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1 OTC Application and Evaluation of a NanoFluid Containing NanoParticles for Asphaltenes Inhibition in Well CPSXL4 R. Zabala, E. Mora, C. Cespedes, L. Guarin, H. Acuna, O. Botero, Ecopetrol; J.E. Patino, Petroraza; F.B. Cortes, Universidad Nacional de Colombia Copyright 2013, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference Brasil held in Rio de Janeiro, Brazil, October This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright. ABSTRACT This paper describes the evaluation of a fluid containing nano-materials with high adsorption capacity for Asphaltenes inhibition in a volatile oil field in Colombia. Asphaltenes precipitation has been identified as one of the most potential damaging mechanism affecting the well productivity in Cupiagua Sur Field. Before this work traditional techniques have been tested in the field for asphaltenes inhibition. The goal of the injection of nano-fluids containing nano-particles of alumina with high surface area (>120 m 2 /g) is to absorb the asphaltenes before being flocculated and carry them with the condensate avoiding precipitation near the well bore. In the first stage of this work, lab testing, the adsorption capacity of alumina nano-particles for the asphaltenes produced in the field was evaluated, next, the nano-fluid containing nano-particles was evaluated in the Mirador formation plug cores by performing flowing core tests at the reservoir conditions. Returns of the relative permeability curves were quite good, improving Ko from 0,09 md to 6,13 md which enabled us to recommend the application of this nano-fluid in the well CPSXL4. For the Field Application, the job was designed by forcing 220 bbl of nano-fluid within a radius of 8 ft. After the job, through monitoring with SARA analysis in produced fluids and the residual tracking of nano-particles as well. Maintenance of asphaltenes concentration in the produced fluids has been observed. Also, significant gain in the oil and gas production has been reported, with more than 100,000 bbls of cumulated gain production in only 4 months after the job was performed. Results may be promising and now it is possible to extend application of nano-fluid in other wells in this field. INTRODUCTION The Cupiagua Sur field is located 110 kilometers northeast of Bogota in the foothills of the eastern mountain chain of the Colombian Andes, close to other fields discovered in this area such as Floreña, Pauto, Volcanera, Recetor, Cupiagua and Cusiana (Fig. 1). Despite the fact that Cupiagua Sur is very close to Cupiagua, this reservoir is totally independent and separate. It is formed by a back thrust structure with several reservoir pressures, properties and contact fluids. Cupiagua Sur is a compositional volatile oil reservoir with an average API gravity of 38 ; there is no free gas cap at initial conditions. The main formations are Mirador and Barco which are quite similar about petrophysical and fluid properties (average permeability is 21 md and average porosity is 6.5%). It is a prolific oil field that in fifteen years of production has recovered 88 MMstb gross out of 189 MMstb original in place from Mirador and Barco reservoirs in a developed area with four producer and two injector wells.

2 2 OTC Ecopetrol assumed direct operation of Cupiagua Sur since July 2010, following by BP operation. Fig. 1-Cupiagua Sur Location and other Foothills fields The current reservoir conditions that can be expected in Cupiagua Sur field are highly dependent on the way the reservoir has been produced and the gas injection support. The first production well CP XP1 started producing in March 1998, while the first gas injector well was put on injection in January 2000; at that time one oil volatile well was on production. Currently in the field there are 6 active wells. The number of producers and gas injectors are 4 and 2 respectively. Figure 2 shows that oil production reached its peak in March 2001, which was 41,722 SBbl/d. The average gas production (at that date) was 175 MMscfd. Today, the recovery factors are (48.2) 48.2%, and 29.5% in Mirador and Barco respectively. Additionally, Fig. 2 shows the historical behavior of production and injection through time, showing the entries of different production wells comprising the field. On this graph it is easy to explain that there was no production plateau in Cupiagua Sur field, a constant decline rate is observed. This indicates the drastic conditions how the reservoir was depleted and also the drastic conditions regarding sources of damage.

3 OTC XA3 Axis 1 CONTRACT: SANTIAGO FIELD: CUPIAGUA_SUR(8) Oil ( bbl/d ) Water ( bbl/d ) Axis 2 Gas ( Kcf /d ) CONTRACT: SANTIAGO FIELD: CUPIAGUA_SUR(8) XL XP XL4 XN XZ Date Fig. 2 Historical production for Cupiagua Sur Field 0 The total cumulative gas injection is GSCF in Mirador and the average gas injection is between 250 and 260 MMscfd lately as Figure 3 indicates Gas_inj ( Kcf /d ) FIELD: CUPIAGUA_SUR Date Fig. 3 Historical Gas Injection for Cupiagua Sur Field

