ Stamatia G. Rokidi,†,‡ John Parthenios,‡ Petros G. Koutsoukos,†,‡ and Christakis A. Paraskeva*,†,‡ †

Department of Chemical Engineering, University of Patras, Patras, GR-26504, Greece Institute of Chemical Engineering Sciences, Foundation of Research and Technology, Hellas, Platani Achaias, Patras, GR-26504, Greece

‡

ABSTRACT: The precipitation and crystal growth of sparingly soluble salts are associated with scaling problems in oil and gas extraction processes. The presence of oleic phases and/or organic solvents added as inhibitors of the gas hydrates often complicates scale formation problems. The pore structure of the oil reservoir walls is of particular interest in the process of scale formation. In the present work the effect of the presence of n-dodecane in the crystallization of CaCO3 was investigated. The study was done in simulated one-dimensional porous media made of Plexiglas, in which supersaturated solutions of various supersaturation ratios (SRinitial) with respect to calcite were introduced. The growth of the precipitated crystals was continuously monitored by optical microscopy and calcium concentration monitoring. At high SRinitial values, larger numbers of small size crystals were grown, mainly near the oil−water interface. The presence of n-dodecane resulted in the reduction of the induction time preceding the appearance of CaCO3 crystals. The measured precipitation rates of CaCO3 were higher with increasing SRinitial values. The presence of n-dodecane in the supersaturated solutions, in the respective two-phase system, favored the stabilization of the thermodynamically less stable polymorphs of CaCO3 (aragonite, vaterite). Mapping analysis of the deposits polymorphs using Raman spectroscopy showed that the precipitation was not homogeneous along the flow channel at the experimental conditions of the present work.

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INTRODUCTION The precipitation and subsequent growth of sparingly soluble salts are encountered in a large number of industrial and environmental applications, including oil and gas production,1,2 desalination of water through membrane filtration,3,4 heat transfer processes,5,6 geothermal energy exploitation,7,8 and CO2 storage.9,10 In oil and gas reservoirs, scales deposited in the porous formations near the wellbore area result in significant reduction of the porosity and permeability of the local rock formations, causing flow blockage.2,11,12 Experimental investigations concerning mineral precipitation in sandstone cores11 and in sand packed beds13 have shown a severe degradation of the permeability ranging from 4 to 23% or up to 60% of the initial value. The formation of scale deposits in oil fields is mainly induced either directly by the existing formation water, from changes of the physical parameters,14,15 or upon mixing of incompatible water streams2,16,17 for the displacement of trapped oil and pressure maintenance during the secondary oil extraction process.18 The formation and injection water streams contain high concentrations of Ca2+, Ba2+, Sr2+, Fe2+, HCO3−, and SO4− ions,19 which at the local high temperature and pressure conditions raise the supersaturation with respect to the salts formed. The most commonly encountered scales in oil fields consist of calcium carbonate (CaCO3) and calcium phosphate (CaSO4) salts, which are mainly produced due to pressure and temperature alterations, respectively.16 Other types of sparingly © 2016 American Chemical Society

soluble salts frequently located in the oil industry include barium and strontium sulfates (BaSO4, SrSO4) and ferrous carbonate (FeCO3).15,20,21 There are several reports in the literature concerning the crystallization of calcium carbonate under various operating conditions. The rates of calcium carbonate nucleation and crystal growth are mainly related to the supersaturation ratio, the temperature, the pressure, the nature of the reservoir rock, the substrate and the flow conditions.22,23 Glass micromodels have been used to study scale formation in porous media, simulating the porous structure and the multiphase flow phenomena that take place in oil reservoirs.24−27 Ghaderi et al. (2009)24 studied the precipitation of calcium sulfate using a water-wet glass micromodel and showed that scale deposition inside the porous medium was random, while the scale tendency increased with temperature, salt composition, and flow rates. Dawe and Zhang (1997)25 investigated the precipitation of calcium carbonate in glass micromodels assessing the effect of the solution supersaturation, of the temperature and of the ionic strength, and they showed that the nucleation of CaCO3 was facilitated by the presence of gas− liquid interfaces. The deposition of calcium carbonate crystals in one- and two-dimensional (1-D and 2-D) porous media Received: July 14, 2016 Revised: October 18, 2016 Published: October 19, 2016 6874

