Effect of Salinity, Resin, and Asphaltene on the Surface Properties of

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Effect of Salinity, Resin and Asphaltene on the Surface Properties of Acidic Crude Oil/Smart Water/Rock System Mostafa Lashkarbolooki, Shahab Ayatollahi, and Masoud Riazi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5015692 • Publication Date (Web): 21 Oct 2014 Downloaded from http://pubs.acs.org on October 23, 2014

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Effect of Salinity, Resin and Asphaltene on the Surface Properties of Acidic Crude Oil/Smart Water/Rock System Mostafa Lashkarbolookia, Shahab Ayatollahi∗b, Masoud Riazia a

Enhanced Oil Recovery (EOR) Research Center, School of Chemical and Petroleum Engineering, Shiraz University, Post Office Box, 7134851154, Shiraz, Iran

b

Enhanced Oil Recovery (EOR) Research Center, School of Chemical and Petroleum Engineering, Shiraz

University, Post Office Box, 7134851154, Shiraz, Iran, Now with Sharif University of Technology, Tehran, Iran.

ABSTRACT It has been already well established hat adjusting the salinity of displacing fluid critically affects the oil recovery efficiency during secondary and tertiary oil recovery processes. In this investigation, systematic experiments are designed and conducted to find the effects of both low and high salinity water on the surface properties of crude oil-brine/solid surfaces. With this respect, the effects of the major salts including NaCl, CaCl2 and MgCl2 are tested in the concentration range of 0-45000 ppm on fluid/solid and fluid/fluid interactions for a crude oil /water/rock system. Two main surface properties including contact angle and interfacial tension (IFT) are measured using pendant drop apparatus. The obtained results demonstrate the critical effects of heavy oil components on the interfacial properties. High film stability in some cases resulted in small contact angle changes, mostly in the range of strongly water wet condition, for different brine salinity. Keywords: Interfacial Tension; Contact angle; Asphaltene; Resin; Salt

∗

Corresponding author, Email: [email protected]. Tel/Fax.: +98 21 66166411.

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1. Introduction As the oil reservoir pressure decreases and production starts to descend substantially, most of the oil will be trapped underground (about 85 % of original oil in place).1 Therefore, it is necessary to utilize methods to enhance the oil production from the depleted reservoirs by producing the residual oil in place. Water-flooding (known as the secondary oil recovery method) is the most popular and well studied technique to increase the oil recovery efficiency.2 This technique could be enhanced by modifying the salt concentrations manipulating viscous forces, fluid/fluid interactions (IFT) and fluid/solids interactions (wettability) to modify the mobility ratio during fluid injection process.3 Previous investigations revealed that higher oil recovery efficiency can be achieved if the ions concentration presented in the injecting waters is manipulated.4 This type of saline water which commonly known as low salinity water is recently gaining attention as a new and efficient EOR technique called in abbreviation “LoSal”, Smart Water (SW), or Advanced Ion Management.5 Although, experimental investigations revealed the effectiveness of the injecting water salinity on oil recovery efficiency, the involved mechanisms still are unsettled due to very complex interactions between different minerals, oil and water. Generally, the Smart Water is referred to the ion compositions adjusting/optimizing of the injected fluid in such a way that the change in the equilibrium of the initial crude oil/brine/rock (COBR) could be modified to the initial wetting conditions which allows the oil to be displaced easily in the porous rock.6 The popularity of this technique raises from several unique advantages of this method including efficiency of displacing light to medium gravity crude oils, ease of injection into oil-bearing formation, water availability and affordability, environmentally friendly, no expensive chemicals are added, low damage problems and lower capital/operating costs especially for the fields were already treated by water flooding process.5,6 Several investigations reported the LoSal process


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efficiency and its related mechanisms which are helpful to extend the laboratory results to the field-scale processes,

5-13

but to the best of our knowledge no LoSal water-flooding potential

has been reported in the literature. 14-16 Since investigating the LoSal process is in its starting phase, vast numbers of efforts have been performed to find the main active mechanisms although wettability alteration introduced as the main key mechanism.

