Wettability Alteration of Mineral Surface during Low Salinity Water

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Wettability Alteration of Mineral Surface during Low Salinity Water Flooding: Role of Salt Type, Pure Alkanes and Model Oils Containing Polar Components Abhijit Kakati, and Jitendra S. Sangwai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03727 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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Wettability Alteration of Mineral Surface during Low Salinity Water Flooding: Role of Salt Type, Pure Alkanes and Model Oils Containing Polar Components

Abhijit Kakati, Jitendra S. Sangwai* Enhanced Oil Recovery Laboratory, Petroleum Engineering Program, Department of Ocean Engineering Indian Institute of Technology Madras, Chennai – 600 036, India

Corresponding Author: *Jitendra S. Sangwai: [email protected] Phone: +91-44-2257-4825 (Office) Fax: +91-44-2257-4802

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ABSTRACT Low salinity water injection is an emerging enhanced oil recovery technique and an area of active research. According to many researchers, the role of brine salinity has been attributed to its ability to change the reservoir rock wettability. The influence of different parameters like total dissolved solid, concentration of individual ions, oil composition, lithology, acid and base number, etc., has been tested previously using different combinations of crude oil and reservoir core samples. However, due to the complex nature of crude oil-brine-rock system, there is no clear idea of the mechanism of wettability alteration and the influence of above parameters on this process. In this work, we have investigated the governing factors affecting the wettability of mineral surface using pure alkane liquids and model oils (containing organic acid and base). The wettability studies were performed through contact angle measurements over a wide range of concentration (1mM to 1M) of monovalent and divalent salts (NaCl, CaCl2 and Na2SO4) to identify the effect of salt types and concentrations of different ions present in the injection water. The use of model systems provided a better understanding of the wettability alteration mechanism in comparison to the earlier studies performed using crude oil and actual reservoir rock sample. The results of this study showed that the wettability alteration with brine salinity is significantly different for pure alkanes, model oils containing polar component and depends on the type of cations (monovalent vs. divalent) present in the system. Scanning electron microscopy and electron dispersive spectroscopy studies showed that the polar oil components like petroleum acids and bases get adsorbed on mineral (quartz) surface in the presence cations and primarily depends on the cationic concentrations in water, affecting the performance of low salinity water flooding process.


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Keywords: Low salinity water flooding; Model Oil; Polar components; Quartz surface; Wettability alteration. 1. INTRODUCTION Significant research efforts have been put forwarded into modifying the surface chemistry of solids to obtain specific wetting characteristics.1 Wetting of rock forming mineral surfaces by crude oil and water is also an important area of research in the field of crude oil recovery from subsurface geological reservoirs. Wettability governs the capillary retention phenomenon which in turn dominates fluid flow in the porous media. Wettability is also the most important factor that controls location and distribution of fluid in the reservoir rock.2 Good wettability of a porous matrix to one liquid generally indicates stronger retention of that fluid and easier displacement of the other fluid.3 Understanding the factors controlling wetting and alteration of wetting is always considered as a crucial aspect in studying most of the chemical enhanced oil recovery (EOR) techniques4-8 and also in environmental processes including contamination of water saturated soil by crude oil and it’s remediation.9-12 Injecting different chemicals in an aqueous phase has become a common practice to enhance crude oil recovery from subsurface reservoirs. More recently, it has been discovered that by tuning the ionic composition (salinity) of the injection water during water flooding process, the crude oil recovery can significantly be influenced. Several researchers have found that the crude oil recovery increases with the reduction in injection water salinity and named this process as ‘low salinity water flooding’.13-19 Currently, this is an area of active research in enhanced oil recovery. Many authors have tried to understand the physiochemical processes involved in the crude oil recovery improvement through this process and postulated different hypothesis based on their observations. One of the most common observation reported in most low salinity water 3 ACS Paragon Plus Environment

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flooding literature20-24 is the change of reservoir rock wettability. Myint and Firoozabadi25 in their recent review underlined that many researchers claimed wettability alteration to be the dominant mechanism behind the success of low salinity water flooding. However, the reason behind wetting alteration at the interfaces of rock-brine-oil in the reservoir during low salinity water flooding is still not clear26 and there is an ample scope in this area needing further investigations. Two types of representative rock surfaces, namely, quartz and calcite, have been used by several researchers to investigate the low salinity water flood performance.3, 27-29 Lashkarbolooki et al.30 examined the effect of NaCl, KCl, MgCl2 and CaCl2 in brine on the contact angle using carbonate rock surface and an Iranian crude oil. Their study concluded that the wettability of the carbonate rock surface can be shifted from strongly oil-wet to strongly water-wet condition by lowering the water salinity. However, they claimed that the monovalent cations showed better performance in altering the wettability of carbonate rock compared to the divalent cations. Yousef et al.31,

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studied the effect of dilution of injection seawater on the wettability of

carbonate reservoir. Their results revealed that the dilution of seawater can change the wettability of carbonate rock surface from oil-wet to water-wet state. Nasralla et al.33 studied the wettability alteration by low salinity water flooding through contact angle measurements on mica substrate using two dead crude oil samples. On the basis of experimental results, they concluded that the mica surface become more water-wet by lowering the salinity of injection water. Aslan et al.27 investigated the effect of NaCl and MgCl2 with varying concentrations in brine on the contact angle using two crude oil samples from two different reservoirs of a Chevron operated field on the quartz and calcite surfaces. They witnessed a non-monotonous trend from their contact angle data. In case of NaCl brine, they observed a decrease in contact angle initially with increase


