Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial

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Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial Resolution Omar Bashir Wani, Chia-Yun Lai, Syed Mohamid Raza Quadri, Matteo Chiesa, and Saeed M. Alhassan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01515 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial Resolution Omar Bashir Wani1 , Chia-Yun Lai 2, Syed Mohamid Raza Quadri3, Matteo Chiesa2 and Saeed M. Alhassan1* 1

Department of Chemical Engineering, Khalifa University of Science and Technology, The Petroleum Institute, Abu Dhabi, United Arab Emirates 2

Laboratory for Energy and NanoScience (LENS), Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates 3

Abu Dhabi National Oil Company (ADNOC), Abu Dhabi, United Arab Emirates

ABSTRACT Wettability alteration of carbonate rocks from oil wet state to water wet or mixed wet state during water flooding is known to enhance the recovery of oil from the reservoirs. Previously, the wettability of porous mediums has been extensively investigated using numerous macroscopic methods. Amongst those, the contact angle is a highly employed method for wettability measurements; however, this technique because of its lower spatial resolution fails to provide a complete chemical understanding of all the factors affecting the reservoir wettability. In an attempt to overcome this, we employed a multiscale approach involving macro-micro-nanoscopic analytical techniques to investigate the wettability of calcite. Studies were performed by aging two different planes of freshly cleaved calcite in ambient atmosphere and De-Ionized water. Contact angle measurements and AFM force profiles were recorded at macroscale and nanoscale respectively. Wettability transition was observed from super hydrophilic to hydrophobic nature in ambient atmosphere and super hydrophilic to hydrophilic nature in DI water. When AFM studies were performed on samples aged in DI water there were always patches of water present which were only observed at the nanoscale. These water patches affect the contact angle measurements and make the macroscopic wettability results inherently ambiguous. This work has shown that the contact angle measurements should not be taken as the absolute measurement of wettability. Keywords: Calcite, Enhanced Oil Recovery, Contact Angle, Atomic Force Microscopy, Smart Water Flooding.

1. INTRODUCTION More than half of the world’s hydrocarbon reserves are located in carbonate reservoirs; however, the oil recovery from these reservoirs is about 35% leaving behind a significant amount of oil. The low recovery from the carbonate reservoirs is due to the high capillary pressure of the mixed wet or oil wet condition of the reservoirs. Thus, rendering natural drainage of pores as an unavailable option in many cases. Another reason for the low recovery is natural fracturing of carbonate reservoirs due to which the injected fluid flows through the fractures and bypasses the oil resulting in an early breakthrough 1-5. The wettability of an oil reservoir is affected by its physical morphology and chemical composition. The physical morphology includes parameters like pore size distribution, pore connectivity, and permeability within the rock whereas the chemical composition includes parameters like the three-way interaction of crude oil with brine and rock (CBR), pH, acid number, base number etc. 6-14. Nevertheless, the fundamental chemical understanding of these interactions has remained elusive 14. Studies have been conducted with an aim to increase the recovery from carbonate reservoirs by altering their wettability towards a water-wet scenario 15. Over the past two decades, injection of low salinity water as an enhanced oil recovery (EOR) method into the carbonate reservoirs has gained significant attention. Studies have shown that injecting water

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with specific ionic composition and concentration (mostly SO42-, Ca2+, and Mg2+) into a carbonate reservoir can alter its wettability towards a more water wet scenario thereby resulting in higher oil recovery 1, 7, 9, 10, 15-23. Despite the extensive research and interest in low salinity water flooding a comprehensive explanation and understanding of wettability alteration mechanism has not yet emerged 15. Results from one reservoir cannot be applied to another because of the difference in crude oil properties from different fields as well as the geological variation of the oil reservoirs. The CBR system has been well studied at the macroscopic scale using contact angle24-28, interfacial tension (IFT)28-31, nuclear magnetic resonance (NMR) studies 32-36 etc. Due to the lack of spatial resolution, they fail to provide information about the wetting behavior at the pore scale which signals the use of techniques with higher spatial resolution. Atomic force microscope (AFM) is considered a viable tool to probe wettability and has been used to investigate the wettability between the CBR system at micro-nano scales 37-42.