12-17

According to the proposed LoSal mechanisms, wettability

alteration from oil wet and mixed-wet states toward more water-wet state during the LoSal water-flooding process is suggested as the main reason for oil recovery enhancement. Based on this fact, it is expected that the best results of oil recovery will be obtained if the LoSal injection performs for the oil-wet toward intermediate-wet rock. In general, there are several possible mechanisms can enhance the oil recovery during the LoSal flooding process including the presence of clay minerals, oil composition, the presence of formation water with high concentration of divalent cations (i.e., Ca2+, Mg2+), and the salinity level of the low salinity water in the range of 1000 ppm-5000 ppm. 2,6 Although, LoSal water-flooding has received growing attention over the recent years, effect of LoSal water-flooding on carbonate rock has not been systematically investigated compared to sandstones.18 Previously performed investigations revealed that most of the carbonate reservoirs are between neutrally to oil-wet states due to the adsorption of the natural surfactants present in the crude oil. On the other hands, since more than half of the world's hydrocarbon proven reserves are stored in carbonate reservoirs, it is necessary to systematically investigate the effect of LoSal water-flooding on oil recovery of this specific reservoirs. Besides, IFT of the aqueous phase and crude oil is also known as the other effective parameter could be manipulated by SW flooding.

8, 15, 19

A close look into the

previously published literature revealed contradicting results urge the necessity of performing more systematic investigations to find unique and reliable mechanisms of action for smart


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water injection. For example, Alotaibi and Nasr-El-Din

20

were measured the interfacial

properties between oil n-dodecane and brines with various salt contents as functions of salinity, temperature and pressure using pendant drop method. Their results indicated that there is a critical salt concentration, which the IFT between brine and oil reaches its minimum value. They believe that changing the salt concentration can act as a key parameter to enhance oil recovery if water-flooding process is being developed for specific reservoir. Also, Cai et al.

21

investigated the effect of carbon number on the IFT of hydrocarbon/brine

systems. They measured the IFT of 5 normal alkanes versus water/brine as well as some hydrocarbon mixtures versus water/brine using pendant drop tensiometry. They reported that the presence of salts in the aqueous phase increases the IFT regardless of the type of the salts. Moreover, Xu

22

studied the effect of the brine composition on IFT by changing the salinity

and salt compositions of the aqueous phase. Five different systems using live crude oil: deionized water, NaCl, CaCl2, formation brine, and 50 % formation brine in deionized water were examined to find the individual influence of each parameter. The dilution of formation brine did not affect the IFT behavior of the aqueous-crude oil system. The highest equilibrium IFT value was obtained for live oil + CaCl2 solution system compared with the others. Recently, Kumar

23

observed that the IFT of a hydrocarbon versus water increases with the

concentration of salt in the aqueous phase; but when a small amount of surfactant is present in the solution, the IFT decreases with salinity. Besides, Bai et al.

24

measured the IFT of

heavy crude oil and its components for different aqueous phases. They have concluded that NaCl concentration had no considerable effect on the IFT. More recently, Moeini et al.

19

measured the IFT between heavy crude oil/NaCl and CaCl2

aqueous solution at 313.15 K and atmospheric pressure. Their obtained results demonstrated that for both salts, the IFT decreases significantly at the beginning while this reduction turn


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over to increasing trend with low gradient. In addition, in all of the concentrations of salt, higher IFT values were obtained using CaCl2 compared with NaCl aqueous solution, which is more intensive and apparent at higher concentrations. There are natural compounds in the crude oil such as, asphaltene, resins, organic acids and solids that significantly affect the physical properties of emulsions and the solubility of some polar organic compounds into the oil-water interface. Xu et al. 22 observed that the IFT of oil/water significantly affected by both the oil and water compositions. In addition, asphaltene played an important role affecting the IFT properties. Considering the performed experiments, many researchers have studied the IFT behavior of surfactant formulations in the presence of only Na+ and Cl- ions, despite the fact that the connate (or interstitial) water in real petroleum reservoirs contains numerous ions (notably Ca2+ and Mg2+).25, 26 Due to this shortcoming, Jennings et al. 27 design different experiments at the presence of Ca2+ and/or Mg2+ ions which demonstrated that extreme effect of ions on the increase of IFT. Also, Kumar et al.

