Influence of Magnesium and Sulfate Ions on Wettability Alteration of

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Influence of Magnesium and Sulfate Ions on Wettability Alteration of Calcite, Quartz, and Kaolinite: Surface Energy Analysis V. Alipour Tabrizy,† A. A. Hamouda,*,† and R. Denoyel‡ † ‡

Department of Petroleum Engineering, University of Stavanger, 4036 Stavanger, Norway CNRS/Universite de Provence, Madirel, Centre de Saint-Jerôme 13397, Marseille Cedex 20, France ABSTRACT: The effects of SO42- and Mg2þ on the adsorption/displacement of stearic acid (SA), N,N-dimethyldodecylamine (NN-DMDA) and asphaltene, as oil soluble additives, onto or from calcite, quartz, and clay minerals are addressed in this paper. Thermal gravimetric analysis, isothermal water vapor adsorption, and contact angle methods are used to determine the extent of surface modification and evaluate the hydrophilicity/hydrophobicity of the modified powders and minerals, respectively. The experimental results of the modified mineral surfaces are analyzed using a suggested wettability index based on water vapor adsorption isotherm and contact angle. It is interesting to observe that SO42- and Mg2þ ions enhance hydrophilicity characteristic of the modified calcite surface while SO42- ions have insignificant effect on adsorption of the tested polar components on the silicate minerals. Mg2þ ions enhance the hydrophilicity of quartz and kaolinite surfaces modified by N,N-dimethyldodecylamine. On the other hand Mg2þ ions increase the hydrophobicity of silicate minerals when asphaltene is the surface modifying component. This may be due to bridging of the hydrated Mg2þ ions with asphaltene. The suggested bridging mechanism is also consistent in the case of alteration of calcite surface with asphaltene, however to lesser extent due to the more affinity toward calcite surfaces.

’ INTRODUCTION Several studies have been reported in the literature on improving oil recovery by spontaneous and forced imbibition at elevated temperature.112 It has been observed that the initial wettability of the reservoir rock and the final wetting state after imbibition process significantly impact the amount of oil that can be recovered from an oil reservoir.1,3,4,6,13,14 Hamouda and colleagues in a series of work dealing with calcite, water, and oil revealed that factors that may affect the initial wettability of the reservoir rock and final wetting state after imbibition should be mainly considered as oil composition, ion composition of initial and imbibing water, pH, rock surface characteristics, and temperature.8,1012,1518 Sea water contains, among other ions, Mg2þ and SO42- which have been shown to alter the initial and final wetting state of water flooded reservoir rocks.8,1012,15,17 Petrovich and Hamouda8 observed an increase of calcium ions and a decrease in the magnesium ion concentrations in produced water as a result of seawater injected into Ekofisk field samples (chalk reservoir). They concluded the possible adsorption/exchange processes between Mg2þ in seawater with Ca2þ in calcite, on a field scale. They reported also, as the temperature increases, the degree of exchange increases.8 Rezaei Gomari et al.15 showed that alteration of the wettability of modified calcite surface in presence of SO42- is not only due to possible reduction of the available active sites on the calcite surfaces but could also be due to a displacement process of various preadsorbed carboxylate ions of the tested fatty acids. This alteration corresponds to the change of the zeta (ζ) potential of the calcite particles when adsorption and/or exchange processes occur in the presence of SO42- and Mg2þ.15 Zeta potential of calcite particles in aqueous solution indicates that the calcite surface is positively charged in a large range of pH. However it may change significantly in r 2011 American Chemical Society

presence of ions.15 Adsorption of anionic surfactant and dissociated acidic components on the calcite surfaces are therefore, expected. Hirasaki and Zhang7 obtained also higher oil recovery in their experiments, by addition of inorganic salts (Na2CO3/ NaHCO3). Adsorption of carboxylate anion is influenced by the ionic composition of the water, ionic strength, and pH. The presence of Mg2þ and SO42- ions is shown to increase the waterwetness of the calcite. The degree of wetting is dependent on the pH.15 At pH > 7 in stearic acid dissolved in an n-decane, water, and calcite system, both ions reduce the contact angle. The measured advancing contact angles (toward the water phase) are 95°, 108° at pH 5, and 88°, 82° at pH 7 in presence of Mg2þ and SO42-, respectively.15 Computation of the disjoining pressure for the modified calcite and three brine systems (ion-free water, SO42-, and Mg2þ) indicates that, in the system containing magnesium, higher critical capillary pressures are needed to rupture the water film compared to that in the other two cases (ion-free water and SO42-). In other words, the presence of magnesium ions indicates a more stable water film and requires an increase in the capillary pressure to rupture the water film as the temperature increases.10 Calculation using DLVO theory and experimental observations may suggest that fine detachment of colloidal particles from rock surfaces is one of the mechanisms that may alter the wettability of the rock at elevated temperatures.4,10 It has been shown that the potential interaction between the calcite surface and modified calcite by fatty acid in presence of Mg2þ are always repulsive. On the other hand, for sulfate and distilled water, the interaction