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NaCl concentration and found a minimum value between 0.1 to 1M for both quartz and calcite substrates. When they further increased the NaCl concentration to 10M, an increase in the contact angle was noticed. They found a different trend in case of MaCl2 brine. Increase in MgCl2 concentration in brine resulted in decrease of contact angle and attained a minimum value between 0.001 to 0.01M. However, further increase in MgCl2 concentration increases the contact angle values which reached a peak at 1M concentration. The contact angle again decreased when MgCl2 concentration was further increased to 10M.27 Yang et al.34 studied the relevance of calcium ions in the wettability alteration process of quartz surface during low salinity water flooding through adsorption and desorption experiments. They found that the wettability alteration behavior is primarily governed by the presence of divalent ions since they help in adsorption of acidic components of the crude oil onto the quartz surface. Increase in divalent ion concentration promotes adsorption of acidic components and increases oil-wetness. Some studies35,

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have also proposed that formation of water-in-oil micro emulsion at low salinity

condition as a reason for wettability alteration. This was proposed based on the experimental observation of flow visualization on micro-models and imbibition experiments using clay free sandstone cores. Bartels et al.37,

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conducted contact angle studies using glass substrate and

micromodel functionalized with clay mineral for crude oil and non-polar decane oil. Their study revealed that low salinity water is responsive in altering wettability towards water wet state in case of crude oil whereas they haven’t observe any change for pure decane. Again, more recent investigations showed that the wettability alteration during low salinity water injection is irreversible.39 The conflicting observations from various experimental studies is due to the complex nature of crude oil-brine-rock interactions. The actual reservoirs systems are too intricate to


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identify the conditions governing the wetting behavior during low salinity water flooding. Therefore, it is necessary to simplify the complex reservoir systems by adequate model system to understand essential details. Only few studies were reported in open literature that utilizes complete model system to understand wetting condition during low salinity water flooding. Mugele et al.3, 40 reported such study using model system to understand the wettability behavior of mica surface which may not closely represent the sandstone reservoirs. They studied the effect of NaCl and CaCl2 with varying concentrations in brine on the wettability of mica surface using only decane as a model oil. In this work, we have investigated the effects of varying ion concentrations of monovalent and divalent ions and its type (Na+, Ca2+, Cl-, SO4 -) in the brine on the wettability alteration behavior of quartz surface which is the most important rock forming mineral for sandstone reservoirs. Various concentration of salts (such as, NaCl, CaCl2, and Na2SO4), such as 0.001M, 0.01M, 0.1M and 1M have been used to prepare various brine solutions. A series of contact angle measurements were conducted on the quartz substrates using pure alkane liquids and model oils. Four different alkane liquids, such as n-octane, n-decane, n-dodecane and nhexadecane have been used in this work in order to relate the wettability alteration with molecular weight of alkane and in the presence of monovalent and divalent salts in the brine. Also, in order to study the role of acidic and basic components of crude oil on the wettability alteration during low salinity water flooding, two model oils were prepared by adding commercially available organic acid and base, such as stearic acid and n-decylamine to the base oil (decane+ toluene). The aging time and temperature were reported to be an influencing factors in the wettability studies by several researchers.41-44 Therefore, in this study, the effect of aging time and temperature has also been incorporated. Adsorption of organic acid and base on the


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quartz surface at different brine concentrations has also been investigated using scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS).

2. EXPERIMENTAL SECTION 2.1 Material Used The details on the chemicals used in this study have been provided in Table 1. As a nonpolar oil phase, four pure alkane liquids were used in this study which are: n-octane (C8), ndecane (C10), n-dodecane (C12) and n-hexadecane (C16). The alkane liquids were used as provided by the supplier. The salts, viz., sodium chloride, calcium chloride and sodium sulphate were used to prepare the brine solutions of various concentrations (e.g., 0.001, 0.01, 0.1 and 1M) by dissolving in deionized water. In addition, two types of model oils were prepared by adding acidic and basic components to the base oil. Model oil-A is prepared by adding small amount stearic acid to the base oil consisting 8:2 volume mixture of n-decane and toluene. Model oil-B is prepared by adding small amount of n-decyl amine to the base oil consisting 8:2 volume mixture of n-decane and toluene. The concentration of the stearic acid and n-decyl amine in the model oils is 2 % by weight. Stearic acid and n-decyl amine have been used in the model oil so as to represent organic acid and basic compounds which occurs in the crude oil. Since alkanes are the most abundant hydrocarbon in the crude oil therefore the base oil used for model oil preparation primarily composed of alkane (decane). Small amount of toluene is also added to dissolve the stearic acid. The salt quantities were measured using LC GC RADWAG AS/X 220 analytical balance (RADWAG Wagi Elektroniczne, Poland; repeatability, ± 0.1 mg; readability 0.1 mg) with ±0.00004 mass fraction of uncertainty. Sudan IV, an oil soluble dye was used to provide color to the transparent oil phases during contact angle measurements.