28

reported that the IFT of crude oil and surfactant

solutions in petroleum reservoirs can drastically be influenced by composition of the connate water. They have also mentioned that, among the different ions, Ca2+ and Mg2+ ions are very detrimental on the IFT behavior of surfactant solutions considering the ionic strength. Bring into account all of these contradicting results, it seems interesting to find the effect of salinity on the IFT and wettability alteration regarding the presence of polar compounds especially resins and asphaltenes. These kinds of experiments are desired and useful since no systematic investigation has been reported the sole effect of these polar compounds on the IFT and wettability variation. Therefore, in this study, the effect of NaCl, CaCl2 and MgCl2 salts concentrations (0-45000 ppm) on the surface properties of both liquid-liquid (IFT) and liquid-solid (wettability) with


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respect to the presence of polar compounds using one of the southern Iranian crude oil and carbonate rocks were examined systematically.

2. Experimental Section 2. 1. Experimental Apparatus In this study, a drop shape analysis apparatus (DSA 100, KRUSS, Germany) working based on pendant drop technique was used not only to measure the IFT between crude oil and saline solution but also to measure the contact angle for different cases. The DSA technique is probably the most advanced and accurate method for measuring the IFT especially for IFT rang of 3-80 mN/m. Compared to the other existing methods, the DSA technique for the pendant drop case is accurate for the IFT measurement (0.05 mN/m), fully automatic, and completely free of the operator’s subjectivity.

29, 30

Therefore, this technique has been

proposed widely as a standard method for measuring the IFT.

23

In the case of IFT

measurements, with the aid of computer-controllable IFT measurement apparatus equipped with image analysis software, an accurate dynamic interfacial tension measurement is possible. The obtained results revealed that the measured data are changed as a function of time however the last values as they approach to a constant IFT was reported in this study as the equilibrium IFT.

2. 2. Crude oil properties The chemical analysis of the used crude oil supplied from one of the Iranian oil reservoir is shown in Tables 1 and 2. Besides, the result of Infrared (IR) spectroscopy (perkin elmer spectrum rx1) of oil sample is shown in Figure 1. IR spectroscopy of crude oil makes it possible to determine the type of functional groups. Spectroscopic investigations in Figure 1 indicated the presence of sulfoxide, sulfone, acid, and carbonyl functions in the crude oil. In 6 ACS Paragon Plus Environment

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detail, the crude oil containing only small amounts of phenolic, amine and amide functionalities while the carboxyl signal has a high intensity. In the aliphatic, C-H stretch range and CH2 stretching dominates over CH3. This corresponds to the cyclic structures and high acid number (TAN=1.5 mg KOH/g). Generally, acidic components in crude oils consist of organic acids, inorganic acids and some other compounds such as esters, phenols, amines and pyrrole series, affecting the oil acidity status.

31

The acidity of a crude oil is most

commonly expressed by its total acid number (TAN), which is the number of milligrams of KOH determined by non-aqueous titration (ASTM D 664-1989) needed to neutralize the acidity in one gram of oil .32 In petroleum industry, if TAN number of a crude oil is higher than 0.5 mg KOH/g, the crude oil considered as an acidic crude oil.

33

Respect to this, the

TAN of crude oil was measured using a potentiometric titration based on ASTM D 664 method and the TAN was obtained about 1.5 mg KOH/g. Table 1 Table 2 Figure 1

2. 3. Extraction of asphaltene and resin A petroleum fluid commonly divided into three parts: oils (saturates and aromatics), asphaltenes and resins.

34-36

Resins and asphaltenes are similar to aromatics but are, larger,

polar, contain more fused aromatic rings and more heteroatoms including nitrogen (N), sulfur (S), oxygen (O), vanadium (V), and nickel (Ni). 37, 38 As the other active component of crude oil, resins can also play a vital rule in the IFT of aqueous phase/ crude oil although their effects are not well understood as well as asphaltene. Because asphaltenes and resins are two contiguous classes of components, they may have


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similar structure. However, the difference in size, aromaticity, polarity, and physical appearance confer different properties for asphaltenes and resins. 39 The results obtained from spectroscopic investigations revealed that not only resins consisted of hydroxyl groups, ester, acid, and carbonyl functions but also long paraffinic chains with naphtenic rings and polar functions are presented on the structure of resins to attribute them more surfactant nature.39-41 In this way, Chang and Fogler 42 have reported that the polarity of the head and length of the tail affect the effectiveness of the amphiphile. Besides, Petrova et al.