DOI: 10.1021/acs.cgd.6b01048 Cryst. Growth Des. 2016, 16, 6874−6884

Crystal Growth & Design

Article

made of Plexiglas and glass was not uniform along the length of the path of the flow, resulting in a large number of small crystals at the inlet of the porous media.28 The crystal growth rates increased with increasing supersaturation, but the supersaturation decreased toward the outlet of the media. The precipitation of calcium carbonate in aqueous media has been investigated in the presence of foreign substrates or ions which showed either inhibitory activity with respect to the kinetics of precipitation,29−31 or they stabilized the growth of unstable polymorphs.32,33 Heterogeneous nucleation of calcium carbonate was studied both on calcite and on quartz crystals, over a range of supersaturations, the kinetics being fastest on calcite.34 The presence of oxalate anions in the supersaturated solutions reduced the crystal growth rate of calcite at pH 8.5 and 25 °C, while the unstable calcium carbonate monohydrate was stabilized.33 Other divalent cations such as Fe2+, Mg2+, Cu2+, and Zn2+ were proven to impose inhibition on calcite crystal growth,35,36 probably due to the adsorption of these impurities on the active sites of the growing crystals. The presence of foreign substrates (sodium dodecyl benzenesulfonic acid, SDBS) has also been shown to stabilize vaterite,37 while in more recent studies38 particles of nanocalcium-diethylenetriamine penta(methylenephosphonate) (Ca-DTPMP) showed that they were used to delay calcium carbonate scaling in water treatment processes. Heterogeneous nucleation of calcium carbonate has also been shown to take place on a number of polymers including elastin,39 styrene−butadiene copolymer,30 sulfonated polysterene or polystyrene divinylbenzene polymers,40 and cellulose41 which stabilized different polymorphs or hydrated salts. The presence of water-miscible alcohols in solutions supersaturated with respect to calcium carbonate has been reported to have different effects from acceleration of the rates of crystal growth to extension of induction times preceding the onset of crystallization.42,43 Recent work concerning the effect of the presence of monoethylene glycol (MEG) on the formation of sparingly soluble salts in sandbeds showed maximum consolidation due to the formation of the respective salts in the space in between the sand grains. Interestingly, the presence of n-dodecane which is water immiscible organic phase accelerated crystal growth of the salts investigated leading to pore clogging at the inlet of the sandbeds.44 In oil field conditions, nucleation and crystal growth of calcium carbonate take place from fluid media, supersaturated with respect to calcium carbonate in contact with solid substrates of various petrological compositions. As a rule, the fluids contain hydrocarbons (water insoluble) and other organic compounds (e.g., alcohols) are freely miscible with the aqueous phase. More specifically, carbonate reservoir rocks composed of carbonate minerals are naturally water-wet,45 but when the recovery of crude oil is initiated, the nonpolar heavy components such as asphaltenes, resins, and carboxylic acids alter the wettability to oil-wet.46 Wettability changes may also be due to other mechanisms such as the diffusion of surfactants47 often added to reduce the oil−water interfacial tension or to promote substitution of Ca2+ by Mg2+ and SO42− onto the chalk.48,49 About 80% of the carbonate reservoir rocks from around the world are mainly oil-wet.50 Wettability changes are important for displacement patterns in porous media and residual oil alterations.51,52 The presence of oily phases in aqueous media supersaturated with sparingly soluble salts may have a significant effect on the nucleation and growth of the respective mineral phases.53

In the present work the precipitation of calcium carbonate from supersaturated solutions was investigated in a model of 1D porous medium, made of Plexiglas under flow conditions, in the presence of n-dodecane, a water-immiscible, nonpolar organic fluid. As stated previously, in oil field conditions, the precipitation and subsequent growth of calcium carbonate take place in the presence of hydrocarbons or other organic compounds. n-Dodecane was chosen to simulate this procedure because it is a pure organic substance without impurities contained in crude oil and can simulate the oleic phase because its viscosity and density values resemble the corresponding properties of the fluids trapped in real oil wells. Additionally porous medium properties (width and depth of the flow channel) and used flow rates are such that simulated the flow conditions in oil wells. However, the scope of this research was a focus only on the evaluation of the effect of an oily phase on the growth of calcium carbonate in a porous medium. The impact of the initial supersaturation values (SRinitial) of the aqueous solutions on the crystallization of CaCO3 was investigated. The rates of crystal growth were measured, and the nature of the polymorphs forming was estimated from the habit of the precipitated crystals. It can be concluded that the presence of n-dodecane led to a decrease of the induction time of CaCO3 precipitation, and the crystals were formed mainly near the interface of the aquatic solutions and n-dodecane, due to the lower interfacial tension. In this case the presence of a foreign substrate (n-dodecane) did not show an inhibitory effect but seemed to stabilize the growth of the unstable polymorphs (aragonite, vaterite) of calcium carbonate.