Figure 1. (a) XRD of calcite plane 1. (b) Time-dependent water static contact angles for calcite plane 1 and plane 2 when aged in ambient atmosphere.

Studies have shown that by tuning the properties at nanoscopic level wettability of substances can be altered from one extreme to another 43-49. For instance, the wettability of Vanadium Pentoxide can be altered from being superhydrophobic to superhydrophilic upon exposure to UV light and storage in dark restores its superhydrophobicity 46. Sun et al.45 used thermo-sensitive polymer poly(N-isopropylacrylimide) on top of roughened silicon where they observed reversible wettability transition from being hydrophilic below the lower critical solution temperature (LCST) to hydrophobic nature of the film above the LCST. The intermolecular hydrogen bonding between the polymer chains and water molecules leads to the hydrophilic nature at temperatures lower than LCST whereas intramolecular hydrogen bonding between the polymer chains results in the hydrophobic nature at temperatures higher than LCST. The effect of atmospheric contaminants on the wettability of graphene has also been studied extensively 50-54. Amadei et al.55 reported the transition of Highly ordered pyrolytic graphite (HOPG) from being slightly hydrophilic when freshly cleaved to hydrophobic upon being exposed to ambient atmosphere. They employed macro-nano investigations and reported the cause of transition to be adsorption of hydrocarbon contaminants along with water molecules from the atmosphere. Upon annealing the sample they managed to reestablish the initial macro-nano surface properties. Calcite is the most stable polymorph of calcium carbonate and a representative of carbonate type reservoir. To reduce the complexity from morphology-chemistry coupled wettability studies to just chemistry related investigations we have used an atomically flat calcite crystal. Unlike macroscopic contact angle measurements, using amplitude modulated AFM, surface wettability alteration and force interactions can be studied at the nanoscopic level where chemical interactions take place thus allowing us to decouple the role of morphology from chemistry. Kendall et al.56 have reported that the carbonate surface of a freshly cleaved calcite surface is atomically rearranged in response to the humid air. It is also well known that wettability transition occurs when surface is exposed to the environment due to the adsorption of contaminants 52.Nevertheless, with abundant literature in hand and calcite being a representative of reservoir rock its hydrophobicity and hydrophilicity at the nanoscale has been rarely discussed. Here in this work, we have employed macroscopic (water static contact angle SCA), microscopic (Fourier Transform Infrared Spectroscopy, X-Ray Diffraction and Inductively Coupled Plasma Mass Spectrometry) and nanoscopic analytical techniques (dynamic Atomic Force Microscopy) to investigate the wettability of calcite in ambient atmosphere and deionized (DI) water. By observing at the nanoscale we have been able to see the development of heterogeneities on the surface of the calcite due to the growth of hydrates as well as observe the transition from being hydrophilic to hydrophobic nature. We have shown that nanoscale measurements are able to verify the macroscale results when aged in ambient at-


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mosphere. Furthermore, we have also shown that macroscopic measurements are not the true representation of calcite’s wettability when aged in DI water.

Figure 2. AFM phase images of Calcite plane 1 from freshly cleaved to when exposed for 12 hours in the ambient atmosphere.