Received: January 7, 2011 Revised: March 4, 2011 Published: March 11, 2011 1667

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potentials are slightly more attractive and become less attractive with the temperature reaching a repulsive status at >70 °C.10 Surface characterizations of minerals are done using thermogravimetric analysis (TGA), water vapor adsorption isotherm, and contact angle measurements.17,18,2025 Thermogravimetric analysis provides a quantification of strongly adsorbed molecules as well as a measure of the strength of the adsorption. This method has advantages compared to other techniques (such as spectroscopy) since it can be applied to all organic adsorbates and it does not require aromaticity or specific functional group for detection.26 From the shape of adsorption isotherms one can determine the wetting state of a solid.23,27 For a high energy solid surface, the amount of adsorbed vapor is infinite at saturation pressure, and the area under the adsorption isotherm is large. Such behavior is defined as complete wetting behavior when vapor at saturation pressure forms a macroscopically thick film and spreads on the solids.23 For a low energy surface, the area under saturation isotherm is small and the amount of adsorbed vapor at saturation is finite. This case is defined as partial wetting when a thin film (with a thickness ranging from one to a few monolayers) is formed.23 This paper addresses the influence of sulfate and magnesium ions on wettability modification of calcite, quartz, and kaolinite surfaces. The investigation is divided into two parts: first, the effect of ions on the adsorption of polar components, and second, the ability of ions to displace the preadsorbed organic materials in presence and/or absence of ions. The wettability of the modified surfaces is checked in presence of sulfate and magnesium ions by water vapor adsorption isotherm experiments. Characterization of the wettability of modified minerals has been performed using a dimensionless index, and the ability of brines to displace the preadsorbed organic materials is estimated by thermogravimetric analysis.

’ EXPERIMENTAL SECTION Materials. Three types of powdered solids were used in this work to represent different types of minerals in reservoir rock. Quartz and kaolinite powders were supplied by Sigma-Aldrich and their chemical compositions were SiO2 and Al2O7Si2 3 2H2O, respectively. Calcite powder was provided from Norwegian Talc AS. Crystalline calcite and silicate (quartz) are used for contact angle measurements. “Island spar” calcite and quartz crystals were obtained from India and supplied by J. Brommeland AS, Norway. In addition, n-decane and toluene were used in this study as solvents for added polar components. Both liquids were supplied by Chiron AS in HPLC grade (purity >99%) and dried over 0.4-nm molecular sieves for 24 h before use. The water was also purified through Milli-Q Millipore. Stearic acid (SA) and N,N-dimethyldodecylamine (NN-DMDA) were used as oil soluble additives to represent natural fatty acid and base in the crude oil. Table 1 summarizes the type and details of these chemicals. Asphaltene precipitated from crude oil in excess of normal heptane (1:40) is also used to represent the polar and heavy fraction of crude oil. After 48 h of equilibrium between crude oil and normal heptane, the

Table 1. Polar Components supplier and components

purity

structural formula

stearic acid (SA)

Aldrich g99%

CH3(CH2)16COOH

N,N-dimethyldodecylamine

Fulka g99%

CH3(CH2)11N(CH3)2

(NN-DMDA)