43

reported that low-molecular-weight asphaltenes have the

intermediate constitution between resin and high-molecular-weight asphaltene. However, it is not known if the polarity of asphaltene is mainly due to the presence of heteroatomic compartments in their structure or it is determined by the entire structure including the polycyclic aromatic core. Similar to the resins, the total amount of heteroatomic compartments in the low-molecular-weight asphaltenes is higher than that in the highmolecular-weight asphaltene. Also, it has been shown that the amount of polar heteroatomic compartments relative to the aromatic structure of resin is higher than in asphaltene.43, 44 On the contrary, Goual and Firoozabadi 45 claimed that the dipole moment increases sharply from oils (0–1 D) to resins (2.4 –3.2 D) to asphaltenes (3.3– 6.9 D). Polar species with a high dipole moment (greater than 2 D) affect molecular interactions in the petroleum fluid and result in the formation of micelles. Therefore, for most of the crude oils contain more saturates and aromatics, even small concentrations of asphaltenes and resin not only affect the quality of the crude oil because of high propensity to aggregate at the crude oil-water interface but also affect their rheological properties. 46-47 Therefore, in this study the effect and contribution of asphaltene and resin on the equilibrium IFT of crude oil/ aqueous solution including NaCl, CaCl2 and MgCl2 salts at the concentrations range of 0-45000 ppm were examined.


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There are several methods to extract the asphaltene and resin from crude oil (e.g., ASTM D893-69, D2007- 80, and modified D2007-80).

48

The used method in this study is based on

the removal of asphaltenes by precipitation using paraffinic solvent (n-heptane, IP 143/90)49 prior to chromatographic separation of the remaining crude oil on attapulgite clay and/or silica gel.

50

In brief, asphaltenes were extracted from the crude oil used for this study, by

dissolving crude oil in n-heptane with the ratio of 20:1 followed by soxhlet extraction for further purification. 51 In addition, resins were extracted from the same de-asphalted oil with the column chromatography method described elsewhere.48, 52 The maltene (deasphalted oil + n-heptane) was adsorbed to a column of silica gel (Merck, 35−70 mesh ASTM); followed by rinsing of the saturates and aromatics by a solution of 70:30 n-heptane and toluene. Finally, the mixture of acetone, dichloromethane and toluene with the ratio of 40:30:30 was used to extract the resins from the column.

51

The point must be mentioned that all of the used chemicals were

supplied from Merck, Germany with purity higher than 99.9 %.

2. 4. Density Measurement Fluids density significantly affects IFT calculations; therefore, it should be measured with high accuracy. The density of brine solution was measured using Anton Paar DMA 35N, (Graz/ Austria), under ambient conditions while density of the crude oil, asphaltene and resin solutions were measured using glass pycnometer (see Table 3 and 4). Table 3 Table 4


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3. Results and Discussion In the following sections, the effects of salts concentrations on the IFT and contact angle were analyzed and discussed. Each measured data point in the current investigation is the average of at least three independent measurements. In addition, along to each data point error bars are depicted for better comparison between the different cases. Besides, the measured data points which are depicted by different marks have been joined to each other by lines to clearly demonstrate the trend of variations and functionalities. Although IFT and contact angle measurements were recorded over one hour and twenty four hours respectively, for the sake of better comparison. Although IFT and contact angle measurements were recorded over one hour and twenty four hours, respectively, the most significant changes of IFT occur at the initial minutes of measurements while contact angle was not significantly changed as a function of time.

3. 1. The effect of monovalent salt concentration on IFT and contact angle In the first series of experiments, the contact angles were measured for the systems contained crude oil, brine (with different concentrations of NaCl (0, 1000, 5000, 15000, 30000 and 45000 ppm)) and carbonate rock (see Figure 2). A close examination on Figure 2 demonstrated that the contact angle is independent of NaCl concentration while the IFT increases as the concentration of salt in the solution increases to 1000 ppm. But, further increase of NaCl concentration to values higher than 1000 ppm cause to a reverse trend. In other words, as the salinity increases above 1000 ppm, the IFT decreases and then it increases again. Figure 2


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To check the sole effects of these natural surface-active agents, the asphaltene and resins of the used crude oil were extracted. Afterward, two different solutions of 8 wt/wt % of these components in toluene were prepared and the IFT measurements were performed. Figure 3 showed that similar trend can be obtained for the solution containing 8 wt/wt % of extracted asphaltene and resin in toluene. For the sake of comparison, the measured IFT of toluene/deionized water system in this study was about 35.4 mN/m close enough to the literature value of 35.8 mN/m.