2. EXPERIMENTAL DETAILS 2.1 Sample Preparation Optically clear Iceland spar variety of calcite crystals were purchased from Educational Innovations USA. Before analyz 4 plane using a hammer so as to avoid the contaminaing the calcite, the crystal was cleaved mechanically along the 101 tion caused by adventitious hydrocarbons. The freshly cleaved surface of calcite was handled carefully by holding it from the edges using a tweezer so as to avoid surface contamination. Calcite, which cleaves along the unit rhombohedron is labelled as plane 1 and plane 2 of calcite was identified by judging the inclination with respect to plane 1. 2.2 Contact Angle Measurements The water static contact angle (SCA) measurements were performed using Kyowa DM 701 machine. A droplet of 1 μL DI water was dispensed on top of the sample surface using a stainless steel needle. A digital image of the liquid droplet and the sample surface was recorded on the computer using the software FAMAS (interFAce Measurement & Analyses System) provided by the company. The contact angle measurements for the samples aged in the ambient atmosphere were taken at different time intervals starting from 0 h i.e., the freshly cleaved surface of calcite followed by 1, 2, 3, 6, 9, 12, 24, 48, 72, 96 and 120 hrs. The calcite samples were aged in ambient atmosphere at a temperature of 25 ± 2° C. The SCA measurements for samples aged in DI water were performed by removing the crystal samples from the DI water and then flushing the sample with a jet of nitrogen so as to remove the excess of DI water. The measurements were then performed in the similar way as of aging in ambient starting from 0 h i.e., freshly cleaved surface of calcite and then followed by 1, 2, 3, 6, 9, 12, 24, 48, 72, 96, 120, 144, 168, 192, 216 and 240 hrs. The samples were aged by first cleaving in the ambient atmosphere and then dropping immediately in a beaker containing 1000 ml of DI water, the exposure time in the ambient atmosphere was less than 60 seconds. The water was obtained by passing through Milli-Q cartridge filters. It is to be noted that different samples were used for different time points in both the cases. 2.3 Atomic Force Microscopy In our study, we have used Cypher S AFM from Asylum research. For the studies on samples which were aged in the ambient atmosphere, we used AC 160 OLYMPUS cantilevers with spring constant k ≈ 40 N/m, Q ≈ 500 and fo ≈ 300 kHz with a scan rate of 0.8 Hz. However, for the samples aged in DI water we used AC 240 OLYMPUS cantilevers with k ≈ 1.7 N/m, Q ≈ 100 and fo ≈ 70 kHz with a scan rate of 0.6 Hz. The mode of operation was Amplitude modulation for both imaging as well as force measurement. The samples were scanned over an area of 2 x 2 μm2. A freshly cleaved sample of calcite


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was probed continuously under the AFM for 12 hours in order to observe the changes occurring with time when aged in ambient atmosphere. However, in case of aging in DI water the sample was first removed from DI water and then flushed with a jet of nitrogen before being probed under the AFM. In our studies, we employed phase imaging by recording the phase lag Φ of the AFM probe. For the force reconstruction, we employed SJK formalism 57, 58 which has been explained in the literature 59. During our experiments, we observed Phase and Amplitude as a function of tip-sample separation distance i.e., APD curves. The radius of the AFM tip R is known to greatly affect the force between the tip and the sample; it was continuously monitored in situ using the critical amplitude method 60. The radius of the tip R was observed to be constant during the scans performed for samples aged in the ambient atmosphere however, it considerably changed when observing the samples that were aged in DI water. A minimum 300 APD curves were collected on each plane of the sample at a given time point when aged in the ambient atmosphere and a minimum of 100 APD curves were collected when aged in DI water so as to analyze statistically.

2.4 Fourier Transform Infrared Spectroscopy and Inductively Coupled Plasma Mass Spectrometry A freshly cleaved sample of calcite was placed in a Praying Mantis Diffuse Reflection accessory and exposed to the ambient conditions for 24 hours. This accessory was placed in the Bruker 80v FTIR spectrometer’s front compartment. The FTIR spectrum was then recorded on averaging 200 scans in absorbance unit. For the samples aged in DI water, we used the ATR accessory of the FTIR spectrometer and performed Inductively Coupled Plasma Mass Spectrometry (ICP-MS) of the DI water in which calcite was aged to observe if any dissolution had occurred.

Figure 3. (a) Calcite plane 1 phase image at t= 9 hrs. (b) Force profiles for phase 1 and phase 2 on plane 1 of Calcite at t= 9 hrs. (c) Calcite plane 2 phase image at t= 9 hrs. (d) Force profiles for phase 1 and phase 2 on plane 2 of Calcite at t= 9 hrs. The experimental data is represented by black and grey dots whereas continuous lines represent the averaged force curves.


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Figure 4. (a) FTIR spectrum showing change in peaks intensity when freshly cleaved calcite is aged in the ambient atmosphere. (b) Time-dependent water static contact angles for calcite plane 1 and plane 2 when aged in DI water.