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2.2 Substrate Preparation The quartz substrates used for contact angle measurement has a dimension of 25mm × 25 mm × 3mm supplied by a local vendor in Chennai, India. The substrates were cleaned by modifying an existing27,

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smooth surface cleaning method. In this method, the washing

procedure has been followed in the sequence as: 5 min of toluene power wash, 5 min suspension in toluene, 5 min acetone power wash, 5 min suspension in acetone, 10 min deionized water power wash, 10 min suspension in deionized water. Here, power wash means washing the substrates under fluid force applied by wash bottle or squeeze bottle. The substrates were aged in the respective brine solutions for 24 hours before doing the experiments to establish an equilibrium between the brine and quartz surface.2, 33

2.3 Contact Angle Measurement Anderson41 reviewed various methods of wettability measurement used in oil industry which can be grouped as: qualitative and quantitative methods. Qualitative methods include imbibition rates, microscopic examination, floatation, relative permeability curves, capillary pressure curves, etc. Quantitative methods include contact angle, Amott test and USBM (US Bureau of Mines) method. Contact angle methods are used to measure wettability of certain surfaces whereas Amott and USBM methods are used to measure wettability of reservoir cores. Contact angle method are particularly useful for measurement of wettability in case of clean surfaces and pure fluids.46 Again, there are different contact angle measurement methods which are described by Adamson and Gast47, Vijapurapu and Rao48, Rao and Girard49. In this work, the 8 ACS Paragon Plus Environment

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contact angle measurements were performed via a sessile drop technique using a very simple custom-build goniometer set-up. A schematic of the experimental set-up is shown in Figure 1. The set-up consists of a doubled walled (having annular space) cylindrical glass cell equipped with window having a planer glass made up of quartz. The planer quartz glass is used at the window to avoid distortion of the shape of the oil droplet that can be caused by unidirectional enlargement if viewed through the cylindrical wall of the glass cell. The glass cell is filled with respective brine solution with varying concentrations and the aged substrate is suspended horizontally in the solution with the help of stainless steel holder. The temperature of the system is maintained by circulating water from the water bath (TC 650, Brookfield Engineering, USA) in the annular jacket of the glass cell. The oil drop was placed onto the substrate with a ‘J’ shape blunt end needle fitted to a syringe. As an oil drop (approximately 0.05 mL or 50µL) is injected from the tip of the needle, the droplet rises towards the lower surface (from the bottom) of the substrate due to difference in density between oil and water. The image of the droplet were captured with the help of high resolution camera (Canon EOS 600D, Taiwan). The drop images were analyzed in ImageJ software using a drop shape analysis plugin called “DropSnake” developed by Biomedical Imaging Group at EPFL, Switzerland.50, 51 The colored drop images are converted to gray scale for the analysis. The contact angle is calculated as the average of right contact angle (RCA) and left contact angle (LCA) as shown in Figure 2. Since the dye (Sudan IV) is mixed with the transparent oil phase, therefore a significant contrast was obtained between the oil droplet and the surrounding water phase even in the gray scale images that has helped in the better image analysis. Additional experiments were done to confirm any effect of the dye used on the wettability. Sudan IV was found to have no effect on the wettability for both pure alkane and model oil case.


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2.4 Adsorption Study of Stearic Acid on Quartz Surface In addition to contact angle measurements, scanning electron microscopy (SEM) imaging and electron dispersive spectroscopy (EDS) studies were conducted to know the adsorption of polar components of model oils on the quartz surface under different brine concentrations when two model oils were used as an oil phase. This is to understand the mechanism of wettability alteration during the process of low salinity water flooding enhanced oil recovery process. The quartz plates (1.5cm×1.5 cm) were cleaned using the same method described in section 2.2 and dried. Clean and dry quartz plates were immersed in CaCl2 solutions (1M, 0.001M and 0M) and aged for 48 hours. The quartz plates were removed from the brine solution and directly placed in a stearic acid solution and left for 48 hours. After that the quartz plates were taken out and dried in an oven. Precaution was taken to avoid accumulation of any dust particle on the surface of it during drying the quartz plates. The morphology of the stearic acid deposits or scales on the quartz plates under different brine environments were studied by a scanning electron microscopy (FEI Quanta 400 Field Emission Gun, Germany) and the elements present in the stearic acid scales on the quartz plate were determine with electron dispersive spectroscopy (Bruker, Germany) technique.

4. RESULTS AND DISCUSSION The wettability states of the mineral surface (quartz substrate) for various pure alkanes and model oil systems in the presence various aqueous brine phases were evaluated through contact angle measurements. Figure 2 represents a schematic of how the contact angle of an oil 10 ACS Paragon Plus Environment

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droplet surrounded by an aqueous phase (brine) was measured from the captured drop images. The contact angle existing between two immiscible fluids is measured through the more dense surrounding phase (water in this case) as the angle at which the oil-water interface meets the solid surface.27, 30, 52-54 It is an indicative of the relative wetting or relative spreading behavior of one fluid over the second fluid on the solid surface. The wettability regimes to interpret the contact angle data were adopted from Anderson41 which considered contact angle between 0o to 75o as a water-wet, from 75o to 115o as an intermediate-wet and from 115o to 180o as an oil-wet. The quartz substrate has been used in this work to represent the sandstone reservoir rock mineralogy because sandstone primarily composed of quartz25 and both has the same chemical composition (silica, SiO2). However, sandstone apart from SiO2 also contains different types of clay, which is considered as an important factor for low salinity effect (LSE).16, 17 However, in a more recent studies of Pu et al.58, it has been observed that LSE can also play a role in a clay free sandstone.