53

After that, the IFT of 8 wt/wt % extracted asphaltene and

resin in toluene respect to deionized water were 23.9 and 24.1 mN/m, respectively. Since the value of IFT for extracted resin and asphaltene in toluene is lower than the value of IFT of just toluene, it can be concluded that the crude oil contains surface-active agents, which can act like a surfactant in the interface of crude oil and water. In detailed, the simultaneous presence of the hydrocarbon skeleton (hydrophobic) and the polar groups (hydrophilic) on a single molecule makes the asphaltene and resin molecules similar to surface active agents. So, thermodynamic stability (i.e. a minimum in free energy or maximum in entropy of the system) will be achieved when they are accumulated at a polar/non-polar (e.g. oil/water or air/water) interface, due to their affinity for both polar and non-polar phases.

19, 54

Therefore,

with the hydrophilic group aligned in the aqueous phase and the hydrophobic group in the organic phase, asphaltene and resin molecules are willingly adsorbed at the water–oil interface leading to more IFT reduction as natural surfactants. 55, 56 It has been observed that the IFT of a pure hydrocarbon (saturates and aromatics) versus water increases with the concentration of salt in the aqueous phase;

23

however the trend of

IFT of used crude oil in this study with NaCl salt solution is significantly similar to the resin and asphaltene. Therefore, it is concluded that the natural surfactants in the crude oil play the dominant role on the variation of IFT of crude oil at the presence of salinity.


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It is also worth mentioning that, in the case of 8 wt/wt % of extracted asphaltene and resin in toluene, no considerable variation of the IFT was noticed as the concentration of NaCl was increased. Therefore, it can be concluded that the studied monovalent salts are not able to promote considerably the natural surfactant molecules in the crude oil to transfer from the bulk-phase to the oil–water interface. Thus, the IFT of crude oil/NaCl is not considerably changed with the salt concentration. At low monovalent salt concentration up to 1000 ppm, the difference between IFT of NaCl/ crude oil and deionized water/crude oil is almost 4.1 mNm-1. It appears that due to the low synergism between monovalent salt and natural surfactant in the crude oil, the effect of salinity on water/hydrocarbon IFT is the dominant mechanism. Hence, when inorganic salts (Na+) are presented in the aqueous phase, the water molecules form a cage-like hydrogen bonded structure around the salt ions. At the interface, water molecules are in contact with different phases and the hydrogen bonding is disrupted creating a higher energy environment for the ions. Therefore, the salts are depleted near the interface and the surface excess concentration (the difference between solute concentration in the bulk and that at the interface) of salts is negative.

19, 23

As claimed by the Gibbs adsorption isotherm,

57

(eq 1),

IFT increases when inorganic salts are added to the aqueous phase. 24, 57

dγ = − RT ∑ Γi d ln ai where

dγ

(1)

is the change in IFT of the solution, R is the gas constant, T is the absolute

temperature, and

Γi

and

ai

are the surface excess concentration and the activity of the ith

component in the solution, respectively. In this case, Γsalt in the eq 1 is a negative parameter and due to negligible synergism between of asphaltene and resin molecules, it is obvious that the IFT increases as the concentration of salt increases.


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Addition of salts into the solutions alters the distribution of natural surfactants (including asphaltene and resin) at the interface, due to the electrostatic forces and consequently they can alter the IFT. It is reported that the presence of salts in water can alter the distribution of surface active component in oil phase toward aqueous phase leads to a salting- in and saltingout effects. Due to salting-in effect, salts accelerate the diffusion of surface active components from bulk solution to the interface at low concentration.

