Understanding the Role of Asphaltene in Wettability Alteration Using ζ

Feb 1, 2016 - Department of Chemical Engineering, The Petroleum Institute, P. O. Box, 2533, Abu Dhabi, United Arab Emirates. ‡. Chemistry Department...
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Understanding the Role of Asphaltene in Wettability Alteration Using ζ Potential Measurements Syed Taha Taqvi,† Ali Almansoori,† and Ghada Bassioni*,‡ †

Department of Chemical Engineering, The Petroleum Institute, P. O. Box, 2533, Abu Dhabi, United Arab Emirates Chemistry Department, Faculty of Engineering, Ain Shams University, Cairo 11517, Egypt

‡

ABSTRACT: Wettability study using the ζ potential technique is a novel approach proposed for carbonate reservoir rocks. As opposed to other existing industrial and laboratory methods, such as the Amott−Harvey, United States Bureau of Mines (USBM), and contact angle methods, the ζ potential technique provides a complete wettability profile of limestone versus relative water/oil content in the presence of water for different oil samples. In addition, the study is extended for oil derivatives (i.e., asphaltenes and maltene). A case study was carried out to understand the effect of asphaltenic solutions on wettability of limestone rocks. From the obtained wettability profiles, it was found that different oil samples show the wettability transition, from water-wet to oil-wet. Moreover, the adsorption of maltenes was limited to the water-wet region whereas asphaltenes were found to be the main reason for heavy-fraction deposition.

1. INTRODUCTION Asphaltenes are the most polarizable petroleum macromolecules that are capable of accepting or donating protons.1−3Their dipole moment can range from 3.3 to 6.9 D, whereas, for crude oil, the dipole moment varies from 0 to 1 D.1These molecules, while retaining their unique ambiguous identity, tend to coagulate.2 Their aggregates represent sites capable of becoming electrically charged.4 Heavy fractions of the polydisperse asphaltenic compounds, in crude oil, exist in the form of micelles or colloidal suspensions incorporating metals and are assumed to be kept stable by resins.1,2,5 According to Goual and Firoozabadi,1 it is believed that, in a micelle, asphaltenes self-associate into an aggregate to form the core, while resins, heavy and mostly aromatic molecules, adsorb onto the core (i.e., asphaltene aggregate) to form a steric shell.1,2 The electrostatic and induction interactions help stabilize the asphaltenes in the petroleum fluid.1 Murgich6 presented a comprehensive analysis on the intermolecular forces in aggregates of asphaltenes and resins and stated that the forces present are only those that originated in the van der Waals, Coulombic (electrostatic), charge transfer, and induction. In support, Gonzalez4 states that asphaltene particles carry their own distinct charge, either positive or negative, depending on different conditions,4 whereas, due to the polar character of resins, they act as natural dispersants.1 On the other hand, limestone, a common reservoir rock in the Arabian Gulf, demonstrates a positively charged character when present in solution and has the potential to adsorb ions onto its surface.7,8 The surface of the limestone, composed mainly of calcium carbonate, allows molecules in the liquid phase to adsorb onto its surface and “wet” it, as illustrated in Figure 1. There are various techniques present in literature that facilitate studying wettability of calcium carbonate. The Amott−Harvey method and the United States Bureau of Mines (USBM) method are the most commonly used industrial © XXXX American Chemical Society

Figure 1. Illustration of competitive adsorption between negatively charged oil particles and water molecules. Due to a higher dipole moment, the oil molecules tend to dominate in adsorbing onto the surface by displacing the water molecules into the bulk.

methods for characterizing wettability of oil/brine/rock systems.9 However, these methods are regarded as an industry standard for comparing the wettability of various core plugs but have no validity as an absolute measurement tool.10 Contact angle testing method also allows measuring wettability of a limestone surface in contact with fluid from the reservoir. However, this method consists of a set of limitations that include its inability to take into account the heterogeneity of the carbonate rock surface, the existence of more than one stable contact angle, and issues with irreproducible data.11 A recently proposed technique demonstrated a wettability study of limestone using the “ζ potential” approach.10 It measures the actual wettability as opposed to portraying it as an absolute measurement. Moreover, it is capable of recording the wettability profile, showing the transition of wettability, from water-wet to oil-wet, as changes are made to the system.10 Therefore, the aim of this work is to study the wettability of limestone for different apshaltenic solutions. Such experiments Received: September 18, 2015 Revised: January 16, 2016

A

DOI: 10.1021/acs.energyfuels.5b02127 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels will help in understanding the behavior of asphaltenes when exposed to a positively charged surface as well as understanding the role of asphaltene in crude oil. Furthermore, the change in ζ potential values due to asphaltene, if any, would help make predictions regarding the heavy-fraction deposition tolerance of such systems.