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MATERIALS AND METHODS

Stock solutions of calcium chloride and sodium bicarbonate were prepared from crystalline calcium chloride dihydrate (CaCl2·2H2O) and sodium bicarbonate (NaHCO3) (Merck, Pro Analisi) using deionized, distilled water. The calcium carbonate supersaturated solutions were prepared by in situ mixing equal volumes of solutions of calcium chloride and sodium bicarbonate prepared by dilution of the appropriate volumes of the stock solutions. The solutions were filtered through membrane filters (0.22 μm, Millipore), while the sodium bicarbonate solution was freshly prepared for each experiment. Sodium chloride was added in order to maintain the ionic strength (IS) of the solutions constant at 0.15 mol/L. The initial supersaturation values (SRinitial) of calcium carbonate with respect to calcite were calculated using MultiScale software.28 The SRinitial values for the experiments done in the present work were 5.06, 10.5, 14.8, and 21.28. The experiments were done in a model simulating 1-D pore at ambient conditions (θ = 25 °C, P = 1 atm). The model was a channel made of Plexiglas, 6 cm length, 1 mm width, and 0.3 mm depth, as shown in Figure 1. In all experiments done in the presence of ndodecane (C 12 H26 , Sigma-Aldrich), the porous medium was presaturated with the oil which was allowed to flow through for 12 h at a constant flow rate of 0.1 mL/h. After this time period, the supersaturated solution of calcium carbonate was introduced into the porous medium by in situ mixing of solutions of calcium chloride and sodium bicarbonate and a displacement of n-dodecane may have occurred, forming drops of the insoluble organic fluid (gagglia) within the flow channel. The main purpose of this work was to observe the formation and growth of calcium carbonate crystals in the presence of the entrapped gagglia with the aid of an optical microscope (Zeiss) and how the presence of this foreign substrate would affect the induction time and the size of CaCO3 crystals. In Figure 3 the entrapped drops of n-dodecane are circled with green color, and as may be seen, they were present in all different experimental conditions. It should also be noted that the SRinitial values of calcium carbonate solutions in all the experiments were calculated taking into account the aqueous phase alone. 6875



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Figure 1. Experimental setup for the in situ monitoring CaCO3 crystal growth in a simulated 1-D pore (Plexiglas channel). The calcium and bicarbonate solutions were mixed at the entrance of the flow channel using a Y-shaped mixer within a Teflon tube of 3 cm in length and 0.5 mm in diameter, and two syringe pumps operating at constant flow rate 1 mL/h. As reported elsewhere,28 the distance between the mixing point and the inlet of the porous medium was 3 cm. This length was found to be appropriate for the complete mixing of the two solutions. Calcium carbonate crystal growth, taking place inside the channel, was monitored through an optical microscope (Zeiss) equipped with a digital programmed video camera (Axis 223 M network camera) connected to a computer. Snapshots of the precipitated crystals were recorded at discrete time intervals, and the size of the first observed crystal in each experiment was measured for the assessment of the rates of crystal growth. The pH of the effluent at the outlet of the channel was measured using a combination glass/Ag/ AgCl electrode calibrated against NIST standard buffer solutions (pH 4.008, 6.864, 7.414, and 9.181). The collected samples from the effluent were filtered through membrane filters (0.22 μm), and the filtrates were analyzed for calcium by atomic absorption spectroscopy (PerkinElmer, AAnalyst 300). Samples from the porous medium were collected for the investigation of the morphology of the precipitated crystals by scanning electron microscopy (SEM, FEI, Quanta FEG 250 and Zeiss V35). The elemental composition of the solid particles was examined by EDS microanalysis (Bruker) and by Raman spectroscopy (Renishaw inVia Reflex Raman microscope) in order to identify the calcium carbonate polymorphs. The Raman measurements were carried out using excitation laser line at 785 nm. The Raman system with Streamline capabilities allowed for rapid spatial mapping of the whole channel by collecting approximately 7 × 105 Raman spectra in the spectral region 100−1200 cm−1. Therefore, a high definition 2-D chemical image of the mineral deposits inside the channel was formed. The mapping area was 60 mm2 and scanned in steps of 30 and 3.6 μm in directions parallel (x-coordinate) and lateral (y-coordinate) to the channel, respectively. The microscope objective was a Leica x20, while the laser power was sufficiently high (∼80 mW) to ensure the detection of any vaterite crystals present. The Raman spectra analyses were focused at the detection and deconvolution of the characteristic peaks corresponding to calcite and vaterite at ∼1077 cm−1 and ∼1083 cm−1 respectively. Two-dimensional false color maps were generated by plotting the intensity of the characteristic peaks as a function of the spatial x, y coordinates, so that for each crystal polymorph the corresponding topological distributions were obtained.

Figure 2. Crystal size evolution as a function of time during the precipitation of CaCO3 from supersaturated solutions in the presence of n-dodecane at SRinitial = 5.06, 10.5, and 21.28, at 25 °C and ionic strength = 0.15 M NaCl.

Figure 3. Sequence of images captured during the precipitation of CaCO3 from supersaturated solutions in the presence of n-dodecane at SRinitial = 5.06, 10.5, and 21.28, at 25 °C and IS = 0.15 M NaCl. Droplets of n-dodecane are encircled with green color.