4 4 OTC Volatile oil reservoirs represent a challenge because although asphaltenes contents are very low, also the largest percentage of this light crude is represented in light saturates in which compounds, usually asphaltenes, have poor solubility. To discuss the precipitation of asphaltenes in volatile oil, it is important to provide a description of volatile oil reservoirs. In such reservoirs, the temperature is slightly lower than the critical point so that the volatile oil has a high content of this gas or light components of the gas. The hydrocarbon mixture at the initial conditions is near the critical point and is not very stable. The typical composition of hydrocarbon samples from such oil volatile reservoirs may contain methane 65%, ethane 7%, butane 4%, pentane 3%, hexane 1%, others 15%. The phenomenon that occurs in a volatile oil reservoir during depressurization of the reservoir has to do with the phase equilibrium and this role is crucial. The reservoir is in its initial phase in a liquid state, because the temperature is below the critical temperature, but as it begins to produce and the system pressure decreases quickly reached bubble pressure, where the first gas bubbles appears and below this pressure creates a free gas phase. What usually happens is that the gas phase flows easily toward the well, but the liquid phase with a different composition, moves slowly into the reservoir and accumulates in the reservoir. Regarding the effect of compositional changes in the liquid phase, the oil generated can submit asphaltenes flocculation. 1 In addition; Gas re-injection is used as a mechanism for sustaining the reservoir energy in a volatile oil reservoir. Gas injection can cause further precipitation of asphaltenes. While the compositional characteristics of the hydrocarbons contained in a volatile oil reservoir show a lower composition of asphaltenes, in some cases less than 1%, it s effects on precipitation in the near well bore can be very visible in terms of impact on well productivity. When precipitation of asphaltenes occurs in a volatile oil reservoir, traditional methods of inhibiting asphaltenes at reservoir level appear to not be effective, this is because asphaltenes inhibitors and asphaltenes dispersants have similar chemical composition that are mixtures of organic acids and derivatives of aromatic hydrocarbons, which when they are squeezed into the reservoir can disperse deposits and dilute into crude but with a little chance to maintain a residual in the reservoir. In Cupiagua Sur where some wells have significant skin damage associated with asphaltenes precipitation, the wells are stimulated with various conventional treatments for the purpose of stabilizing and reducing the flocculation pressure of asphaltenes. Currently the reservoir is subjected to gas injection, leading to compositional changes in the volatile oil at the reservoir. This directly affects asphaltenes stability. Usually organic stimulations are performed for the purpose of periodical cleaning or removing asphaltenes deposits that precipitate in these wells, even when attempting to manage the pressure drops in all the wells have large draw downs which accelerates flocculation and precipitation of asphaltenes. The high flow rates and high draw downs impede good retention of asphaltenes inhibitors at the formation for the conventional type of inhibitors. It is expected that the adsorption of asphaltenes inhibitors based on nano-particles such as nano-alumina, have a better affinity with the mineral structure of the reservoir rock, and it can be retained for extended periods. BACKGROUND Many important works have been developed, focusing on better understanding the nature of asphaltenes and the way they remain peptized in oil. Since these early studies may better understand how the asphaltenes remain as a heavier fraction of crude and being insoluble in saturated or paraffinic of crude oil fractions could be soluble in aromatic fractions such as toluene or benzene. Early works for inhibiting asphaltenes were developed based on amino compounds, based resins, and some solvents with high aromatic contents. Some research has shown amphiphilic behavior of asphaltenes in the fact that they contain polar groups and non-polar groups in the molecule and in the form of colloidal aggregates 2,3. Although there is no precedent of asphaltenes inhibition works with nano-particles. There is important work in the study of the adsorption of asphaltenes on surfaces of metal oxides, and based on this work, developed since long ago, catalytic cracking technology was widely used in the oil refining. Work on the adsorption of asphaltenes on minerals exhibit behavior of the Langmuir isotherm.4 Langmuir model 5 has been used on