3. RESULTS AND DISCUSSIONS Plane 1014 which is hereafter referred to as Plane 1 of calcite was identified by using X-Ray Diffraction. From figure 1 a, a single sharp peak at ~29.5° confirms plane 1 we obtained corresponds to 1014 cleavage plane of calcite. Due to the rhombohedral structure of the calcite crystal, plane 2 was identified by judging the inclination with respect to plane 1, i.e., the angle between plane 1 and plane 2 of calcite is ~75° 61.

3.1 Aging in ambient atmosphere 3.1.1 Macroscopic Studies The first study was performed by aging freshly cleaved calcite in ambient atmosphere at 25° C. Macroscopic studies were conducted by measuring the water static contact angle on freshly cleaved calcite i.e., at time = 0 hours and subsequently up to 120 hours of exposure to the atmosphere. The results of macroscopic studies in ambient atmosphere are shown in figure 1 b. Both planes of Calcite showed superhydrophilic behavior with contact angles less than 5 degrees when freshly cleaved. Upon exposing to the ambient atmosphere, the contact angles increased to ~82.8 ± 4.2° for plane 1 and ~ 90.3 ± 5.4° for plane 2. Both planes showed a change in wettability behavior upon being exposed to the ambient atmosphere and followed a similar trend. The changes in the contact angles of calcite samples upon being exposed to ambient atmosphere indicate some surface changes have occurred over the course of time. 3.1.2 Nanoscopic Studies To understand the contact angle results with higher resolution we employed AFM to investigate the surface changes at the nanoscale. In order to observe the changes occurring while aging in the ambient atmosphere, we performed Phase imaging. Figure 2 shows the phase images of calcite plane 1 whereas phase images of plane 2 are provided in the Supporting Information. The samples were continuously scanned for 12 hours and Phase images were recorded. It can be observed that freshly cleaved calcite remains pristine up to three hours as no phase contrasts can be seen; this pure calcite is called as the first phase. Upon being exposed to the ambient atmosphere there is a growth of foreign material from t = 3 h which is lighter in color this is known as the second phase. From the phase images, it can be observed that the pattern of growth for the second phase is different on both the planes. On plane 1 it grows orderly along the steps/terraces of calcite whereas on plane 2 it grows all randomly. To quantify the amount of growth for the second phase we calculated the percentage of area covered by the second phase which is shown in the Supporting Information. The macroscopic results showed us the change in wettability behavior of calcite but only AFM phase imaging confirmed the cause for this transition, therefore, suggesting the need for macro-nano investigations into the wettability studies. To further quantify the changes that occur on the calcite surface as the second phase evolved we performed AFM Force spectroscopy. We measured the force between the tip and the sample for both phases on both the planes. The force of adhesion between the tip and the sample can be estimated by a sphere flat plane model 62, 63: | | 4 Where γ is the surface energy and is the radius of the tip. From figure 3 b and d, it can be clearly seen that there is a difference in force of adhesions between phase 1 and phase 2 on both the planes. Since the radius of the tip was kept constant by measuring it in situ, the only parameter changing was surface energy i.e., phase 1 and phase 2 present on calcite surface are different in their chemical composition. These quantitative force measurements are in good agreement with


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the phase images that show the evolution of the second phase with time which changes the surface properties and results in the change of wetting behavior. The correlation between macroscopic and nanoscopic results is presented in the Supporting Information.