CO32 − + Ca 2 + ↔ CaCO3 (s)

Equations 1 and 2 imply that the carbonate species concentrations in the solution are a function of the CO2 partial pressure and the pH values. The supersaturation ratio with respect to CaCO3 (calcite) is defined as in eq 4: αCa 2+αCO32− SR = 0 K sp,CaCO (4) 3 In eq 4, α represents the activities of the subscripted ions and K0sp,CaCO3 is the thermodynamic solubility product of calcite, which is the most stable form of calcium carbonate at ambient conditions.54 From eq 4 it may be concluded that for SR > 1, the thermodynamic driving force for the precipitation of CaCO3 is favorable. The supersaturation ratio is directly related to the change in Gibbs free energy for the transition from the supersaturated solution to equilibrium.34 The type of calcium carbonate polymorph formed depends on parameters including the solution supersaturation, pH, CO2 partial pressure, temperature, and presence of foreign substrates.54 Calcite,

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RESULTS AND DISCUSSION The precipitation of calcium carbonate in water solutions may be described by the following equilibria,43 which determine the relative amounts of Ca2+ and CO32− ions in the supersaturated solutions: H 2CO3 ↔ HCO3− + H+

(1)

HCO3− ↔ CO32 − + H+

(2)

(3)

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Table 1. Experimental Data for the Initial Time for Visual Observation, the Initial Crystal Size and the Final Crystal Size during the Precipitation of CaCO3 from Supersaturated Solutions in the Presence of n-Dodecane and for the Blank Experiments at SRinitial = 5.06, 10.5, and 21.28, at 25 °C and IS = 0.15 M NaCl SRinitial

solvent

5.06 5.06

water water /n-dodecane water water /n-dodecane water water /n-dodecane

10.5 10.5 21.28 21.28 a

Figure 4. Crystal size evolution as a function of time for the INITIAL stages of the precipitation of CaCO3 from supersaturated solutions in the presence of n-dodecane at SRinitial = 5.06, 10.5, and 21.28, at 25 °C and IS = 0.15 M NaCl.

vaterite, and aragonite are the three anhydrous crystalline forms of calcium carbonate, while monohydrate and hexahydrate calcium carbonate are reported as two hydrate forms considered to be thermodynamically less stable.55 A detailed description of the solubility products of the various CaCO3 polymorphs and the conditions for their stabilization and transformation to more stable forms are given elsewhere.28 A set of experiments was done for the assessment of the effect of SRinitial on the crystal growth of calcium carbonate in the presence of n-dodecane. Figure 2 shows the growth of a single crystal as a function of time for different supersaturated solutions with SRinitial = 5.06, 10.5, and 21.28 at ambient temperature, in the presence of n-dodecane. At higher SRinitial value (21.28) the growth rates were lower and the final size of the first formed crystal of CaCO3 after 110 h (or 4.5 days) was 37 μm. Past 60 h, the increase of the crystal reached a plateau value, suggesting approach to equilibrium. At lower SRinitial values (5.06 and 10.5) the rate of the crystal growth was higher and the final size of the crystals was 71 and 50 μm, respectively. For low SRinitial values only a few but constant number of crystals were observed, while for higher SRinitial values the number of CaCO3 crystals formed was larger. The size of crystals formed at higher SRinitial value was significantly lower in comparison to the size of the crystals formed at lower SRinitial

t (first formed crystals)/h

crystal size /μm

final crystal size past 110 h of growth/μm

n/aa 5.5

n/aa 7.5

n/aa 71

6.5 4.5

6.1 5.2

57 50

3 1

4.5 3.8

32 37

n/a: not available.

Figure 6. Total calcium concentration in the effluent as a function of time for SRinitial = 10.5 and 21.28 in the absence and in the presence of n-dodecane; 25 °C, IS = 0.15 M NaCl.

values. This observation was confirmed in experiments done in the absence of the oleic phase (blank experiments) reported earlier at SRinitial = 10.5 and 21.28.28 It may therefore be suggested that at high SRinitial values, in the presence of ndodecane, secondary nucleation is significant because of the adequately elevated supersaturation with respect to calcium carbonate polymorphs. The interface between the entrapped

Figure 5. (a) Crystal size evolution as a function of time during the precipitation of CaCO3 from supersaturated solutions for SRinitial = 10.5 in the presence of n-dodecane and for the blank experiment, (b1) sequence of images captured during the precipitation of CaCO3 from supersaturated solutions in the blank experiments, and (b2) in the presence of n-dodecane, at SRinitial = 10.5, at 25 °C and IS = 0.15 M NaCl. Droplets of ndodecane are encircled with green color. 6877