solution was filtered through a 0.22-mm filter (Millipore) to remove the asphaltene and then it was dried for 24 h under vacuum at room temperature. Na2SO4, MgCL2, and NaCL were dissolved in distilled water to prepare 0.1 M brine solutions in order to investigate their influence on the interaction between organic adsorbates and mineral surfaces. Methods. Adsorption of Polar Components on Mineral Powders. Two types of modifications were carried out to investigate the extent of modification by polar components and the presence of ions on initial wetting. In the first case, both solid powders and liquids (n-decane or toluene) were dried. The dried solids were placed in a desiccator in the presence of a saturated solution of K2SO4 for a period of 10 days at 25 °C. The saturated solution of a salt provides constant relative humidity and vapor pressure.16,28 For K2SO4 solution, the relative humidity reaches near 97% at 25 °C which provides a sufficient amount of water on the solid surface to investigate its effect on subsequent adsorption of organic adsorbates.28 Two grams of solid were introduced in a glass tube and 20 mL of stearic acid, and N,N-dimethyldodecylamine dissolved in n-decane were added to the solid. For these experiments, the initial concentration of acid or amine in n-decane was 0.01 M. To investigate adsorption of asphaltene molecule, 0.35 wt % of asphaltene was dissolved in toluene. For the second procedure, 1 g of powder was dispersed in 10 mL of distilled water (presence or absence of ions), and then 10 mL of dissolved polar components in n-decane and toluene were added to the system. In that case, a two-phase system was obtained. For both modifications, the tubes were stirred with a slow rotating agitator (50 rpm) for 24 h. This is based on results by Madsen et al.,29 Kokal et al.,30 and Rezaei Gomari et al.16 where no alteration of adsorption was observed after 20 h. The suspension was then centrifuged for 30 min at 4500 rpm. The liquid phase was separated from the solid, and the solid phase was washed with toluene and centrifuged again for 10 min at 4500 rpm. Finally the solid was dried under vacuum system (in order to accelerate the evaporation of liquid) for complete dryness. Displacement of Polar Components by Ions from Mineral Powder Surfaces. To determine the effects of different ions for displacement of preadsorbed polar components over minerals, humidified samples modified with organic adsorbates were immersed in distilled water, 0.1 M Na2SO4, and MgCL2 solutions. Then the sample was stirred with a slow rotating agitator (50 rpm) for 24 h. To separate the solid from liquid phase, the solution was centrifuged for 30 min at 4500 rpm and then dried under vacuum condition and room temperature. Gas and Vapor Adsorption Isotherms. The specific surface areas of the unmodified solid powders were determined by nitrogen and krypton adsorption at 77 K using manometric apparatus, ASAP 2010 from Micromeritics and AUTOSORB-1 from Quantachrome, respectively. Vapor adsorption isotherm experiments also were determined by gravimetry at 25 °C using TGA Q 5000 apparatus for both unmodified and modified samples to determine the surface areas and obtain further information in terms of surface energy. The temperature for heating treatment of unmodified and modified samples before water vapor adsorption was adjusted to 60 °C. The accuracy of TGA Q 5000 apparatus is about 105 mg. The average experimental error is approximately 11% with 5% standard deviation. Thermogravimetric Analysis (TGA). The amount of polar components adsorbed onto the surface mineral was estimated from the weight loss measured by thermogravimetric analysis (TGA). High-resolution thermogravimetric analyzer (TGA Q500 from TA Instruments) was applied as a rapid procedure to scan a wide variety of potential adsorbates. The sample was placed in an alumina pan, equilibrated at 30 °C, and then heated at 10 °C/min under dried air flow (40 mL/min) up to 600 °C using a value of 6 for the high resolution parameter. The accuracy of TGA Q 500 apparatus is about 105 mg. Contact Angle Measurements. Pieces of calcite and quartz chunks were filed using silicon carbide grinding powders from 120 grit to 1668

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4000 grit. The water-wetness of the crystalline pieces was checked by placing a drop of water on the surface before and after filing. After filing, the samples were washed with distilled water and dried for 24 h under vacuum and the temperature of 120 °C. The prewetted mineral samples in DW were aged in dissolved polar component solutions for 24 h, and then reimmersed in individual solutions of 0.1 M MgCl2, Na2SO4, and DW overnight. After this period of time, the modified sample was taken out and washed with n-heptane and then dried under vacuum at room temperature for 3 h. To measure the contact angle between water, n-decane, and mineral interface, two pieces of the modified crystallites were placed in two holders (upper and lower) mounted in the contact angle cell facing each other and surrounded by distilled water. A drop of n-decane (without additives) was introduced by a needle onto the surface of the upper calcite. Plated holders were carefully moved upward to touch the hanging n-decane drop and horizontally to produce advancing and receding contact angles, which were then measured by microscopic optical system. The standard deviation for the measurements is estimated to be about 3°. The detail description of the contact angle cell and procedures is published elsewhere.10,24

’ RESULTS AND DISCUSSION Unmodified Minerals. The specific surface area of powders can be calculated from gas or vapor adsorption data by applying the BET equation. Table 2 summarizes the estimated surface areas using the BET equation. From this table, it can be seen that variations of surface areas are less significant for quartz (≈ 3%) and kaolinite (≈ 13%) compared to calcite (≈ 69%). In the case of calcite, the calculated specific surface areas vary from 0.76 (m2/gr) for N2 to 0.64 (m2/gr) for Kr and 0.23 (m2/gr) for H2O. The difference in the specific surface areas using N2 and Kr for calcite is near 15%, which may be considered to be within the calculated difference of about 13% that is obtained in the case of kaolinite (including the water vapor adsorption isotherm). The