2. MATERIALS AND METHODS Experiments were performed with limestone, crude oil samples (A, B, and C), and maltenes and asphaltenes extracted from these samples at RTP conditions. The studied CaCO3 has medium particle size D50 of 7.06 μm and specific surface area of 8.628 m2g−1. The density was found to be 2.66 g/cm3. According to saturate-aromatic-resinasphaltene (SARA) analysis, as seen in Table 1, all three samples of

Table 1. SARA Analysis of the Crude Oil Samples A, B, and C sample

saturate

aromatic

resin

asphaltene

crude oil A crude oil B crude oil C

68.9 70.6 68.6

21.8 21 22.4

7.1 5.5 6.6

0.5 0.1 0.8

crude oil can be regarded as light oils. However, crude oil C has the highest asphaltene content whereas crude oil B has the least asphaltene content. The experimental error lies at 3%, which is acceptable worldwide. The asphaltenes are n-heptane asphaltenes. 2.1. Limestone Slurry Sample Preparation. A 200 g amount of limestone was poured into 200 g of water over a period of 1 min in a clean beaker. The mixture was allowed to sit for a minute and then vigorously stirred for 2 min with a spatula.7 This suspension was transferred to a mixing cell for studying the wettability. 2.2. Maltene and Asphaltene Extraction. n-Heptane was added to the crude oil sample with a ratio of 4 to 1, volumetrically. Centrifuge 5810R (Eppendorf AG, Hamburg, Germany) was used to centrifuge the mixture for 25 min at a speed of 4500 rpm (∼3509g). Maltene, the lighter fraction of the mixture, was separated by filtration into a clean vial with n-heptane in the maltene fraction. The precipitated phase was washed with n-heptane several times to increase the purity of the asphaltenes. Toluene was used to dissolve the asphaltene residue. The asphaltene−toluene solution was transferred to an evaporating dish and left to evaporate overnight, under the fume hood. Solid asphaltene residue was mechanically removed using a spatula and weighed. 2.3. Wettability Experiment. Wettability experiments were performed on CaCO3 using ζ potential measurements, as proposed in the earlier study.10 Model DT-1200 electroacoustic spectrometer (Dispersion Technology, Inc., Bedford Hills, NY, USA) was used to determine the surface charge of calcium carbonate particles in water. The ζ colloidal vibration current (CVI) probe was placed in an external mixing cell where the limestone−water mixture is continuously stirred using an electronic stirrer, as seen in Figure 2. Measurement was made initially to determine the ζ potential value of the limestone−water slurry. The studied fluid, for example crude oil, was added milliliter-wise, and ζ potential measurements were made after each addition. Experiments conducted were carried out using distilled water since wettability effects become very important when brine saturation is lowered.12 Additionally, the temperature and pressure were kept constant since the water-wet character of carbonate reservoirs increases as temperature increases.13

Figure 2. Schematic diagram of the apparatus used in the ζ potential technique to measure wettability.

Figure 3. ζ potential measurements for aqueous limestone suspension with the addition of crude oil.

charge for aqueous limestone suspension is found to be about +30 mV. At this stage, the CaCO3 is completely wetted by water. Upon addition of crude oil, the surface charge of the aqueous limestone suspension falls steadily until around +21 mV beyond which a steep fall is observed. This limit differs for each type of oil, as shown in Figure 3. Because oil is comprised of negatively charged particles,10 oil droplets tend to adsorb to the positively charged CaCO3 surface as a result of electrostatic interaction, displacing the water particles to the bulk. Once the entire surface is completely wet with the oil droplets, the wettability curve experiences a steep decrease in the ζ potential value, signifying a complete oil-wet surface. The curve flattens out to indicate the end of the adsorption process. It can be seen from Figure 3 that all crude oil samples experience a similar trend upon the addition of crude oil. However, crude oil C tends to have a slightly different trend. This behavior will be explained in section 3.4.