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Table 2. Composition of Solutions before and after the Mixing Point, the Final Measured at the Effluent and the Calculated Equilibrium Composition Using MultiScale at 25 °C measured calcium concentration at the outlet (mM)

SRinitial

total calcium concentration after mixing (mM)

equilibrium composition (mM)a

ndodecane

blank

5.06 10.5 14.8 21.28

4.25 6.5 8 10

3.98 5.77 6.87 8.27

3.5 7.3 5.8 7.5

n/ab 5.1 6.5 7.4

a

Calculated. bn/a: not available.

grow mainly near the interface created by the flowing solution and the entrapped n-dodecane. The growth evolution of an isolated calcium carbonate crystallite for different SRinitial values (5.06, 10.5, and 21.28) over the first 14 h is shown in Figure 4 where induction times and initial crystal growth sizes can be observed. The induction time is defined as the time period elapsing between the achievement of supersaturation and the appearance of the first crystals, and it is influenced by parameters such as supersaturation, impurities, or flow rate. Stamatakis et al. (2005)56 showed that the induction time for CaCO3 precipitation in porous media under flow conditions depends strongly on the velocity. It was found that the induction time increases with decreasing averaging velocity. In the present work, the flow rate within the medium was constant at 2 mL/h for all the experiments so only the effect of the presence of n-dodecane on the induction time could be investigated. The first crystals were observed past 6.0, 4.5, and 1.0 h respectively, from the onset of the experiment. The rate of crystal growth calculated from the size-time profiles for the initial stages of the crystallization was higher with increasing SRinitial. Specifically, for high SRinitial values, the total number of the growing crystals was greater, as it is depicted in Figure 4. This results in covering more space inside the porous medium and preventing the growth of the primary formed crystals. The impact of the oleic phase on the crystal growth of CaCO3 for SRinitial = 10.5 is presented in Figure 5, in which the blank experiment is compared with the corresponding experiments conducted in the presence of n-dodecane. The

Figure 7. Precipitation of calcium carbonate in pore model: Total calcium concentration in the effluent as a function of time for SRinitial = 5.06, 10.5, 14.8, and 21.28 in the presence of n-dodecane; 25 °C, IS = 0.15 M NaCl.

ganglia of n-dodecane and the supersaturated solution seemed to favor the nucleation of CaCO3 probably because of the reduction of the surface energy. It should be noted however that in the presence of ndodecane, secondary nucleation was appreciable past the initial stages of the crystal growth of CaCO3. The results presented in Figure 2 do not take into consideration secondary nucleation. For high SRinitial values, the total number of the growing crystals was significantly larger, as may be seen in Figure 3. The formation of a large number of small crystallites is typical for high supersaturations. The series of snapshots, shown in Figure 3 were taken for SRinitial = 5.06, 10.5, and 21.28 in the presence of n-dodecane. The total number of the growing crystals was larger at higher respective solution supersaturation. The crystals encircled with blue color in each value of supersaturation are those that their size was measured as a function of time. The crystal encircled with red color at the last snapshot for SRinitial = 10.5 is an example for the crystals that were precipitated by secondary nucleation. Droplets of n-dodecane may be distinguished along the path of flow of the supersaturated solution in the model pore and are encircled in Figure 3 with green color. The majority of the primary and secondary precipitated crystals, shown in blue and red circles, respectively, in Figure 3, seem to

Figure 8. Rate of precipitation of calcium carbonate in the pore model in the presence of n-dodecane as a function of SRinitial (a) from measurements in the effluent (b) from the precipitate formed inside the channel; 25 °C, IS = 0.15 M NaCl. 6878



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Table 3. pH Values Experimentally Measured and Calculated by MultiScale for All the Experiments at 25 °C SRinitial

solvent

calculated initial pH

measured initial pH

calculated equilibrium pH

measured final pH

5.06 10.5 10.5 14.8 14.8 21.28 21.28

water/n-dodecane water water/n-dodecane water water/n-dodecane water water/n-dodecane

7.95 7.91 7.91 7.89 7.89 7.86 7.86

8.08 8.27 8.13 8.04 7.96 8.00 8.16

7.31 7.03 7.03 6.90 6.90 6.77 6.77

7.57 7.42 7.61 7.52 7.52 7.37 7.40

Figure 9. SEM pictures of precipitated (a) calcite (SRinitial = 21.28), (b) aragonite (SRinitial = 14.8), and (c) vaterite (SRinitial = 14.8), in the presence of n-dodecane; 25 °C, IS = 0.15 M NaCl.

Figure 10. SEM pictures of precipitated CaCO3 in the presence of n-dodecane (a), (b) aggregates (c) rhombohedral calcite crystals with steps; SRinitial = 10.5, 14.8 and 21.28, 25 °C, IS = 0.15 M NaCl.