observation of the difference in the obtained surface areas for calcite by water and nitrogen adsorption isotherms has also been reported by Hansen et al.24 and Rezaei Gomari et al.16 Water vapor adsorption isotherm experiments at 25 °C can also be used to test the degree of hydrophilicity of unmodified quartz, calcite, and kaolinite powders (Figure 1). The isotherm for quartz surface appears to be type II according to IUPAC classification, where the isotherm curve is concave to the P/Po axis, then almost linear and finally convex to the P/Po axis. The sharp point between the concave and linear part of the curve indicates a formation of monolayer at low relative vapor pressure.31 The isotherm for kaolinite shows lower affinity to adsorb water and appears between type II and type III in IUPAC classification. Nevertheless the measured adsorption isotherm in this plot is calculated based on surface area (mg/m2) while the comparison based on weight (mg/g) is misleading as the surface area of kaolinite is 15 times higher than quartz. In contrast to quartz and kaolinite, the calcite surface shows type III and the interaction of water and surface seems weak.20 To examine the magnitude of water vapor adsorption on the solid surface in details, the number of water layers is compared when the relative vapor pressure (P/Po) is equal to 0.15.16,20 At this relative vapor pressure, the number of adsorbed water layers is estimated to be 1.13, 0.52, and 0.19 for quartz, kaolinite, and calcite, respectively. This may be interpreted that quartz surface is more hydrophilic than kaolinite which is itself more hydrophilic than calcite. Adsorption of Polar Components in Presence of Ions. In this section, the effect of ions (magnesium and sulfate) on hydrophilicity/hydrophobicity is addressed for modified surfaces of calcite and silicate (quartz and kaolinite) minerals. From the water vapor adsorption, wettability index may be estimated using eq 1, which is a ratio between the estimated surface areas, from the adsorbed water vapor, for the modified to that of the unmodified surface of the minerals.

Table 2. BET Surface Area (m2/g) Calculated from Nitrogen, Krypton, and Water Adsorption Isotherms N2

Kr

H2O

quartz

0.62

0.67

0.65

kaolinite calcite

8.56 0.76

9.47 0.64

9.95 0.23

WI ¼

RmH2 O ðBETÞ RuH2 O ðBETÞ

ð1Þ

O H O Where RH m2 (BET) and Ru 2 (BET) are measured surface areas covered by monolayer of water over modified and unmodified samples, respectively. In this equation, WI = 1 indicates highly hydrophilic surface and WI = 0 indicates highly hydrophobic

Figure 1. Comparison of water adsorption isotherms on unmodified minerals. 1669


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Figure 2. Comparison of water adsorption isotherms on unmodified and modified calcite with stearic acid in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Figure 3. Comparison of water adsorption isotherms on unmodified and modified calcite with asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

surface; hence it may be used to characterize and rank the treated surfaces from a hydrophilicity standpoint. The suggested wettability index (WI) needs to be further developed and tested for various systems to identify its applicability and limitations, however it used here for qualitative assessment of the surface hydrophilicity/hydrophobicity. The wettability index indicates the fraction of solid surface which is not affected by surface treatment. Calcite Mineral. Figures 2 and 3 show adsorption isotherms of water on unmodified calcite and modified calcite by stearic acid (SA) and asphaltene (Asph.) in presence of DW, SO42-, and Mg 2þ. The strength of modification in terms of hydrophilicity and hydrophobicity can simply be deduced from the relative position of the adsorption isotherms. As it can be seen from these

figures, in terms of hydrophilicity qualitative classification may be presented as follows: unmodified calcite> modified calcite with SA (or Asph.) in presence of Mg 2þ > modified calcite with SA (or Asph.) in presence of SO42- > modified calcite with SA (or Asph.) in presence of DW. It has been demonstrated also by Rezaei Gomari et al.15 that the affinity of water for calcite surface increases in presence of sulfate. They have also reported, even with fatty acid (18-cyclohexyl-octadecanoic acid) adsorbed onto the surface, that the affinity of water vapor for the sample modified in presence of sulfate is improved as compared to the case of unmodified calcite.15 From their observations it may be concluded that the presence of sulfate not only can decrease the amount of fatty acid but also 1670


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Figure 4. Comparison of wettability index for unmodified and modified calcite with asphaltene and stearic acid in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

makes the unmodified calcite more hydrophilic. In contrast to SO42-, which shows a significant effect on adsorption of SA, hence on wettability of calcite, DW has minor effect. This may be explained by the change of the surface charge of the modified calcite in presence of SO42- from positive to a negative charge while such effect is not occurring in presence of DW alone. It is interesting to see that Mg2þ ions enhance the hydrophilicity of modified calcite surfaces with SA and asphaltene more than that in the case of SO42- and DW. The exact mechanism(s) by which magnesium ions change the wettability of calcite surface is not fully understood, however it has been shown by Petrovich and Hamouda,8 that an interaction between Mg2þ and calcite surface takes place. The preferential interaction of calcite with magnesium ions reduces the possible interaction with other competing ions or molecules for the calcite surface. Figure 4 compares the wettability index for modified calcite by stearic acid (SA) and asphaltene (Asph.) in presence of DW, SO42-, and Mg 2þ. The wettability index of modified calcite by stearic acid is reduced from 0.63 in presence of Mg 2þ to 0.47 in presence of SO42- and to 0.32 in presence of DW. For treated calcite with asphaltene also the wettability index is reduced from 0.59 in presence of Mg 2þ to 0.40 in presence of SO42- and to 0.19 for DW. In other words, less SA and asphaltene adsorbed on the calcite surface in presence of magnesium ions compared to that for sulfate ions, as explained above. Silicate Minerals. In this section, the effect of SO42- and Mg2þ on modification of the hydrophilic silicate mineral surfaces (quartz and kaolinite) is investigated. An approach similar to that done in the previous section for calcite is followed here for comparison of the behavior of the different mineral surfaces. Mineral silicate surfaces are modified by NN-DMDA and asphaltene. Adsorption isotherm of water vapor of unmodified and modified quartz with NN-DMDA in presence of ion free water (DW), 0.1 M SO42-, and 0.1 M Mg2þ is shown in Figure 5. As shown, there is inconsiderable difference between the modification in presence of DW and SO42-. Similar to calcite/SA