6, 58

Contrary, the

aqueous solubility of petroleum hydrocarbon species decreases with increasing salinity at high salt concentration (salting-out effect).59-61 Based on this fact, the obtained results revealed that as the concentration of monovalent salt (NaCl) was increased to higher than 1000 ppm, the activity coefficient of the salt increases and the salt molecules transfer to the oil phase,62 the cations near the interface tend to interact with the natural surfactant, especially asphaltenes, leading up to a reduction of IFT. With the cations present at the interface, negative surface excess concentration for salt turns to be positive results in positive Γasphaltene, which consequently lowers the IFT according to the Gibbs adsorption isotherm (eq 1). However, further increase of concentration makes an increasing trend in IFT takes place leaving an optimum salinity behind. The dominant mechanism in this region is suggested to be the salting-out effect. The natural surfactants will be depleted near the interface and migrate to the oil phase and break the balance of oil–water interface adsorption.63 Consequently, the concentration of salts and natural surfactants at the interface are depleted and Γasphaltene, Γresin and Γsalt become negative and IFT increases subsequently. According to the obtained results, it can be concluded that there is no systematic tendency between the salinity and IFT variation while contact angle was remained almost unchanged for all the measurements. Figure 3


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3. 2. The effect of divalent salt concentration on the IFT and contact angle In the next series of the experiments, the effect of CaCl2 and MgCl2 concentration on the equilibrium IFT and contact angle of crude oil versus salt concentration at ambient conditions are shown in Figures 4 and 5, respectively. A close look into these results revealed that the IFT initially experiences a sharp reduction as the concentration of MgCl2 and CaCl2 increases and then remained unchanged as the concentration further increases. Figure 4 Figure 5 Since the wettability of the reservoir is very important on the multiphase flow in the reservoir rock as well as the oil recovery efficiency, contact angle technique was used to find the alteration of carbonate rock wettability. In the case of measuring contact angle, results revealed that in contrast to the monovalent ions (NaCl), the contact angle initially increased with increasing salt concentration and then remained unchanged. However, the variation of wettability (contact angle) for MgCl2 salts (18.4°-24.7°) compared to CaCl2 salts (18.9°22.1°) as a function of concentration is slightly greater. Lingthelm et al.64 proposed a LoSal mechanism related to the thickness of the water-film referred as the double layer thickness between crude oil and reservoir rocks directly related to the bridging effect. However, two different possible mechanisms including bridging effect 6567

and salting out effect

6, 60

resulted in wettability alteration toward more oil-wet under the

influence of multivalent ions. Generally, there are three different interactions, which can take place if divalent cations (Ca2+ used as example) are presented in the solution: crude oil ---- Ca2+---- crude oil carbonate mineral ---- Ca2+ ---- carbonate mineral crude oil ---- Ca2+ ---- carbonate mineral


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Among the proposed interaction mechanisms, the first and second are responsible for wettability changes in some limited extend, however the third one can accelerate the wettability alteration.18 Commonly there are two ways which ions can move toward surface and change the wettability. The first one is ionic interaction mechanism which the opposite charged oil/brine interface force the surfactants to bind to the solid surface which leading to surface wettability alteration. The second one is the adsorption of ions at the solid surface follows by the attraction of opposite charged surfactants at the oil/brine interface. On the other hand, the strong binding between the divalent cations to the interfaces change the surface to be more positively charged and impact the pure acid/base interaction of the system.66 On the other hand, the previously published results in the literature has reported that in contrast to the divalent ions monovalent ions such as Na+ can cover the rock surface without destroying the pure acid/base interactions.68 In addition, the relative strength of divalent ions as structure breakers can be reflected in their hydration energy. Therefore, the organic materials are more soluble at the presence of divalent ions.6 In more details; the salting out effect not only keeps the organic material soluble in the oil phase but also can increase the tendency of organic materials to adhere to the solid surface. Thus in this case, wettability changes towards more oil-wet conditions. 18, 60 In this regard, NaCl does not change the wettability for different salt concentrations (see Figure 2), while divalent ions, due to bridging and salting out effects, slightly influence the wettability toward less water-wet condition (see Figures 4 and 5). At last, the existence of electrostatic forces can explain the stability of the water-films between oil and solid in oil reservoir, as these interactions also appear in colloidal dimensions.69 Finally, the effect of salt concentrations (CaCl2, and MgCl2) on the equilibrium IFT of crude oil and 8 wt/wt % extracted asphaltene and 8 wt/wt % extracted resin in toluene are shown in Figures 6 and 7. A closer examination on the results revealed that the IFT of crude oil,