Figure 11. (a−c) SEM pictures of precipitated CaCO3 crystals forming aggregates in the presence of n-dodecane; SRinitial = 14.8 and 21.28, 25 °C, IS = 0.15 M NaCl.

and Figure 5 (b2) (SRinitial = 10.5, in the presence of ndodecane). The number of crystals precipitated in the blank experiment was higher than the respective in the presence of ndodecane. It should be noted that in the latter case the crystals were found to form mainly near the n-dodecane ganglia/water interface. The initial conditions and the results of the experiments done in the presence of n-dodecane are summarized in Table 1. As may be seen, the presence of the oleic phase resulted in the reduction of the time lapsed for the observation of the precipitated calcium carbonate crystals, while the size of the grown crystals at the end of the experiment was smaller, in comparison with those measured in blank experiments. Supersaturation gradient generated within the porous medium was the main driving force for the precipitation and

crystal size evolution as a function of time (Figure 5a) suggested that the growth rate of an isolated calcium carbonate crystallite at SRinitial = 10.5 was reduced when n-dodecane was entrapped within the porous medium. However, the presence of the heterogeneity (n-dodecane) led to a significant reduction of the time required for the visual observation of the first formed crystal and the final size of the crystal. In the blank experiment, the first crystal was observed past 6.5 h from the onset of the experiment, as may be seen in Figure 5 (b1), and the final size corresponding to the end of the experiment was 61 μm (Table 1). In the presence of n-dodecane the observation time of the first crystal was past 4.5 h (Figure 5 (b2)), and the size at the end of the growth process was 52 μm (Table 1). Sequences of snapshots for the two experiments are illustrated in Figures 5 (b1) (SRinitial = 10.5, blank experiment) 6879



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Figure 15. Calcite (blue) and vaterite (green) detection from two points in zone A (close to the inlet) of the flow channel, for SRinitial = 10.5 in the presence of n-dodecane.

concentration values and the values measured in the effluent. Calcium concentration decreased with time in all the experiments, as a result of the formation of calcium carbonate crystals. In Figure 6, the calcium concentration ([Ca]t) in the effluent as a function of time for SRinitial = 10.5 and 21.28 for the blank experiments and for the experiments done in the presence of n-dodecane is shown. The drop of calcium concentration with time was higher for the blank experiments, indicating that the total consumption of calcium ions within the porous medium was higher, despite the fact that a large number of calcium carbonate crystals precipitated in the presence of ndodecane, as already shown (Figure 5). The change of calcium concentration as a function of time at the outlet of the pore model for four different initial supersaturation values (SRinitial = 5.06, 10.5, 14.8, and 21.28) in the presence of n-dodecane is shown in Figure 7. The higher the SRinitial value, the larger was the drop rate of [Ca]t in the effluent. This behavior has also been reported earlier28 for experiments done in the absence of organic substance and was attributed to the supersaturation gradient along the flow channel, which was higher as the SRinitial value increased. For each of the SRinitial values studied in the present work, from the slope of the total calcium variation as a function of time profiles ((d[Ca]t/dt)t=0), the respective rates of crystal growth of calcium carbonate were calculated. Plots of the measured rates as a function of SRinitial are shown in Figure 8a. As may be seen, higher rates of CaCO3 formation corresponded to higher initial supersaturation. The rate of precipitation of CaCO3 within the pore model as a function of SRinitial is shown in Figure 8b. The precipitation rate within the flow channel was calculated from the slopes of the calcium concentration difference just after mixing ([Ca]0) from the calcium concentration values at the outlet ([Ca]t) as a function of time for the various SRinitial values, i.e., from d([Ca]0-[Ca]t)/dt. As may be seen in Figure 8b, the rate of precipitation of calcium carbonate within the porous medium was higher for higher initial supersaturation values, following an exponential increase as a function of the solution supersaturation. The composition of the solutions before and after the mixing point, the calculated composition at equilibrium at 25 °C using

Figure 12. (a, b) SEM and (c, d) SEM/EDS pictures of precipitated calcite crystals for SRinitial = 10.5 and 21.28 in the presence of ndodecane; 25 °C, IS = 0.15 M NaCl.