4.1 Effect of Aging Time on Wettability Aging time is an important factor to be considered in wettability studies.42 Many authors have not incorporated the effect of aging time on the contact angle in their studies or many of them have not specified the aging time.31, 33 To determine the effect of aging time, contact angle experiments have been performed for a long time upto 25 h for all the pure alkane systems in 1 M NaCl solution on quartz substrate. The findings from Figure 3 converges into the fact that with increase in aging time, the contact angle increases. Contact angle versus time plot in Figure 3 shows that the contact angle changes considerably within first five hours, and then subsequently the oil droplet reaches equilibrium represented by the flat plateau in the contact 11 ACS Paragon Plus Environment

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angle curve. The change in the contact angle with time is observed to be more for lower molecular weight hydrocarbons than the high molecular weight hydrocarbons. For example, in case of octane, the contact angle was 19.10o at close to 0 h, which was changed to 21.76o after 5 h (Table S1 of supporting information). Whereas from Table S1 it can be seen that in case of hexadecane the contact angle has changed from 35.75o (at close to 0h) to 36.51o (at 5h). From this study, we have determined that the adequate aging time for the present study is 5 hours to obtain an equilibrium contact angle. The transformation of shape of the oil drop with time is probably due to the tendency of drop to minimize the free energy.56 Therefore, during subsequent experiments an aging time of more than 5 hours was provided before contact angles are measured.

4.2 Effect of Temperature on Wettability Several studies dealing with low salinity wettability (or contact angle) investigations have been performed for oil-water-solid systems at atmospheric conditions.3,

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However, in this

study, a few contact angle measurements have also been conducted at elevated temperatures (25, 40 and 55 oC) to understand the variation in wettability behavior with increasing temperature. Figure 4 portrays comparison of contact angle at different temperatures for pure alkane liquids on the quartz surface in the presence of 1M NaCl brine. The contact angle is found to increase for all the pure alkanes with increase in temperature and molecular weight of the alkanes. For example, from Table S2 (supplementary information), it can be observed that in case of octane the contact angle was 21.76o at 25oC which was increased to 38.56o at 55oC, while for hexadecane, it was 36.51o at 25 oC and 61.46o at 55oC. The increase in contact angle with temperature is observed to be significant for heavier alkane (e.g., hexadecane) as compared to 12 ACS Paragon Plus Environment

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lighter alkane (e.g., octane). In case of hexadecane, the change in contact angle was 24.95o when temperature is increased from 25oC to 55oC; whereas it was 16.8o in case of octane for the same temperature variation. This implies that the quartz surface becomes less water-wet under high temperatures and with increase in molecular weight of pure alkanes. This behavior is similar to that observed by Rajayi and Kantzas46 for quartz/water/bitumen system. However, the variation in contact angle observed by them for the bitumen system were insignificant as compared to the one noticed for quartz/water/alkane systems of this work. Rao58 investigated the effect of temperature on the wettability for thermally enhanced oil recovery processes. The results of his study projected a similar increasing trend of oil-wetness with increase in temperature for crude oil/brine/quartz system. However, they reported a reverse trend for calcite surfaces.

4.3 Effect of Aqueous Ions and Polar Components of Oil on Wettability Figures 5 and 6 and Tables 2 and 3 show the information on contact angles for various pure alkane systems and model oils in the presence of brines with varying concentrations, respectively. The wetting behavior of quartz surface in the presence of pure alkanes and model oils in the presence of brine phase has been found to be sensitive to the composition of brine and the oil phase. From Figure 5, relatively lower values of contact angle can be seen at sub-molar concentration as compared to high salt concentration (1M) for all the salt types. Also, from Table 2 (and Figure 5), we can observe that in case of octane and NaCl brine, the contact angle is 17.25o at 0.001M (or 1mM); but at 1M the contact angle is found to be 21.76o. Similarly in case of octane and CaCl2 brine, the contact angle is 30.43o at 0.001M (or 1mM); but 34.03o at 1M. This trends has been observed to be the same for all alkane-brine combinations investigated in this study. It means that, at low salt concentration the surface of silicate mineral exhibits a more


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water-wet nature as compared to the one under high salt concentration. This is one of the possible reasons of wettability alteration of rock towards more water-wet in the presence of low salt concentrations improving oil recovery. From Figure 5, we also have observed that, though at lower ionic concentrations the contact angle values are low; but a minimum value was found at 0.1 M (e.g. 20.91o at 0.1 M in case of decane-NaCl systems). On the other way, as the concentration of salt increases from 0.001M (1mM) to 0.1M (100mM) the contact angle decreased gently to a minimum value and increased suddenly to a higher value at 1M concentration. For example, from Table 2 (and Figure 5a) it has been observed that the contact angle for decane in NaCl brine is 26.02o at 0.001 M (1mM) and 24.57o at 0.01M (10mM), which is further decreased to 20.91o at 0.1M (100mM). But when the concentration is increased to 1M, the contact angle value got sharply increased to 26.56o. Even though the degree to which the contact angle varies is different but the trend is same for all the alkane-brine investigated in this study (see Table 2). According to Young’s equation47, contact angle can be related to oil-water interfacial tension. According to this relation reduction in oil-water IFT can result in lower oilwater-solid contact angle. In our previous work59, we have observed a decreasing IFT trend with increase in salt concentration at low concentration range near 0.1 M. Figure S2 shows near 0.1M salt concentration the IFT exhibited a minimum value. This is one of the possible reason why a minimum contact at 0.1M was observed in case of n-alkanes in the present study. From Figure 5, it can be observed that the higher molecular weight alkanes showed more tendency to wet the solid mineral surface (oil-wet) in comparison with the lower molecular weight hydrocarbon, though the total readings of the contact angles are in water-wet regime. This is in-line with the behavior observed in experimental works by Bertrand et al.8; Blunt and Zhou60 and Fenwick et al.61 Their gravity drainage experiments on sand-column have reported that the lighter alkanes