3.1.3 Surface Analysis To investigate the composition of the second phase which evolved when the samples were aged in ambient conditions, we performed FTIR spectroscopy. We exposed a freshly cleaved sample of calcite to the IR beam and allowed it to age in the ambient atmosphere. The spectrum obtained at t =0 h was taken as the baseline signal and we then evaluated the change in spectra after 24 hrs of aging. The absorbance spectra of plane 1 is shown in Figure 4 a. It can be observed that there is a variance from 3200-3600 cm-1 which corresponds to the O-H stretching signal. The C-O stretching and bending signals which occur at 1400 cm-1 have not shown any difference. An instrument artifact signal at 2400 cm-1 was observed during each measurement which was independent of the sample or user. The increase in O-H stretching signals can be caused by the adsorption of water vapor on the surface of calcite. The water vapor adsorbed from the atmosphere can remain as molecular water or the dangling bonds present on the surface of calcite can be satisfied by hydrolysis after hydration. When a mineral is cleaved, its atomic structure is interrupted which will lead to broken bonds that will need to be balanced or relaxed by restructuring, hydration or chemical reaction. The newly created surface is out of equilibrium and will react with the surrounding medium. Water vapor is the most abundant polar material available for minerals cleaved in ambient atmosphere. To balance the dangling bonds of calcite which are formed when it is cleaved in ambient atmosphere, calcite adsorbs water vapor from the atmosphere to form hydrates in order to reach thermal equilibrium 56, 64-72 . Furthermore, it has also been reported that the energy dissipation for the second phase is less as compared to the original calcite surface which implies that the viscoelasticity or hydrophilicity of the second phase/ hydrates is lower in comparison to the original surface 73. The second phase that evolves on top of calcite is calcium carbonate hydrates that are formed in response to balance the dangling bonds. Therefore, when freshly cleaved calcite is exposed to the ambient atmosphere, it adsorbs water vapor from the atmosphere to form hydrated calcium carbonate film that causes the wettability change with time.


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Figure 5. AFM phase images of Calcite plane 1 when aged in DI water for (a) 1 hour, (b) 3 hours, and (c) 168 hours. (d) Phase image and (e) Force profiles of calcite plane 1 when aged in DI water for 48 hours. The experimental data is represented by black and grey dots whereas continuous lines represent the averaged force curves.

3.2 Aging in DI Water 3.2.1 Macroscopic studies In the second study, the freshly cleaved calcite samples were immersed in a beaker containing DI water and stirred using a magnetic stirrer for up to 240 hours. The samples were superhydrophilic when freshly cleaved i.e., the contact angles measured on both the planes were less than 5 degrees at time = 0 hours. Both the planes of calcite showed a similar trend of initially increasing and followed by a decrease in the measurement of contact angles before stabilizing at ~ 15.4°±3.6° for plane 1 and ~18.4°±4.3° for plane 2 as shown in figure 4 b. It can be observed that the calcite samples largely remained water wet with some changes in the values for contact angles indicating changes in surface chemistry during the process of aging. 3.2.2 Nanoscopic Studies We performed AFM phase imaging on calcite samples that had been aged in DI water. Unlike the case of imaging in the ambient atmosphere where one sample was continuously probed we took different calcite samples at different time points and flushed them with a jet of nitrogen to remove excess of water before being scanned under the AFM. Plane 1 of DI aged calcite was scanned at t= 1h, 3 h, 48 h and 168 h whereas Plane 2 of DI aged calcite was scanned at t= 1 h, 3 h, 48 h and 96 h. From the phase images of plane 1 in figure 5 one can observe two different phases exist when aged in DI water (see Supporting Information for Phase images and force profile of plane 2). The grey part corresponds to the calcite mineral whereas the black island-like structure corresponds to the water attached to the surface. The presence of water patches is also confirmed by the force profiles taken on top of the water patch which has a long plateau (figure 5 e), this has been reported in the literature to be the characteristic of force profile taken on top of water 59, 74. The terraces of calcite have