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extracted asphaltene and resin were reduced because of the presence of both divalent ions. This observed trend is related to this fact that polar organic component of asphaltene and resin react with the divalent cations (Mg2+ and Ca2+) consequently produce complex ions, which are easier soluble in water phase and resulted in IFT reduction.70 In general, complex ion, charged molecular aggregate, consisting of a metallic atom or ion to which is attached one or more electron-donating molecules. Many complex ions, however, are only loosely aggregated and tend to dissociate in a water solution until equilibrium is established between the complex ion and its components. In particular, many complex ions constructed by divalent ions (calcium and magnesium salts) and of polar organic component, which have Nand O- bearing moieties, are soluble in aqueous solutions which can consequently enhance the surface excess concentration. In this case, Γasphaltene and Γresin in equation 1 is a positive parameter due to presence of asphaltene and resin at the interface of aqueous solution and crude oil. Therefore, based on Gibbs adsorption isotherm (eq 1), IFT decreases as divalent salt ion concentrations increases in the aqueous solution. Figure 6 Figure 7 The results presented in Figures 6 and 7 showed that the lower concentration of salinity introduce a greater effect on the reduction of solution contained extracted asphaltene compared with the solution contained resin. However, at the higher concentration of salts the trend was reversed and the effect of resin on the IFT reduction is more apparent. This observation is related to this fact that the resins molecules are smaller than asphaltene molecules. Therefore, at the same weight ratio concentration, the higher number of resins molecules are available to orient themselves at the crude oil/water interface compared to the asphaltene molecules. Besides, the smaller size and the presence of more heteroatoms in the resin molecules introduce higher polarity for the resins compared to the asphaltene. In more detail, since the molecules of asphaltene is bigger and need no solubilization for orienting in 16 ACS Paragon Plus Environment

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the interface, low amount of asphaltene molecules can fully occupy the interface. Consequently, the neutralization of surfaces charges for the case of asphaltenes occurs at the lower concentrations of salts. In addition, at high divalent ion concentration, high affinity of divalent cation to the oxygen of resin (obtained by IR spectroscopy) mitigated the salting out effect; therefore IFT was stabilized in the range 15000 to 45000 ppm.

3. 3. Comparing the effect of different salts on IFT The obtained results in this section are depicted in Figures 8 to 10 to clearly compare the effect of the different salts on the IFT of resins, asphaltene and crude oil. Based on the obtained results (see Figure 8), it can be concluded that the monovalent ions introduce slight effect on the IFT of resin while the effect of divalent ions on the IFT is more apparent. Moreover, MgCl2 showed higher effect on the IFT reduction. This observed trend can be related to the higher affinity of Mg+2 toward oxygen (present in the resin molecules) compared with Ca+2. Hence, magnesium, as the central atom, is capable of coordinating oxygen compounds in the crude oil which are in the resin structure. Figure 8 On contrary, in the case of asphaltene (see Figure 9) reverse trend for calcium and magnesium ions were obtained. This observed trend can be related to this possible mechanism that calcium ion is slightly bigger compared with magnesium. On the other hand, since the asphaltene has bigger molecules in size, it is easier for calcium ions to form complex with asphaltene molecules compared with magnesium. Therefore, for asphaltene case, CaCl2 reduces the IFT slightly more than MgCl2 salt. Also, similar to the resin, monovalent ions introduce slight effect on the variation of IFT of asphaltene . Figure 9


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Finally, the effect of salt type on the IFT of crude oil and water was investigated. As it is demonstrated in Figure 10, the effect of salt on the IFT reduction follows MgCl2>CaCl2>NaCl. This observed trend can be related to this phenomenon that NaCl has no significant effect on resin and asphaltene. On the other hand, the results revealed that a reduction in IFT using magnesium is more evident compared with calcium. Also, it has been reported that the effectiveness of crude oil surface active components in lowering the IFT is more dominant if they act together, compared with the case which they act individually. In other words, the surface active components in crude oil show synergistic effects.71,

72

According to this fact, since the magnesium show considerably better interaction to resin for IFT reduction compared to calcium, better results was obtained for IFT reduction of crude oil at the presence of magnesium. Figure 10