Table 4. Contact Angle, Surface Tension, and Interfacial Tension Measurements properties

measured values

θ of CaCO3 saturated solution on Plexiglas (deg) θ of CaCO3 saturated solution in n-dodecane on Plexiglas (deg) σCaCO3 saturated solution (mN m−1)

46.8 80.8

σn‑dodecane (mN m−1) γn‑dodecane/sat. CaCO3 solution (mN m−1)

17.14 24.11

71.46

Figure 13. Flow channel divided into five zones for mapping using Raman spectroscopy (1: inlet point, 2: point where the snapshots of the visual observation experiments were recorded, 3: outlet point, A, B, C, D, E: scanning zones).

subsequent growth of CaCO3 crystals. The initial supersaturation values at the inlet of the flow channel, just after the mixing of the CaCl2·2H2O and NaHCO3 solutions, were calculated using MultiScale software. The supersaturation within the flow channel could, therefore, be estimated indirectly by the concentration changes between the initial calcium

Figure 14. Spectral region of the precipitated CaCO3 crystals in which the characteristic peaks of calcite (blue) and vaterite (green) are included. 6880



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Figure 16. Calcite (blue) and vaterite (green) detection in zones B (a), C (b), and D (c) of the flow channel, for SRinitial = 10.5 in the presence of ndodecane; 25 °C, pH 8.25, ionic strength = 0.15 M NaCl.

Table 5. Values of Ratio, r, for Each Zone of the Flow Channel, for SRinitial = 10.5 in the Presence of n-Dodecane; 25 °C, pH 8.25, Ionic Strength = 0.15 M NaCl zone in the medium

r

A B C D

0.38 1.18 1.25 2.34

Heterogeneous nucleation proceeds through the formation of the nucleating phase on a substrate. The energy barrier for the formation of a new nucleus on a substrate depends on the surface tension of the new nucleus, in a similar sense as wetting of the respective surface. Wettability of a solid surface by a liquid is the result of interactions between the molecules of the liquid and the solid surface and depends on the surface mineralogy, pH, the salinity of the aqueous phase, and the nature of the oil.57,58 When a droplet of the liquid is placed on the surface it may spread on it, and the solid surface is well wetted by the liquid or the droplet may shrink and the solid surface is then poorly wetted.53 The thermodynamic definition of wettability, γm, is given by eq 5 and corresponds to the energy loss in a liquid−solid system when the solid surface is wetted by the liquid. The wettability of a solid surface may be estimated from measurements of the contact angle, θ. For θ = 0° the surface is completely wet by the fluid while for θ = 180° completely nonwet.53 In reservoir rock formations, they may exhibit mixed (fractional) wetting, where different regions of the rock surface show different affinity for the existing liquids (oil, water).59,60

MultiScale and the final measured total calcium concentrations for the blank experiments and for the experiments in the presence of n-dodecane are summarized in Table 2. pH values experimentally obtained and calculated for the initial and equilibrium state are summarized in Table 3. As may be seen, the experimental pH values were close to the calculated. The drop of the pH between the initial and the final state was associated with the precipitation of CaCO3 crystals, since H+ ions were released in the solution as a result of the precipitation process (eqs 1 and 2). All three anhydrous polymorphs of calcium carbonate were identified in the morphological examination of the precipitate by SEM. Raman spectroscopy confirmed the findings. Wellshaped calcite crystal for SRinitial = 21.28 in the presence of ndodecane is shown in Figure 9a, while aragonite and vaterite crystals formed at SRinitial = 14.8 are shown in Figures 9, panels b and c, respectively. The presence of n-dodecane seemed to favor the stabilization of the thermodynamically less stable polymorphs (aragonite, vaterite), even 6 days past the mixing of the solutions and the establishment of supersaturation, while in the absence of n-dodecane only calcite was identified.28 The size of the crystals as shown in typical SEM images confirmed the findings concerning crystallite size from optical microscopy monitoring experiments. The presence of the oleic phase favored the formation of aggregates as shown in Figures 10a and 11a,b. The vaterite crystal shown in Figure 9c consisted of a large number of smaller calcite crystals, as the magnification of its surface shows in Figure 11c. Different forms of CaCO3 crystals were grown by a layer-by-layer mechanism as illustrated in Figures 10 and 11a, for various SRinitial values in the presence of n-dodecane. The formation of advancing steps on the calcite rhombohedral crystals may also be seen in the SEM of Figure 10c. The presence of n-dodecane during the crystallization process affected the texture of the grown calcite crystallites as Figure 12a−d shows. Figure 12, panels a and b show SEM pictures where foreign impurities cover partially the formed crystals of calcite and can be attributed to small droplets or ganglia of dodecane. In Figure 12c,d SEM/EDS was used to identify the composition of the foreign material on the surface of the crystals; however the detection of carbon elements with SEM/EDS analysis was difficult because elements lighter than sodium cannot easily be detected and identified by SEM techniques.