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disappeared indicating the dissolution of calcite while aging. During the process of imaging and measurement of forces, it was observed that the tip radius changed considerably. The change in the tip radius was caused due to the presence of water patches on top of the sample surface. While scanning, the tip would interact with the water patches present on the sample surface. Consequently, some water drops may attach themselves to the tip surface thereby causing the tip radius to change. From figure 4 b, we can observe that there are large deviations in the macroscopic results when calcite is aged in DI water as compared to when calcite was aged in ambient atmosphere (figure 1 b). The patches of water that are attached to the calcite samples cannot be seen with the naked eye and go undetected at the macroscopic level. When the droplet from the contact angle machine is deposited on top of the calcite sample aged in DI water, depending upon where the droplet interacts with the sample, the results would be affected i.e., if the droplet from the contact angle machine interacts with the water patches it will spread and give a lower value whereas if it interacts with the calcite surface it would give a different value leading to large deviations (see figure 6 of Supporting Information for illustration). Leeuw et al. 75 used atomistic simulation techniques to model the adsorption of water on 1014 calcite surface. The calcite surface consisted of calcium atom layers and two differently oriented yet equal carbonate layers. In the crystal structure, the carbon, calcium and one of the carbonate oxygen atoms lie in the same plane whereas the other two carbonate oxygen atoms protrude above and below the plane. They reported that cleaved calcite surface is stabilized by associative adsorption of water molecules. The position of calcium and carbonate oxygen atoms allows them to be easily accessible to the adsorbing molecules from a solution. The lattice spacing between calcium atoms is 4.0 Å and 4.8 Å, which is large enough for water molecules to adsorb by bonding its oxygen atom to each calcium atom in a herringbone pattern. They also reported that hydrogen atoms from water molecules are bonded to lattice oxygen atoms through hydrogen bonds. Kerisit et al. 76 observed that water molecules prefer to adsorb associatively on calcite surface. Their results also showed that it is energetically unfavourable for water molecules to adsorb dissociatively on 1014 surface of calcite. Similar to earlier results 75 they reported that the water molecules adsorbing onto the calcite surface showed a regular pattern with opposite orientations. They also reported that the oxygen atom from water molecule is coordinated with the calcium atom at the surface of calcite whereas the two hydrogen atoms form hydrogen bonds with two oxygen ions present on the surface i.e., Carbonate oxygen-Hydrogen. Their study also indicated that the first step to the dissolution of calcite mineral is the reaction of water with the surface carbonate groups, the dissolution would begin at the edges/terraces of freshly cleaved calcite surface and this phenomenon cannot take place on an uncleaved 1014 calcite surface. Sand et al. 77 reported that the top layer of the calcite relaxes and thereby yields a different bond distance between Ca and CO3 at the top in comparison to the bulk of the solid. Similar to earlier studies 75, 76 Sand et al. 77 also reported that oxygen from water molecules points towards calcium and hydrogen from water molecules positions itself in the direction towards carbonate ions. The first adsorbed layer on top of calcite surface comprises of oppositely oriented water molecules; one where oxygen is pointing towards the calcite surface whereas the other is pointing away from the surface. The two orientations of water molecules were also supported by the density profile of Oxygen in water molecules interacting with the calcite surface as it had two peaks. AlMahri et al. 78investigated the effect of different sample preparation methodologies on water static contact angles of calcite. They observed higher contact angles for samples freeze-dried in comparison to samples that were dried using nitrogen stream which indicate the presence of water patches on the surface of samples that go undetected at the macroscopic level.

3.2.3 Removal of Water Layers In order to remove the water patches from the calcite sample, they were heated in an oven initially at 60°C for One hour. Figure 6 shows the phase and height images of the samples before and after heating for one hour that were initially aged for 24 hours in DI water. The grey part in the phase image (figure 6 b) corresponds to calcite whereas the black region is water. Even after heating for 1 hour there are traces of water still present which are in the form of rivers suggesting the need to heat at higher temperatures for longer periods of time (figure 6 c). After heating for one hour the height of water patches has gone down from 15 nanometers to 0.6 nanometers (600 picometers as shown in figure 6 d). Figure 6 e and f show the Phase and height image of calcite that was heated at 100°C for 24 hours. The patches of water have been removed and there appears a second phase in the form of clusters which are salts that have been left after the evaporation of water. The SCA of the sample after heating for 24 hours was measured and was observed to be ~ 30° ± 2° for plane 1 and ~ 35° ± 4° for plane 2 which lies in the range of SCA measured for calcite aged in ambient for 1 hour.


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Figure 6. (a) Height and (b) Phase images for calcite plane 1 when aged in DI water for 24 hours. (c) Phase and (d) Height images of calcite plane 1 after heating at 60°C for one hour. (e) Phase and (f) height images of calcite plane 1 after heating at 100° C for 24 hours.

Figure 7. FTIR spectrum of DI water in which calcite was aged for 96 hours.