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(e.g., hexane or octane) spread onto the water surface to form continuous layer. On the other hand they observed oil layer disruption in case of heavier alkanes (e.g., dodecane) in the sand column. This also implies that the silicate minerals exhibits higher wetting tendency for heavier alkane as compared to lighter alkanes. Figure 5(a-c) also depicts the effect of monovalent and divalent salts on the contact angle of various alkane systems. It has been observed that in case of all pure alkane oils, although contact angle values are varying with salt type (monovalent, NaCl; and divalent, CaCl2, Na2SO4), all of them lies in the water-wet regime. It has also been observed that the contact angle values of alkane oil droplets in NaCl brine were lower than the contact angle of alkanes in CaCl2 at the same concentration. For example, the contact angle of decane droplet in 0.1 M NaCl brine is 20.91o whereas the same in 0.1 M CaCl2 is 33.62o. To ensure whether it is due to the cations or anions (of salts) responsible for this behavior, similar measurements were performed in Na2SO4 aqueous solution. The results in case of NaCl [Figure 5(a)] and Na2SO4 [Figure 5(c)] are almost similar (e.g., for decane the contact angle is 21.57o in 0.1M Na2SO4) which implies that the difference in contact angle was due to the different cations (Na+ and Ca2+) present in an aqueous phase. Why this happens with different cations has been explained subsequently using a proposed mechanism. Although sometimes crude oil reservoirs contains paraffinic base oil59,

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; but pure

paraffinic oils does not occur in natural crude oil reservoirs; rather crude oil contains several other hydrocarbons and different polar components like petroleum acids, bases, resins, asphaltenes, etc. Since studying the role of acidic and basic components present in the crude oil in low salinity water flooding is one of the major aspect of this work, therefore contact angle (wettability) studies have also been performed using separate model oils consisting of organic 15 ACS Paragon Plus Environment

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acid (e.g., stearic acid) and base (e.g., n-decyl amine). The idea of using model oil instead of crude oil was immerged from the fact that crude oil obtained from petroleum reservoirs contain several components; and hence it is difficult to isolate and analyze their individual effects on the contact angle. The two model oils used in this study are named as model oil-A which contains an organic acid (stearic acid) and model oil-B which contain an organic base (n-decyl amine). Experimental section has already provided the details on the composition. Figure 6 shows results of the contact angle measurements of model oils on quartz surface in the presence of two aqueous salt solutions of different concentrations (0.001, 0.01, 0.1 and 1M of NaCl and CaCl2) . The wetting behavior of model oils with varying ionic concentrations are very different from the one we observed for pure alkane oils. From Figure 6(a), we can see that the contact angle of the model oil-quartz-brine system varies with change in NaCl concentration. The contact angle values increases with increase in NaCl concentration for both acidic (model oil-A) and basic oil (model oil-B). From Table 3, it can be noticed that the contact angle for model oil-A in NaCl brine changed from 49.37o at 0.001M (1mM) to 89.44o at 1M, while for the model oil-B in NaCl brine, the contact angle changes from 45.44o to 97.06o over the concentration range of 0.001M to 1M. This implies that, as we increase the NaCl concentration, the quartz surface become more oil-wet in the presence of both types of oils. The change in contact angle is quite significant for model oils as compared to pure alkane liquids (Figure 6). Initially, at ˂ 0.1 M concentration, the wettability state of the quartz surface was found to be in the water-wet regime. With increase in concentration of the salts (> 0.1 M), the wettability got shifted and becomes intermediate-wet at 1 M concentration. As compared to the two model oils, at lower salt concentrations the quartz surface was more preferentially wet by the acidic oil (model oil-A, 49.37o at 0.001M NaCl) than the basic oil (model oil-B, 45.44o at 0.001M NaCl),


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but with increase in the salt concentration, the basic oil (model oil-B) adhere more as compare to the acidic oil, which is reflected by its higher contact angle values (97.06o for model oil-B and 89.44o for Model oil-A, at 1M). Wu et al.63 reported that an amino group (-NH2) exhibited larger adhesion force to the mica surface as compared to the carboxyl group (-COOH) in NaCl aqueous solution. Probably because of this fact, we also have observed higher contact angle values for the model oil-B than the model oil-A at higher salt concentrations (0.1 and 0.01M of both salts); since the model oil-B contains polar component with amine group (-NH2) and model-A contains carboxylic group (-COOH). It has also been reported that the adhesion force is also existed between mica surface and the alkyl group (-CH3) in the presence of NaCl brine. The adhesion force is less for alkyl group as compared to highly polar amine and carboxylic group.63 Therefore, we also have probably observed lower contact angle values or more water-wet nature for pure alkane liquids than the model oil systems (Figures 5-6). Also, at lower salt concentrations, alkyl group might have shown lower adhesion (lower values of contact angles) than at higher salt concentration with higher adhesion (higher values of contact angles) and so is the case for model oils. Figure 6(b) portrays the values of contact angle with model oils on the quartz surface surrounded by varying concentrations of CaCl2 brine. The contact angle trends are similar to that observed for the model oil-A in NaCl brine (Figure 6a). Higher CaCl2 concentration can make the silicate surface less water-wet or shift the wettability towards oil-wet state. Figure S1 (supporting information) shows image of droplet of model oil-B on the quartz surface at 0.001 M (low concentration) and 1M (high concentration) CaCl2 solution. However, the contact angle values are higher for model oils in the presence of CaCl2 brine as compared to NaCl brine (Figure 6a) at all the concentrations. For example, in case of model oil-A in 1M NaCl, the