⎛ ∂G ⎞ γm = −⎜ ⎟ ⎝ ∂s ⎠T , P

(5)

In eq 5 G stands for the Gibbs free energy, s for the surface of the solid phase and T and P denote temperature and pressure, respectively. In the present work, it was attempted to associate the different findings of the nucleation of calcium carbonate in the presence of oil, with the wetting behavior of the oil used on the formation of calcium carbonate nuclei in the mixed aqueous-oil fluid. The measurements of the Plexiglas-oil-supersaturated solutions contact angle were measured with sessile drop technique, while interfacial tension (γ) of n-dodecane with saturated CaCO3 solution was measured with a tensiometer (Sigma 70, KSV) by the Wilhelmy plate technique.61 The interfacial tension of n-dodecane/saturated CaCO3 solution was found 24.11 mN m−1. The interfacial tension of docane/ water was found 24.47 mN m−1.62 The experimental data for the surface tension (σ) of n-dodecane and the saturated CaCO3 solution and their interfacial tension are summarized in Table 4 along with the contact angle measurements. The dramatic drop of the interfacial tension in the presence of n-dodecane is perhaps responsible for the catalytic effect of the presence of oil in the nucleation of calcium carbonate. The investigation of the calcium carbonate polymorphs deposited inside the channel of the pore model was done by Raman spectroscopic examination of the deposits inside the channels. The Raman experimental setup allowed for the spatial mapping of the entire flow channel, providing 2-D images of the chemical content as already described in the Methods section. In particular, Figure 14 shows the corresponding peaks for calcite at 1078 cm−1 and vaterite at ∼1083 cm−1, while Figures 15 and 16 show the false color maps are recorded with well-resolved intensity. In the false color maps, calcite and 6881



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vaterite crystals are blue and green respectively in a magenta background representing the channel material (PMMA) which is free of crystals after the flow. The channel was divided into five zones as shown in Figure 13, where zone A is the entrance zone of the pore model, point 1 represents the entrance point, zone E represents the outlet, and point 3 is the outlet point, while point 2 shows the point of the flow channel where the snapshots were recorded during the experiments. Two channel images were used for the estimate of the relative mineral formation inside the channel. Image processing (SigmaScan Pro) was used to estimate the total area of each zone occupied by the two CaCO3 polymorphs. Figure 15 shows images from an the zone A which is close to the inlet of the porous medium while Figure 16a, b, and c shows images taken from zones B, C, and D respectively. The ratio, r, of the area covered by calcite over the respective due to vaterite in each zone for SRinitial = 10.5 in the presence of n-dodecane is shown in Table 5, while the characteristic peaks of the two polymorphs are shown in Figure 14. As may be concluded from the data of Table 5, the precipitated vaterite crystals cover more area near the inlet of the flow channel were the values of ratio r are smaller compared to the respective values for zones C and D in which more calcite was found. Data are not available for region E because only traces of calcite and vaterite were detected. Near the inlet of the flow channel more CaCO3 crystals precipitated, as may also be seen in Figures 15 and 16, suggesting that precipitation during the experiments done in the presence of n-dodecane was not homogeneous along the flow channel. Similar results have been found from experiments in the absence of the oleic phase.28,63

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected], Tel: +30 2610 997252, Fax: +30 2610 997574. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially funded by the European Union (European Social Fund-ESF) and Greek National Funds through the Operational program “Education and Lifelong Learning” under the action Aristeia II (Code No 4420) and the action KRIPIS/PEFYKA. We would like to thank the Department of Material Science at University of Patras for providing the SEM EVO MA 10 and Dr. Vagelis Karoutsos for the technical assistance concerning the SEM/EDS images. The assistance of Giorgos Barmparis with the crystallization experiments is acknowledged.

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REFERENCES

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CONCLUSIONS The precipitation of CaCO3 in model 1-D porous media in the presence of n-dodecane was found to depend strongly on the initial supersaturation values. For high SRinitial values, at the initial stages of the formation process of the new crystallites the growth rate of individual crystals increased with increasing SRinitial. For low SRinitial values the number of crystals formed spontaneously was small and constant. These crystals grew constantly reaching relatively large sizes (ca. 60 μm). At high SRinitial values, the initially formed number of crystallites was higher and their population increased rapidly, while the average crystal size was significantly smaller than the corresponding size at low SRinitial. The oil−water interface favored calcium carbonate nucleation, probably because of the reduction of the interfacial energy. In the presence of n-dodecane in the model pore system the rate of crystal growth of individual crystallites was smaller, in comparison to the solutions in the absence of the oil phase at the otherwise same conditions. In the presence of n-dodecane, the time preceding the appearance of the first supercritical crystallites was shorter in comparison with the same solutions in the absence of the oil phase, because of the lower interfacial energy in this case. All three anhydrous polymorphs of CaCO3 were found to form in the model channel at the conditions of the present work. Raman spectroscopic mapping of the precipitates formed inside the channel showed higher concentrations of stabilized vaterite close to the entrance point of the supersaturated solutions, while at higher distances further from this point the thermodynamically most stable calcite was dominant. 6882



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Precipitation of Calcium Carbonate in Porous Media in the Presence

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