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3.2.4 Surface Analysis To identify the surface and chemical changes occurring in our way of aging in DI water we again performed the FTIR spectroscopy. Freshly cleaved calcite crystal was aged in DI water for 96 hours. DI water by composition contained no ions. Instead of using the DRIFT accessory, we used the ATR accessory of the FTIR spectrometer. The accessory was mounted on top the spectrometer and a droplet from the solution in which the sample had been aged was placed on the sample spot. The FTIR spectrum was obtained by averaging 120 scans in absorbance unit using the reflective mode. Figure 7 shows the spectrum obtained. Two peaks can be observed; one between 3200 to 3600 cm-1 and the other occurring between 1500 to 1700 cm-1. The first peak on the spectrum belongs to O-H stretching that occurs from the DI water itself, the second peak belongs to carbonate stretching and arises from the dissolution of calcite. The sample in water gets dissolved into Ca2+ and CO32- ions. The calcium ions could not be detected by the FTIR because the IR beam detects the stretching or vibration among the bonds, the fact that calcium ions are not attached/bonded shows why they cannot be detected by the IR beam. We performed inductively coupled plasma mass spectrometry (ICP-MS) on the water in which calcite was aged for 96 hours. The results showed 1556 ppb of Ca2+ present, which supports the fact that calcium carbonate dissolves into calcium and carbonate ions respectively. The ICP-MS and FTIR results are also in agreement with the AFM phase images which were obtained after heating the sample for 24 hours where we could observe localized calcium carbonate salts left behind after evaporation.

4. CONCLUSIONS Compared to the previous documented works which have studied the wettability of oil reservoirs classically using macroscopic methods like NMR, Zeta Potential, Core flooding and Contact angle measurements 11-13, 23, 79-81 this work has employed macroscopic, microscopic and nanoscopic analytical techniques to understand the wettability of calcite in ambient atmosphere and DI water. We have shown that in ambient atmosphere freshly cleaved calcite is superhydrophilic and becomes hydrophobic when exposed for 120 hrs. The wettability change in calcite occurs by adsorption of water from the atmosphere to form calcium carbonate hydrates in order to balance its dangling bonds. The growth of hydrates was also supported by the FTIR results. The macroscopic study provided the evidence for wettability alteration; however, the cause for this inversion was proved only by macro-nano investigations. In comparison to work present in the literature which investigates the wettability of carbonate rocks directly from an oil-wet state, our work has taken into consideration the effect that the ambient atmosphere and DI water can have on calcite’s wettability. From the macroscopic results of calcite when aged in DI water, the sample largely remained hydrophilic with large deviations. The AFM images showed that even after flushing with a jet of nitrogen there are always patches of water present at micro-nano scale on top of the sample surface which go undetected at the macroscopic level. These patches of water affect the macroscopic results and cause large deviations. Our studies have shown that the results obtained from the classical method of measuring reservoir wettability using contact angle measurement 81-85 should not be taken as the absolute measurement of wettability when calcite is aged in liquid mediums like DI water. The calcite samples undergo dissolution when aged in DI water which was shown with AFM imaging as well as FTIR and ICP-MS studies. We have shown that slight changes occurring at the nanoscopic level can have a great impact on the macroscopic results. Further studies are being conducted to develop a method by which rock wettability can be measured at the nanoscopic level when it is aged in liquid environments. This work has shown that macroscopic studies alone are not sufficient to understand wettability of calcite and therefore should be supported with additional macro-micro-nano investigations. This work provides a baseline for the investigation of calcite wettability and the results can be applied for further studies on low salinity water flooding EOR.

AUTHOR INFORMATION Corresponding Author * Saeed Alhassan E-mail: [email protected]

Supporting Information. AFM images of calcite and the correlation between macroscopic and nanoscopic data.

ACKNOWLEDGMENT We are thankful to The Abu Dhabi National Oil Company (ADNOC) for providing financial support. The authors would also like to thank Mohammad Shoaib for useful discussions and comments.


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(Graphical Abstract)


Understanding the Wettability of Calcite (CaCO3) Using Higher Spatial

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