system, modification in presence of Mg2þ shows more hydrophilic surface than that in the case with SO42-. The effects of ions for treated quartz by asphaltene are shown in Figure 6. It is interesting to see, unlike all the studied cases until this point in the paper with modification of the mineral surfaces, the presence of Mg2þ makes the modified quartz with asphaltene more hydrophobic. Presence of SO42- ions shows almost similar effect on the quartz surface hydrophilicity to that with DW. Presence of Mg 2þ in initial water may interlock the negatively charged asphaltene molecules to the negatively charged quartz surface; hence it may act as a bridging ion during wettability alteration mechanism. This is in agreement with previous work reported by Chukwudeme and Hamouda.11 Yan et al.32 reported that as the cation valence in irreducible water saturation is increased from one to three, the bridging mechanism is enhanced and the initial wettability of the mineral is modified to more oil-wet. However. silica surface and sulfate ions have negative surface charges and this ion has insignificant change on wettability modification as there is no clear difference between adsorption isotherms. The effect of 0.1 M SO42- and 0.1 M Mg2þ ions on adsorption of NNDMDA and asphaltene (Asph.) on quartz surface, in terms of the wettability index, is shown in Figure 7 with DW taken as a reference. It is a consistently more hydrophobic surface in case of modification of quartz surface in both cases of NN-DMDA and asphaltene in presence of SO42-, even to a larger degree of hydrophobicity than that in the case of DW. It is interesting to see that in the case of asphaltene in presence of DW, the quartz surface became more hydrophilic than that in the case of both ions SO42- and Mg2þ, where a reduction of the hydrophilicity was observed by 10 and 67%, respectively. In the case of kaolinite surface modification with NN-DMDA and asphaltene, as shown in Figures 8 and 9, the adsorption isotherms of water in presence of SO42- and DW are almost the same, while in presence of Mg2þ less water vapor adsorbed on the surface compared to the case of DW and SO42-. From a wettability index point of view, the effect of ions on the degree of surface modification by NN-DMDA and asphaltene 1671


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Figure 5. Comparison of water adsorption isotherms on unmodified and modified quartz with N,N-dimethyldodecylamine in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Figure 6. Comparison of water adsorption isotherms on unmodified and modified quartz with asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

taking DW as a reference, may be summarized as such (Figure10) that SO42- has almost similar effect on kaolinite surface as that for DW, where the differences are 10 and 6%, respectively. In the case of Mg2þ, kaolinite surface becomes more hydrophobic about 36% and 53% for NN-DMDA and asphaltene, respectively, than that for DW. This is i line with the explanation given above, where asphaltene/magnesium bridging may have occurred at the kaolinite surface. The exact mechanism is not fully understood,

however, our work11 and the work reported by Yan et al.32 explained, in different experiments, possible bridging as an explanation. In other flooding experiments for enhanced heavy oil recovery by alkaline flooding, it was found that oil recovery was greatly affected as wettability alteration of water-wet grains to preferentially oil-wet due to magnesium ion bonding is observed in micro model tests.33 1672


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Figure 7. Comparison of wettability index for unmodified and modified quartz with NN-DMDA and asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Figure 8. Comparison of water adsorption isotherms on unmodified and modified kaolinite with N,N-dimethyldodecylamine in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Effect of SO42- and Mg2þ on modified calcite and silicate surfaces, taking DW as a reference, is investigated also utilizing contact angle measurements. The investigated systems are calcite/SA, calcite/asphaltene, quartz/NN-DMDA, and quartz/ asphaltene. The measured contact angles are shown in Figures 11 and 12. As it can be seen, modifications in presence of SO42- and Mg2þ in the case of calcite systems (calcite/SA and calcite/asphaltene) are in line with the results obtained from WI estimation. It is very

interesting to see that the effects of sulfate and magnesium ions on the wettability consistently reduced the advancing contact angles by about 20 and 41% and about 21 and 49% from that for DW, for calcite and quartz minerals. In other words, in presence of SO42- and Mg2þ less efficient modification of calcite surface to more oil wet is occurring by SA and asphaltene. On the other hand, the effect of ions on modification of quartz surface to more oil wet with NN-DMDA shows a trend similar to that for calcite, where SO42- and Mg2þ reduced the advancing 1673