3. RESULTS AND DISCUSSION 3.1. Different Crude Oils. Figure 3 shows the changes in ζ potential of the limestone−water suspension as different crude oils are added to the system. Wettability study of CaCO3 was carried out for three different oil samples (A, B, and C) from three different geographic locations within the UAE. Initially, the surface B

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Energy & Fuels As seen in Figure 3, it is observed that there is a general trend in the wettability curve for all crude oil samples. This trend comprises regions where the ζ potential steadily decreases, followed by a steep decrease and a plateau. However, the curve appears to be shifted vertically. As per our investigation10 and as reported by another study,4 C7 asphaltene particles were found to be negatively charged.4 Therefore, the crude oil with the most negatively charged particles, including resins as well, results in a lower ζ potential. Hence, the vertical shift is observed. During these experiments, the ζ potential value of the limestone−water slurry is observed not to change over time as repetitive readings are taken. However, if the mixture is left for a much longer time, the water from the mixture evaporates, leaving behind solid clusters of limestone. 3.2. Varying Concentration of Asphaltene−Toluene Solutions. Asphaltene is extracted from crude oil A and different asphaltene−toluene solutions of varying concentration (0.625, 1.25, 2.5, and 5 wt %) are prepared. Figure 4 shows the wettability curve for each solution.

Figure 5. ζ potential measurements for aqueous limestone suspension with the addition of 5 wt % asphaltenic solution prepared from asphaltene extracted from different crude oils.

As seen from Figure 5, the ζ potential value decreases as asphaltenic solution is added until the decrease is no longer steady. The curve tends to flatten out toward the end. For each ζ potential curve, it can be seen that the curve flattens out at a different ζ potential value. Asphaltenic solution prepared from asphaltene extracted from crude oil A results in the highest decrease in ζ potential value. On the other hand, asphaltenic solution prepared from asphaltene extracted from crude oil C has the lowest decrease in ζ potential value. In all, a similar trend is observed for all asphaltene samples. The experiments were halted for asphaltenic solutions of B and C earlier due to the formation of asphaltene−limestone clusters, as seen in Figure 6. These clusters made the mixture nonuniform and prevented further ζ potential readings. 3.3. Maltene vs Asphaltenic Nature of Crude Oil. In addition to the asphaltenes, wettability of limestone due to maltene, extracted from crude oil A, is studied. Figure 7 shows the wettability curve for maltene, asphaltene, and crude oil. For the purpose of clarity, the wettability curve of 5 wt % asphaltene−toluene solution is demonstrated alongside the

Figure 4. Wettability of aqueous limestone mixture for asphaltenic solutions varying the concentrations of crude oil A.

It is observed that, for all concentrations of asphaltene, a decrease in the ζ potential value is observed. This signifies the negative character of the asphaltene molecule in toluene as well as oil, as studied by Bassioni and Taqvi.10 However, as the concentration of asphaltene is increased, the ζ potential falls to a more negative value, provided there is a calcium carbonate surface available for asphaltene molecules to adsorb on. As seen from Figure 4, the ζ potential value of 0.625 wt % asphaltenic solution falls to about 36 mV whereas the ζ potential value of 1.25 wt % asphaltenic solution falls to about 35 mV. In addition, the ζ potential curve for 5 wt % follows that of 2.5 wt % until about 31 mV. For the ζ potential curve for 5 wt %, the ζ potential falls to a more negative value with fluctuating ζ potential values. The reason behind the change in trend of 5 wt % asphaltenic solution will be explained in section 3.4. Initially, the limestone surface in the limestone−water system is completely water-wet. Upon addition of asphaltenic solution, asphaltene particles tend to adsorb onto the limestone surface. The wettability alteration is complete once the wettability curve flattens out and the plateau, at the end of the wettability curve, marks a completely asphaltene-wet limestone surface. Asphaltene was extracted from all crude oil samples, and asphaltene−toluene solutions of 5 wt % are prepared using each asphaltene sample. Wettability experiment was carried out on these samples to compare with those of crude oil A. Figure 5 shows the wettability curve for each asphaltenic solution.