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contact angle is 89.44o, whereas it is 105.58o in 1M CaCl2. It has been found that the resulting change in the contact angle with varying brine concentrations is more in case of CaCl2 in comparison with NaCl brine for both the model oils. For model oil-A in CaCl2 brine, the net change in contact angle over 0.001M to 1M is 48.49o, whereas for model oil-A in NaCl brine, it is 40.07o over the same concentration range. This implies that the divalent CaCl2 salt has significant impact on the wettability alteration for model oils than the monovalent NaCl salt. The above results shows that the cations (for e.g., Na+ and Ca2+) in the brine phase can promote adsorption of polar components of oil (acidic and basic components) to the mineral surface (quartz) resulting in the oil-wet nature of the mineral surface. Ca+2 helps more (than Na+) in the adsorption of polar components of the model oils to the mineral surface. The pH values of the brine solutions used in this work were in between 6-7. The influence of the brine pH on wettability has not been investigated in this work. However, from literature40, increasing contact can be seen with increase in brine pH for decane/CaCl2/mica system. 4.4 SEM and EDS Studies In order to have a more direct evidence on the adsorption behavior of oil polar components (acidic and basic components) of model oils onto the mineral surface in the presence of aqueous ions, few SEM and EDS analysis have been carried out for sample CaCl2 brine systems as explained earlier in the experimental section. Figure 7 shows the SEM images of surface morphology of the quartz plates aged in CaCl2 brine solutions for two different salt concentrations and also in deionized water and later in each case was immersed in a stearic acid solution. The surface morphologies of the quartz palate in Figure 7 (A-C) are different from each other. The adsorption of stearic acid on the quartz surface can be observed from Figure 7(A), and was found to be much higher when the plate was aged in 1M CaCl2 solution as compared the 18 ACS Paragon Plus Environment

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plate which was aged in 0.001 M CaCl2 solution. This can be identified by the scaly phase adsorbed on the surface of the quartz plates. From Figure 7(B), we can see only a little amount of stearic acid adsorbed on the surface of the quartz plate. Again, in case of quartz plate which was aged in deionized water before immersing into the stearic acid solution, almost no adsorption of stearic acid has been found [Figure 7(C)]. This can be identified by the dark and smooth surface of the SEM image in Figure 7(C). Figure 7(a-b) shows the enlarged images of the quartz surfaces of Figure 7 (A-B), respectively, and indicates stearic acid scales on the respective quartz surfaces. Figure 8 (a-c) represents the EDS spectra taken on the surface of the each quartz plates of Figure 7 (A-C), which show different elements present on the quartz plate. Figure 8(a-b) shows the presence of C (carbon) and Ca (calcium) where as in Figure 8(c) there are no traces of C and Ca. The Si accounts for the SiO2 that constitute the quartz plate and O (oxygen) accounts for both SiO2 and carboxylic group (-COOH) of stearic acid. This implies that the Ca2+ ion promoted adsorption of stearic acid on the surface of the quartz mineral. However, elemental composition obtained from EDS has not been reported here because it gives only micro chemical composition on a particular point on the surface and does not gives the overall composition of the quartz plate. Since, the whole quartz surface is not uniformly coated with stearic acid, the elemental composition obtained from EDS does not have much significance.

4.5 Mechanism of Wettability Alteration during Low Salinity Water Flooding Reservoir formation water contains different cations and anions. Also, a significant amount of naturally occurring organic acid and bases are found in the crude oil reservoirs from various parts of the world represented by their high acid and base numbers.64 From the present investigation, it can clearly be understood that the presence of higher amount of cations 19 ACS Paragon Plus Environment

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particularly divalent cations (e.g., Ca2+ and Mg2+) in the reservoir rock-pore water resulted in the adsorption of various acidic and basic components present in the crude oil which are polar in nature. Figure 9 shows the schematic representation of the proposed mechanism for wettability alteration due to desorption of oil components from the rock surface during low salinity waterflood. Silica has an isoelectric point at pH value of 2. In the presence of aqueous medium and pH above 2, the surface of quartz (or silicate mineral) possesses a negative charge65, 65 due to the ionization of silanol group (Si-OH). Reservoir brine or injection brine normally occurs at a pH value above 2. The negatively charged surface of sandstone plays a significant role in the wettability alteration mechanism. The cations present in the aqueous phase acts as a bridge between the negatively charged silicate mineral surface and negatively charged functional groups of the polar components in oil, resulting in the formation of organo-metallic complexes. The mechanism by which the polar oil components (particularly the compounds having carboxylic group) adsorb on the mineral surface through calcium has been proposed earlier by several studies.67-70 Their studies claimed that the divalent cation (e.g. Ca2+ and Mg2+) plays a significant role in this process. When low salinity water is injected into the reservoirs, the equilibrium is disturbed which results in the breakdown of organo-metallic complexes and releasing of crude oil from mineral surface of the reservoir rock.