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Figure 9. Comparison of water adsorption isotherms on unmodified and modified kaolinite with asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Figure 10. Comparison of wettability index for unmodified and modified kaolinite with NN-DMDA and asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

contact angles by about 4.5 and 23%. Again the obtained contact angle in the case of modifying quartz surface to more oil wet with asphaltene seems to be consistent, where the contact angle is reduced by about 5%, which is almost the same as that in the case of NN-DMDA. In the case of modification of quartz with asphaltene in presence of magnesium ions contact angle increased by about 42%. This is not in agreement with WI measured by the water vapor adsorption in the case of modified quartz by NN-DMDA and asphaltene in presence of SO42-. From comparison of contact angle values, it can be seen that modified quartz in presence of 0.01 M SO42- becomes slightly water-wet

compared to modified quartz in presence of DW. The validity of the suggested WI based on BET method is dependent on regression parameter of fitting linear region of BET curve. In the original work of Brunauer, Emmett and Teller (1938), it was found that the type II and III isotherms on various adsorbents give linear plots over the approximate range of P/Po between 0.05 and 0.35.34 The WI is shown to be consistent with contact angle measurements for all experiments, except for quartz modified by NN-DMDA and asphaltene in presence of SO42-. The associated regression parameters of BET linear region for all minerals and modification are greater than 0.9, however the regression 1674


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Figure 11. Comparison of contact angle for unmodified and modified calcite with asphaltene and stearic acid in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

Figure 12. Comparison of contact angle for unmodified and modified quartz with NN-DMDA and asphaltene in presence of 0.1 M Mg2þ, 0.1 M SO42-, and DW.

parameters of BET linear region for quartz modified by NNDMDA and asphaltene in presence of SO42- are about 0.86 and 0.68, respectively; hence, the validity of WI, based on BET method, is limited for a regression parameter >0.9. Displacement of Polar Components by Ions. To determine the effect of SO42- and Mg2þ on displacing the adsorbed organic materials on the surface of the mineral powders, after modification of the humidified mineral surfaces to more hydrophobic, minerals were then immersed in 0.1 M Na2SO4 and 0.1 M MgCl2 following the modification procedure Thermal gravimetric analysis (TGA) was then performed. TGA results of unmodified

and modified samples (wet samples) by stearic acid (SA) and asphaltene (Asph.) is illustrated in Figures 13, 14, and 15. Three main steps for weight loss are identified as the temperature increases from 30 to 600 °C. For temperatures between 30 to 195 °C weight loss may be related to elimination (decomposition/evaporation) of the excess adsorbed modifying organic compound which has not been completely removed by toluene during the washing procedure of the mineral surfaces.20 The major weight loss step, which is used in calculation of surface coverage, occurs at temperature >195 °C. In the case of the modified calcite mineral surface with stearic acid (SA), the weight 1675


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Figure 13. Thermogravimetric analysis for modified and unmodified calcite mineral.

Figure 14. Thermogravimetric analysis for modified and unmodified quartz mineral.

loss occurred between about 195 and 300 °C, whereas for calcite sample modified by asphaltene the weight loss occurred between about 300 and 450 °C. The final weight loss at about 580 °C corresponds to the decomposition of calcite mineral. The observation of these three parts is in a good agreement with the literature.14,16 These figures also show the thermal desorption behavior of quartz and kaolinite modified by N,N-dimethyldodecylamine and asphaltene. Thermal behavior of these minerals is comparable to that for calcite weight loss. Thermal desorption of N,Ndimethyldodecylamine occurred at temperatures between 195 and 250 °C. This is a low temperature interval compared to that for stearic acid. This is in a good agreement with the observation reported by Tadros et al.26 They observed that the thermal desorption of amines occurs at lower range compared to carboxylic acids. Thermal decomposition of kaolinite is shown to be close to 500 °C, whereas for

quartz the decomposition is above the temperature range used in this study. The amount of adsorbed material on the surface depends on the type of polar components and the presence of ions during the surface modification. To obtain the surface area coverage per molecule of the adsorbent (i.e., the cross sectional area of adsorbate) from the TGA results, the following equation is used:16,35 σ ¼

SA Γ NA

ð2Þ

In eq 2, σ (Å2 /molecule) is apparent surface area coverage per molecule of adsorbent, SA (m2/gr) is the specific surface area of mineral, Γ (μmol/m2) is the adsorbed amount of organic compounds over the solid surface, and NA (molecule/mol) is the Avogadro number. Table 3 shows the calculated cross-sectional surface area per molecule of stearic acid, N,N-dimethyldodecylamine, and asphaltene. It also shows the calculated cross-sectional 1676


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Figure 15. Thermogravimetric analysis for modified and unmodified kaolinite mineral.