Conclusion In the current investigation, the effect of salts namely NaCl, CaCl2 and MgCl2 on the surface properties of crude oil, brine solution and solid carbonate rock surfaces were investigated. Different concentrations of the salinities in the range of 0-45000 (de-ionized water, 1000, 5000, 15000, 30000 and 45000 ppm) were selected for the tests. The results revealed that there are three dominant parameters which affect the IFT including a) the presence of natural surface-active agents in the crude oil, b) the type of salts, and c) salt concentration. Due to the presence of resins and asphaltenes a dual effect can be observed for divalent salt concentrations. For the case of low divalent salt concentration, the asphaltene content leads to a greater reduction in the IFT compared with the resins, while as the concentration was increased; the effect of resin solution for IFT reduction is dominant. In addition, the results revealed that NaCl concentration in the brine solution introduce no evident effect on the 18 ACS Paragon Plus Environment

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contact angle of CBR. However, MgCl2 and CaCl2 are able to slightly modify the wettability of the carbonate rock surface toward neutral wet.


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Novel Water flooding Strategy by Manipulation of Injection Brine

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Figure Caption: Figure1. IR spectroscopy of used crude oil Figure 2. Effect of NaCl concentration on IFT of water and crude oil and contact angle between NaCl brine, crude oil and carbonate reservoir rock Figure 3. Effect of NaCl concentration on IFT of crude oil, extracted asphaltene and resin from crude oil Figure 4. Effect of CaCl2 concentration on IFT of brine and crude oil and contact angle between CaCl2 brine, crude oil and carbonate reservoir rock Figure 5. Effect of MgCl2 concentration on IFT of brine and crude oil and contact angle between MgCl2 brine, crude oil and carbonate reservoir rock Figure 6. Effect of CaCl2 concentration on IFT of crude oil, extracted asphaltene and resin from crude oil Figure 7. Effect of MgCl2 concentration on IFT of crude oil, extracted asphaltene and resin from crude oil Figure 8. IFT of extracted resin and different brine as a function of salt concentration. Figure 9. IFT of extracted asphaltene and different brine as a function of salt concentration. Figure 10. IFT of used crude oil and different brine as a function of salt concentration.


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Figure1. IR spectroscopy of used crude oil


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Figure 2. Effect of NaCl concentration on IFT of water and crude oil and contact angle between NaCl brine, crude oil and carbonate reservoir rock


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Figure 3. Effect of NaCl concentration on IFT of crude oil, extracted asphaltene and resin from crude oil


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Figure 4. Effect of CaCl2 concentration on IFT of brine and crude oil and contact angle between CaCl2 brine, crude oil and carbonate reservoir rock


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Figure 5. Effect of MgCl2 concentration on IFT of brine and crude oil and contact angle between MgCl2 brine, crude oil and carbonate reservoir rock


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Figure 6. Effect of CaCl2 concentration on IFT of crude oil, extracted asphaltene and resin from crude oil


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Figure 7. Effect of MgCl2 concentration on IFT of crude oil, extracted asphaltene and resin from crude oil


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Figure 8. IFT of extracted resin and different brine as a function of salt concentration.


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Figure 9. IFT of extracted asphaltene and different brine as a function of salt concentration.


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Figure 10. IFT of used crude oil and different brine as a function of salt concentration.


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Table Caption: Table 1. Crude oil analyzes Table 2. Results of GC analysis Table 3.Density (g/cm3) of salt solutions Table 4. Density (g/cm3) of extracted asphaltene and resin in toluene solutions


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Table 1. Crude oil analyzes Specification

Result

Test method

Specific Gravity @ 20 °C

0.9224

ASTM D-40452

°API

21.49

ASTM D-40452

Water Content vol%

1.5

ASTM D-95

Sulphur Content wt%

3.62

ASTM D-2622

Kinematic Viscosity @ 20 °C c.St.

215.8

ASTM D-445

Asphaltenes %wt

11

IP-143

Wax Content %wt

3.25

BP-237

Molecular Weight

310

IP-86

Molecular Weight of Residue C12+ gr/gr.mole

529.5

IP-86

Density of C12+ Fraction

0.9917

ASTM D-40452


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Table 2. Results of GC analysis Hydrocarbon type (C

Effect of Salinity, Resin, and Asphaltene on the Surface Properties of

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