Yang et al.34

has also proposed that the

desorption of polar oil components from rock surface could lead to alteration of reservoir rock wettability from oil-wet to water-wet during low salinity water flooding. In this study, alteration of wettability towards water-wet at low salt concentration is not only observed for polar oil but also observed to some extent for non-polar alkane oils. This is probably due to the fact that nonpolar alkane oils slightly tend to adsorbed on the negatively charged silicate surface (or quartz surface) due to induced dipole interaction caused by the cations in intervening aqueous phase.


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In case of pure alkane-brine-quartz system the contact angle change is possibly dominated by liquid-liquid interfacial tension. But in case of systems contained polar model oil the solidliquid (model oil) interaction are dominant over IFT changes. Hence, in case of crude oil with polar components or with significant acid and base number, the wettability alteration may be a dominant mechanism. In such situations, the wettability change is attributed to the interactions among charged mineral surface, aqueous ions and oil polar molecule. 5. CONCLUSIONS The present work investigate the effect of various monovalent and divalent brine systems with varying concentrations on the wettability of quartz surfaces in the presence of various pure alkane systems, and model oils containing polar components. The efforts has been made in linking aqueous ion concentrations on the wettability behavior of rock forming mineral surface. The main findings of this work are as follows: (1) Aging time has a considerable effect on the contact angle. Contact angle increases with aging time and reaches an equilibrium values after a certain time period (5 h in this case). (2) Quartz surface becomes more oil-wet when temperature increase. (3) In case of pure alkane liquids when we initially increases the salt concentration the contact angle decreases or quartz surface becomes more water-wet followed by an increase in the contact angle or a shift toward oil wet regime. Heavier alkanes has more wetting tendencies for quartz surface. The quartz surface is found to be less water-wet in the presence of calcium ions in water and more water-wet in the presence of sodium ion in water in case of all the pure alkane liquids. (4) Presence of polar (acidic and basic) components in an oil phase changes the wettability behavior of the quartz surface. When the oil phase contains a significant amount acidic or basic components, the wettability of the quartz surface shifted from water-wet toward oil-wet regime as the concentration of cation in an aqueous phase increases. (5) Lowering brine 21 ACS Paragon Plus Environment

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or cationic concentration of injection water, particularly divalent ions concentration can increase the water wetness of the sandstone reservoir rock surface, if it contains crude oil with significant polar content. The increase in water-wetness can help in oil mobilization during low salinity water flooding resulting in enhanced oil recovery during low salinity water flood operation. This study, in general, provides significant insights into the role of pure alkane, polar components and types of ionic salts and their concentrations on the wettability of reservoir rock. (6) In case of paraffinic or non-polar oils the contact angle change with brine concentration is primarily due to associated oil-water IFT variation. But when the oil contains polar molecules the variation in contact angle with brine concentration is dominated by the solid-liquid interactions.

Acknowledgement The authors are grateful for the financial support by project, ICS/16-17/831/RFIE/MAHS, IC&SR, IIT Madras. Dr. Jitendra S. Sangwai would also like to acknowledge the financial support from IIT Madras as part of the Institute Research and Development Award (IRDA) – 2017 for the work.

Supporting Information The supporting information containing the sample figure for contact angle measured for model oil-B and CaCl2 brine, and tables containing contact angle values for the alkane oils in 1M NaCl with varying aging time and the contact angle values of alkane oils in 1M NaCl at different temperature is available free of charge on the ACS Publication website.


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60. Blunt, M.; Zhou, D. Effect of spreading coefficient on the distribution of light non-aqueous phase liquid in the subsurface. Transp. Porous Media 1997, 25, 1–19. 61. Fenwick, D.H.; Blunt, M.J. Network modeling of three-phase flow in porous media. SPE J 1998, 3, 86-97. 62. Kakati, A.; Jha, N.K.; Kumar, G.; Sangwai, J.S. Application of low salinity water flooding for light paraffinic crude oil reservoir. Proceedings of the SPE Symposium: Production Enhancement and Cost Optimization; Kuala Lumpur, Malaysia, 7-8 Nobember, 2017; SPE189249-MS. 63. Wu, J.; Liu, F.; Yang, H.; Xu, S.; Xie, Q.; Zhang, M.; Chen, T.; Hu, G.; Wang, J. Effect of specific functional groups on oil adhesion from mica substrate: Implications for low salinity effect. J. Ind. Eng. Chem. 2017, 56, 342–349. 64. Lochte, H.L. Petroleum acids and bases. Ind. Eng. Chem. 1952, 44, 2597–2601. 65. Buckley, J.S.; Liu, Y. Some mechanisms of crude oil/brine/solid interactions. J. Petrol. Sci. Eng. 1998, 20, 155-160. 66. Hirasaki, G.J. Wettability : Fundamentals and surface forces. SPE Form. Eval. 1991, 6, 216227. 67. Buckley, J.S.; Liu, Y.; Monsterleet, S. Mechanism of wetting alteration by crude oil. SPE J. 1998, 3, 54-61. 68. Fjelde, I.; Omekeh, A.V.; Sokama-Neuyam, A. Low salinity water flooding: effect of crude oil composition. Proceeding of the SPE Improved Oil Recovery Symposium; Tulsa, OK, USA, 12-16 April 2014; SPE-169090-MS. 69. Liu, Q.; Dong, M.; Asghari, K.; Tu, Y. Wettability alteration by magnesium ion binding in heavy oil/brine/chemical/sand systems -Analysis of electrostatic forces. J. Pet. Sci. Eng. 2007,


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59, 147-156. 70. Morrow, N. R.; Cram, P. J.; McCaffery, F. G. Displacement studies in dolomite with wettability control by octanoic acid. SPE J. 1973, 13, 221−232.