Table 3. Calculated Surface Area Coverage σ (Å2) per Molecule of Organic Adsorbate for Different Minerals and Displacing Ions σ (Å2/molecule) type of modification

calcite

quartz

kaolinite

modification in DW SA

16.58

N,N-dimethyldodecylamine asph.

30.36

18.68

16.97

68.26

61.25

modification in 0.01 M Mg 2þ SA

23.54


40.85

28.01 99.01

20.76 76.33

modification in 0.01 M SO42SA

18.35


34.62

18.64

16.92

68.01

61.03

surface area per molecule in presence and absence of Mg2þ and SO42- ions. Rezaee Gomari et al.16 and Wright and Pratt35 suggested that the areas occupied by organic functional groups are between 22 and 26 Å2 and larger than 51.5 Å2 for perpendicular and parallel orientations with the mineral surface, respectively. A mixture of perpendicular and parallel orientation of adsorbent on mineral surface gives an intermediate value.35 Accordingly, it may be concluded that SO42- and Mg 2þ ions may displace the preadsorbed polar components and affect the packing system of adsorbate depending on the characteristics of mineral surface and the type of polar component. From Figures 16, 17, and 18 it can be seen that SO42- has insignificant effect on displacing the adsorbed asphaltene and amine from the silicate minerals (quartz and kaolinite), whereas it can partly remove the adsorbed stearic acid and asphaltene from the calcite surface. This may be explained by the like surface charge of silicate minerals and SO42; hence low interaction. In the case of the calcite modified with stearic acid and

asphaltene, the adsorbed organic molecules may be displaced by SO42-. Table 3 summarizes the apparent surface coverage. It can be seen that the apparent surface coverage of the modified calcite powders changed from about 16.58 and 30.36 Å2/molecule in the case of displacement with DW to 23.54 and 40.85 Å2/molecule in the case of displacement with 0.01 M of Mg2þ, and to 18.35 and 34.62 Å2/molecule in the case of displacement with SO42- for SA and asphaltene, respectively. In the case of silicate minerals, the effect of SO42- displacement on the occupied areas by individual component, i.e., NN-DMDA and asphaltene, is almost identical to that case with DW displacement. During displacement of adsorbed components on the silicate surface with Mg2þ, in the case of quartz, respectively for NNDMDA and asphaltene, the occupied areas increased from 18.68 to 28.01 and from 68.26 to 99.01 Å2/molecule. In the case of kaolinite, the occupied areas by molecules increased from 16.97 to 20.76 and from 61.25 to 76.33 Å2/molecule. In summary, it is very interesting to see that the percentage increase of the occupied areas by molecules in the case of Mg2þ are about 33% ((2%) and 20% ((2%), for quartz and kaolinite minerals, respectively. It may be stated, in general, that the effect of Mg2þ in both cases of minerals has larger effect than that in the case of SO42-; in addition, SO42- ions have almost no effect in displacing the adsorbed molecules (NN-DMDA and asphaltene) from silicate surfaces. Possible Mechanisms. Two main cases are addressed in this paper. The first case is the modification of the mineral surfaces with organic components after being exposed to ion free water (DW) and water with 0.01 M Mg 2þ and 0.01 M SO42-. The second case is the reversed case of the first one, where the exposure to ions is after the modification of the mineral surfaces. In the first case mineral surfaces are totally exposed to the ions such as 0.01 M Mg 2þ and 0.01 M SO42-. In this case, there are two subdivided cases, where the type of mineral plays main role. In the case of calcite, extensive work has been done in our laboratory as well as observations from a North Sea operated chalk field (Ekofisk). The exact mechanism is not well understood yet, however, Petrovich and Hamouda,8 Rezaei Gomari and Hamouda,17 and Karoussi and Hamouda10 demonstrated 1677


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Figure 16. Comparison among weight losses due to displacement of adsorbed asphaltene from different minerals by distilled water and water containing 0.01 M SO4 2- and 0.01 M Mg 2þ.

Figure 17. Comparison among weight losses due to displacement of adsorbed NN-DMDA from quartz and kaolinite minerals by distilled water and water containing 0.01 M SO4 2- and 0.01 M Mg2þ.

the possible exchange between Mg 2þ and Ca2þ. Introduction of Mg2þ ions disturbs the existing equilibrium at the calcite solid surface with possible exchange/precipitation modifying the calcite surface, hence reducing the interaction/adsorption of SA and asphaltene on the calcite surface, i.e., less hydrophobic surface. Mg2þ ions increase the hydrophobicity of quartz and kaolinite when asphaltene is used as a modifying component compared to NN-DMDA, which may be due to network formation between magnesium hydroxyl compound and asphaltene that enhances the hydrophobicity of the silicate surface. This is in agreement with the observation by Chukwudeme and Hamouda11 for the interaction between asphaltene and Mg2þ ions in the work done on oil recovery by imbibition. Liu et al.33 reported the combination of Mg2þ and ionized organic acid/base

at oil/water interfaces and the adsorption of Mg2þ on the sand surfaces reduced the negative charges at both the oil/water and water/sand surfaces, resulting in an improved probability of interaction between the organic acid and sand surface while reduces the interaction of base groups and quartz mineral. For the second case (displacement), magnesium and sulfate to a lesser extent affect the displacement/occupied area by molecules in the case of calcite, due to their affinity to the calcite. However in the case of silicate minerals, sulfate ions have almost no effect, when compared to the DW (reference), while magnesium ions have a larger effect on quartz than in the case of kaolinite. For silicate minerals (especially quartz with smaller surface area compared to kaolinite) the ability of Mg2þ ion to remove the preadsorbed organic groups is less pronounced 1678