Figure 6. Formation of asphaltene−limestone cluster due to excessive addition of asphaltenic solution. C

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solution coincides with the plateau of the wettability curve for crude oil A, over the range 50−80 mL fluid added. In addition, both wettability curves seem to coincide at a similar value of about +12 mV. The crude oil wettability curve was compared with that of the 5 wt % asphaltenic solution since asphaltenic solution of 2.5 wt % asphaltene content follows a similar trend with a difference in the plateau region. Similarly, asphaltenic solution of content within the range 2.5−5 wt % can be expected to follow a similar trend with plateau regions between the two. Moreover, the crude oil sample has asphaltene content within that range. Hence, in this case, the wettability curve for the crude oil sample can be compared with either 2.5 or 5 wt % asphaltenic solution. All results were found to be reproducible as the experiments were repeated twice each. In addition, the error found is less than 0.5 mV and therefore the ζ potential deviation was considered negligible. The behavior can be explained such that when crude oil is added to the limestone−water mixture, the maltenes, in the crude oil, readily adsorb onto the surface of the limestone particle. After the surface is completely maltene-wet, asphaltenes start adsorbing onto the surface of calcium carbonate. The wettability curve approaches a plateau when the limestone surface is completely asphaltene-wet. The complete wettability process is described in the following section. 3.4. Wettability Alteration due to Asphaltene. All wettability studies that were conducted in this work indicate the adsorption of oil particles when crude oil or its individual components are added to a limestone−water mixture. However, a detailed explanation is required for such behavior and the role of asphaltene in the wettability alteration process. Figure 8 provides a mechanism in which the oil components go through the wettability alteration process, at a microscopic level. Among the four constituents of crude oil (i.e., SARA), resins and asphaltenes are the two fractions that have polar substituents.14 Saturates consist of nonpolar material whereas aromatics are polarizable. In order to observe a surface charge, the material/medium under investigation needs to be polar/

Figure 7. Wettability profile of limestone for maltene, asphaltene, and crude oil A.

wettability curve of crude oil on calcium carbonate. However, all other asphaltene−toluene solution wettability curves are reflecting the general trend these curves follow. Initially, the limestone surface in the limestone−water system is completely water-wet, at a ζ potential value of about 30 mV. When maltene is added to the system, the ζ potential value decreases to 20 mV, as seen in Figure 7. Maltenes adsorb onto the surface and less ζ potential value changes are observed as the limestone surface is completely maltene-wet. It could be argued that the presence of possible light asphaltenes in the maltene could affect the ζ potential drop. However, light asphaltenes, if any, would be kept afloat by the repulsive forces of resins. Since these light asphaltene particles would remain in bulk while resins are being adsorbed onto the calcium carbonate surface, the drop in ζ potential can be attributed to them solely. When compared to the wettability of limestone for crude oil A, the ζ potential values of the maltene wettability curve seem to coincide with those of crude oil, within the region of 0−30 mL of fluid added. Moreover, the maltene wettability curve observes less potential value changes around +20 mV, corresponding to the wettability alteration in the wettability curve of limestone for crude oil. On the other hand, the region approaching the plateau of the wettability curve for asphaltenic

Figure 8. Mechanism of oil particles during a wettability alteration process. D

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Energy & Fuels polarizable. Yet, their polarizability with respect to ζ potential has not been studied. Therefore, the mechanism is discussed based on resins and asphaltenes. Initially, the limestone surface in the limestone−water system is completely water-wet. Stable asphaltenes exist as nanoaggregates in crude oil and are kept in bulk by repulsive forces between resins and asphaltenes. Therefore, when crude oil is added, the resins, component in the light fraction of oil (i.e., maltene), competitively adsorbs onto the limestone surface. Resins are negatively charged in oil with a dipole moment of 2.4−3.2 D.1 Due to electrostatic attraction between the positive limestone surface and negatively charged resins, the resins adsorb onto the limestone surface. At the end of the steady decrease of ζ potential, a monolayer of resins is formed, removing the water molecules to the bulk, as indicated in Figure 8. Following the steady decrease of ζ potential, the limestone surface experiences a steep decrease in ζ potential value. This is attributed to the competitive adsorption between the resins and the asphaltene molecules. Asphaltene molecules, as reported earlier, have a dipole moment over the range 3.3−6.9 D.1 This signifies the ability of asphaltene molecules to carry a greater charge than resins. Therefore, during the wettability alteration process, asphaltenes carry a greater negative charge than resins and competitively adsorb onto the limestone surface, displacing the resin molecules to the bulk. There is a possibility that the asphaltene molecules may directly adsorb on top of the monolayer of resins. If that were to be the case, fluctuations would be observed with no sharp decrease in the ζ potential. In an earlier study, a wettability experiment was conducted where water was added milliliter-wise to a limestone−crude oil mixture.10 The initial ζ potential value of the oil-wet limestone was negative. As water was added, similar fluctuations were observed and the remainder of the experiment showed large chunks of heavy fractions of oil being deposited onto the solid limestone. However, in this study, there is a sharp decrease observed, followed by fluctuations at a later stage. This behavior indicates that there is a competitive adsorption between the asphaltenes and resins. The wettability curve flattens out when the asphaltene molecules form a monolayer on the limestone surface. Wettability studies of limestone for crude oils A and B tend to follow this behavior. Moreover, wettability of limestone for all asphaltenic solutions except 5 wt % is believed to follow this mechanism. According to a study conducted by Plank and Bassioni,7 CaCO3 was found to correspond to a Type II isotherm. The Type II isotherm, observed in physical adsorption, represents the formation of a monolayer near the point of inflection and further layers could form.15The additional phenomenon, indicated in Figure 8, is believed to be observed in the wettability curve of limestone for crude oil C and is the reason behind the difference between the wettability of limestone for all asphaltenic solutions and the 5 wt % asphaltenic solution. As the concentration of crude oil increases in the limestone− water−crude oil system, another steep decrease in the ζ potential value is observed. The curve flattens out, and fluctuations are observed toward the end of the wettability curve. Discussing the mechanism following the monolayer formation of asphaltene molecules on limestone, it is believed that more asphaltene molecules approach the monolayer and adsorb onto it. This results in the formation of multilayers, leading to asphaltene deposition. This behavior can be attributed to the adsorption isotherm IV. As described by Gregg and Sing,15 the