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Figure 1. Schematic of the experimental set-up for contact angle measurements.

Figure 2. Schematic illustration of contact angle measurement of an oil drop (gray) surrounded by water (blue) from drop image along with wettability regimes. The contact angle reported is an average of the right and left contact angle.


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Figure 3. Contact angle measurement of pure alkane oil droplet on the quartz substrate in 1 M NaCl aqueous solution at 25oC plotted as a function of aging time.

Figure 4. Contact angle measurement of an oil droplet (n-decane) on the quartz substrate in 1M NaCl aqueous solution at varying temperature.


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(a)

(b)


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(c) Figure 5. Contact angle measurement of pure alkane liquids versus concentrations of: (a) NaCl brine; (b) ClCl2 brine; (c) Na2SO4 brine at 25oC. Line represents guide lines to the eyes.


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(a)

(b) Figure 6. Contact angle measurement of the model oils versus concentration of: (a) NaCl brine; and (b) CaCl2 brine at 25oC. 36 ACS Paragon Plus Environment

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Figure 7. SEM images of quartz plate immersed in: (A) 1M CaCl2 brine; (B) 0.001M CaCl2 brine; (C) deionized water and interacted with stearic acid. (a) and (b) are enlargement of A and B, respectively.


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Figure 8. EDS spectra of the quartz plates immersed in: (a) 1M CaCl2 brine; (b) 0.001M CaCl2 brine; (c) deionized water and interacted with stearic acid.


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Figure 9. Schematic representation of the proposed mechanism for wettability alteration due to desorption of oil components from the rock surface during low salinity waterflood. Inset shows the crude oil droplet on the quartz surface.


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Table 1. Details on the Chemicals Used in This Study Chemicals

Chemical Formula

Purity* (%)

CAS No.

Manufacturer

n-octane

C8H18

≥ 98

111-65-9

n-decane

C10H22

99

124-18-5

n-dodecane

C12H26

≥ 99

124-18-5

n-hexadecane Stearic acid (octadecanoic acid) n-decylamine

C16H34

99

544-76-3

Alfa Aesar, England Alfa Aesar, Great Britain Alfa Aesar, Great Britain Alfa Aesar, England

CH3(CH2)16COOH

≥ 98

57-11-4

TCI Co. Ltd., Japan

CH3(CH2)9NH2

≥ 98

2016-57-1

TCI Co. Ltd., Japan

Sodium chloride

NaCl

99

7647-14-5

Merck, India

Magnesium chloride

CaCl2

93

10043-52-4

Alfa Aesar, England

Na2SO4

99

7757-82-6

Merck, India

C24H20N4O

99

85-83-6

SRL Pvt. Ltd., India

Sodium sulphate Sudan-IV

*As stated by the manufacturer


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Table 2. Contact Angle (CA) of Pure Alkane Oil on the Quartz Substrate in Different Aqueous Salt Solutions Measured at 25 oC

Sat Type Concentration (M) Octane (C6)

Decane (C10)

Dodecane (C12)

Hexadecane (C16)

Average CA (o) Uncertainty (±) Average CA (o) Uncertainty (±) Average CA (o) Uncertainty (±) Average CA (o) Uncertainty (±)

NaCl

CaCl2

Na2SO4

0.001

0.01

0.1

1

0.001

0.01

0.1

1

0.001

0.01

0.1

1

17.25

17

13.06

21.76

30.43

28.66

24.5

34.03

18.65

17.71

16.02

22.84

0.34

0.52

0.92

1.15

0.81

0.30

0.99

0.92

0.84

0.91

0.90

1.10

26.02

24.57

20.91

26.56

37.08

35.02

33.62

43.26

26.38

24.56

21.57

28.18

0.89

0.26

0.30

0.95

0.34

0.19

0.52

0.72

0.60

0.44

0.62

0.97

32.65

31.89

28.33

32.40

57.11

54.33

50.54

58.24

37.70

37.04

35.81

38.74

0.14

0.88

0.60

0.90

0.90

0.56

1.19

0.72

0.24

0.07

0.66

1.18

33.59

32.58

30.21

36.51

58.58

57.50

55.21

62.39

33.59

32.58

30.20

36.51

0.44

0.22

0.20

0.40

0.36

1.11

0.96

0.38

0.54

0.53

0.69

1.14


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Table 3. Contact Angle (CA) of Model Oils on the Quartz Substrate in Different Aqueous Salt Solutions Measured at 25 oC Sat Type Concentration(M)

NaCl

CaCl2

0.001

0.01

0.1

1

0.001

0.01

0.1

1

Average CA (o)

49.37

56.37

67.88

89.44

57.09

65.84

78.94

105.58

Uncertainty (±)

1.56

0.63

1.10

2.43

1.78

0.71

1.80

0.66

Average CA (o)

45.44

56.16

72.75

97.06

53.56

63.15

81.10

110.38

Uncertainty (±)

0.42

0.81

1.80

0.93

0.56

1.75

1.00

0.99

Model Oil-A

Model Oil-B


Wettability Alteration of Mineral Surface during Low Salinity Water

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