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Figure 18. Comparison among weight losses due to displacement of adsorbed SA from calcite mineral by distilled water and water containing 0.01 M SO4 2- and 0.01 M Mg2þ.

compared to calcite. However, it may cover the negative sites of silicate minerals due to cation exchange capacity and improving the wettability of modified minerals. It is important to mention that interpretation of the adsorbed asphaltene molecular orientations and apparent surface coverage on the mineral surfaces in presence or absence of different ions are affected by the molecular weight used; for example, based on eq 2, reducing molecular weight increases Γ (μmol/m2), hence lower apparent molar surface coverage (σ). This leads to a possible formation of mixed orientation on the surface (perpendicular/parallel). In this work, the value of 1000 mol/g has been used as an average for molecular weight of asphaltene to estimate the cross-sectional area of asphaltene over the minerals.36 The molecular orientation of asphaltene toward mineral in presence and absence of ions is less clear due to the uncertainty of molecular weight.

’ CONCLUSIONS Modification of calcite and silicate (quartz and kaolinite) surfaces with SA, NN-DMDA, and asphaltene, to more hydrophobic, in presence of Mg 2þ and SO42- showed reduction of the hydrophobicity of the mineral surfaces, compared to modified calcite surface with SA/asphaltene in presence of DW. This may be explained based on the reduction of the available adsorption sites. In the case of modification of quartz surface with NNDMDA, almost no difference is observed in presence of SO42and DW, while Mg2þ ions reduced the available adsorption sites (for NN-DMDA) on the quartz surface by possible blocking of the hydroxyl groups on the quartz surface. This is consistent also in the case of modification of kaolinite with NN-DMDA. It is interesting to see that in the case of modification with asphaltene in presence of Mg2þ, silicate (quartz and kaolinite) surfaces become more hydrophobic compared to that in presence of DW and SO42-, which show almost no difference. This may be due to that hydrated magnesium ions and asphaltene form a network at the silicate surfaces, hence reducing the hydrophilicity of the silicate surface.

The validity of the suggested wettability index based on water vapor adsorption isotherms and BET approach is limited for a fitting parameter >0.9.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ47 51 83 22 71; fax: þ47 51 83 17 50; e-mail: aly. [email protected].

’ ACKNOWLEDGMENT This paper has been financially supported by the University of Stavanger. We acknowledge Inger Johanne Olsen from University of Stavanger for getting the chemicals needed in time. ’ REFERENCES (1) Cuiec, L.; Morrow, N. R. 1st International Phenomena in Oil Recovery; Marcel Dekker, Inc.: New York, 1991; p 319. (2) Morrow, N. R.; Lim, H. T.; Ward, J. S. SPE Form. Eval. 1986, 1, 89–103. (3) Buckley, J. S. Ph.D. Thesis, Heriot-Watt University, September 1996. (4) Akin, S.; Schembre, J. M.; Bhat, S. K.; Kovscek, A. R. J. Pet. Sci. Eng. 2000, 25, 149–165. (5) Zhou, X.; Morrow, N. R.; Ma, S. SPE J. 2000, 5 (2), 199–207. (6) Babadagli, T. J. J. Pet. Sci. Eng. 2003, 37, 25–37. (7) Hirasaki, G.; Zhang, D. L. SPE J. 2004, No. 9, 151–162. (8) Petrovich, R.; Hamouda, A. A. Water-Rock Interaction; Arehart, G. B., Hulston, J. R., Eds.; Balkema: Rotterdam, 1998, pp 345348. (9) Hamouda, A. A.; Rezaei Gomari, K. A. In SPE/DOE Symposium on Improved Oil Recovery, Tulsa, OK, April 2226, 2006; Paper 99848. (10) Karoussi, O.; Hamouda, A. A. Energy Fuels 2007, 21 (4), 2138. (11) Chukwudeme, E. A.; Hamouda, A. A. Colloids Surf. A 2009, 336,174182 (12) Hamouda, A. A.; Karoussi, O. Energies 2008, 1, 1934; DOI: 10.339/en1010019. (13) Anderson, W. G. J. Pet. Technol. 1986, 38, 1125–1144. 1679


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Influence of Magnesium and Sulfate Ions on Wettability Alteration of

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