points of inflection in the adsorption isotherm type IV show the completion of monolayer as well as the onset of multilayer adsorption. Moreover, capillary condensation is associated with this isotherm where the adsorbate fills the small pores of the solid. In the case of the wettability alteration by crude oil, the pores can be assumed to be filled by asphaltenes which causes fluctuations. The wettability study, of the limestone−oil system with water added, shows fluctuations at a significant level where oil is destabilized by water in the system.10 In this study, the fluctuations can be clearly seen toward the end of the wettability curve of limestone for crude oil C, as seen in Figure 3. The fluctuations are also evident in the wettability study of limestone for 5 wt % asphaltenic solution, as seen in Figure 4. When a concentrated liquid coats a porous solid, it tends to permeate through the solid rock and fills its pores. As observed from the wettability study of the limestone−oil system with water added, water presence in such a system causes deposition of heavy fractions, leading to instability in the system. Commonly, physical adsorption gives rise to such an isotherm resulting in multilayer adsorption. Components, comprised of aromatic rings, in the crude oil adsorb on to the monolayer of oil particles adsorbed on the CaCO3 surface. As multilayers adsorb, the system experiences a steady decrease in the ζ potential value until a plateau is reached. Due to the multilayer adsorption, the thickness is believed to increase causing the CVI to measure a potential value at a distance beyond the electric double layer.

4. CONCLUSION AND FUTURE WORK ζ potential technique was used in this work to understand the wettability alteration (i.e., from water-wet to oil-wet) of limestone oil reservoir rock. Industrial methods determine the wettability of a rock at a certain state (i.e., either water-wet or oil-wet). However, they are unable to demonstrate the ability to observe in situ wettability transition (i.e., going from water-wet to oil-wet or vice versa). Unlike these methods, the transition period can be clearly explained and the phenomena can be easily understood with the help of using the ζ potential technique. As a preliminary step to deposition, precipitation of asphaltene is thermodynamically essential. Furthermore, according to research, light components are found to be strong precipitants for asphaltenes.16 Therefore, asphaltene deposition in both porous media and production tubing is a major problem for lighter crude oils. The experiments conducted in this study on light crude oils show asphaltenes are deposited onto calcium carbonate forming clusters. This is in good agreement with previous studies.16 Maltenes were found to readily adsorb onto limestone surface, displacing water. Asphaltenes were found to displace maltenes and adsorb onto the surface. Their deposition led to multilayer formation and wettability reversal. Many aspects of this study have areas for further research potential where wettability can be investigated over variable parameters such as time, temperature, and pressure. This could aid in understanding wettability alteration at reservoir conditions. Moreover, since ζ potential technique for measuring wettability is a novel technique and measurements were carried out under a certain induced electric field, future work could be carried out by studying the behavior of asphaltenes in the presence of different electric fields. E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 00201001832728. Fax: 0020222630470. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. Dalia Abdullah (ADCO) for her help with SARA Analysis data. REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02127 Energy Fuels XXXX, XXX, XXX−XXX