5 OTC a frequent basis after publication in 1961 to correlate absorption values at equilibrium. From experimental results obtained in this model assumes that adsorption occurs as a homogeneous monolayer on the surface of the material. Pernyeszi et al.6 assessed the adsorption of asphaltenes on quartz and clays such as kaolinite, bentonite and cliptonite. Nanotechnology has allowed synthesizing materials with higher surface area and better adsorption capacity. There are a number important works showing the behavior of the adsorption of asphaltenes on nanoparticles of metal oxides such as Fe3O4, Al2O3, NiO, MgO. Cortes et al.7 (2012) and Franco et al.8 (2012) performed studies of adsorption of Colombian asphaltenes onto nano-silica and nano-alumina respectively, The authors evaluated the adsorption capacities, the adsorption kinetics and the thermodynamic properties. They found that use alumina as support provides more adsorption capacity than silica and reaction times of less than 2 minutes. The calculated thermodynamic properties for the absorption of asphaltenes onto the nanoparticulated SHS confirmed the spontaneity and exothermic nature of this process. Studies conducted with DC conductivity 9 indicate that asphaltenes form nano-size aggregate close to 2 nm. We know that this happens when the asphaltenes are peptized in crude oil, but once the colloidal stability conditions have been affected by a pressure drop or other factors, it begins to appear flocculation of the asphaltenes and subsequent precipitation, often making alterations of wettability of the near well bore and sometimes large aggregates which generate down hole plugging, however in all instances generates production losses. What is intended with the development of nano-materials for inhibition of asphaltenes is that contrary to the common notion of adsorbing asphaltenes to force the cracking of these, is rather help them to be transported once are adsorbed by nano materials and since adsorption occurs in monolayer form prevents the formation of large agglomerates of asphaltenes which are responsible for the plugging at the well bore and also in installed down hole equipment. In wells of Cupiagua Sur Field, asphaltenes precipitation has been identified as one of the main skin damage factors. The increase in CO2, the compositional changes, pressure drop, and the re-vaporization due to gas injection are the main factors favoring the asphaltenes destabilization and precipitation. Ten Periodical cleaning jobs are performed in those wells, using a chemical blend composed of an aliphatic-aromatic mix. These stimulation jobs have been effective, increasing oil production. However, the effective life of organic stimulation is very short and the wells need to be stimulated again in a few months. Chemical inhibition jobs also have been performed with polymers that act like resins for the asphaltenes inhibition mechanism, these treatments helped to increase the life of the stimulations results in few days. The method of nano fluids containing alumina nanoparticles is expected to effectively extend the life of organic stimulations, preventing asphaltenes precipitation at down hole and perforations while maintaining a sufficient residual of nanoparticles within the reservoir, for inhibiting the new oil that is flowing to the well. With the implementation of the nanotechnology now it will be possible to compare results between this novel method and the polymer inhibition method. EXPERIMENTAL DESCRIPTION Asphaltenes Isolation: First stage of the study was the asphaltenes extraction from a sample of oil, using traditional methodology for asphaltenes flocculation, adding n-heptane. The mixture n-heptane/oil was prepared in a volume ratio 40/1, and then the mixture was sonicated for 2 hours at 25 C, then by a centrifugal process for two hours to complete decantation. The precipitated was then filtered through 8 microns watman paper, repeatedly washing with n-heptane to obtain a clean leachate. Finally the filter containing asphaltenes cake was dried at vacuum for 24 hours. After extracting a sufficient amount of asphaltenes, it proceeds to start the next stage. Adsorption test: For adsorption testing a stock solution of 1 gram of asphaltenes in 0.5 liter of toluene was prepared. Then, different dilutions were prepared from this stock solution in order to produce a calibration curve using a spectrophotometer UV-VIS Genesys 10S, measuring runs at a wavelength of 295 nm.

6 6 OTC To construct isotherms curves, three different dilutions were prepared with concentrations of 250 ppm, 750 ppm and 1500 ppm and adding nanoparticles 0.1 mg per 10 ml of solution. The vessels containing the solutions were taken to a magnetic stirrer for 15 minutes, and allowed to stand for 5 minutes post stirring. A sample of the supernatant was then taken and absorbance measurement carried in the spectrophotometer. This cycle was repeated continuously until two consecutive equal values were achieved, which indicates that we have reached the equilibrium point. The time was recorded and we elaborated a curve of amount of adsorption vs. time, for the nano-material. After knowing the kinetics of adsorption with alumina nano-particles, and proved that adsorption rates are sought. The nano-fluid was prepared using a mixture of solvents as carrier fluid for alumina nano-particles. Physical properties of this carrier fluid or mixture of solvents should have enough viscosity to maintain nano-particles in suspension, low surface tension in order to maintain appropriate dispersion of the nano-particles. The solvent mixture also must have good compatibility with the nano-material avoiding any further reaction to degrade or affect the nano-particles. Aromatic solvents were not included in the nano-fluid, because we wanted to keep it free of aromatics compounds for environmental purposes. So finally the nano-fluid was prepared and we proceeded to evaluate the effectiveness of this nano-fluid in a core plug obtained from the Cupiagua Sur Field. Asphaltenes sorption measure Adsorbed Asphaltenes by the nano-material was measured for the change in asphaltenes concentration in toluene solutions. Figure 4 shows asphaltenes absorption in alumina nano-particles vs time for each of the evaluated Asphaltenes/toluene solutions 250 ppm, 750 ppm, and 1500 ppm. Asphaltenes sorption on nano-alumina shows better performance for higher asphaltenes content. Fig. 4 - Asphaltenes adsorbed onto Alumina nano-particles vs. time Core flooding test: These tests are fundamental to evaluate the nano-particles effectiveness for asphaltenes inhibition in a porous media and also determine returns of permeability in a porous media after each flooding test stages. The following procedure was performed for the flooding test: 1. Preparation of core plug. 2. Injecting 10 vp of water. For measuring absolute permeability. 3. Injecting several pore volumes of oil until pressure was kept constant. Measure Ko at Swr.

7 OTC Injecting 20 vp of water. To Measure effective permeability of water at Sor. Construction curves of Kr and Np. 5. Injecting 10 vp of oil, for oil saturation at Srw. 6. Injecting 0,5 vp N-heptane for induce asphaltenes precipitation and skin damage generation. 7. Injecting 20 vp of water. For making Kr and Np curves. Measure of effective permeability for water. 8. Injecting 2 vp of DAX (diesel, alcohol; Xylene) 9. Injecting 10 vp of oil 10. Injecting 10 vp of Water 11. Injecting 3 vp of oil 12. Injecting 0,5 vp of nanofluid containing alumina nano-particles 13. Evaluation of inhibition: Injection 0,5 vp of n-heptane 14. Injection of 0,3 vp of nanofluid containing alumina nano particles 15. Injection 50 vp of water for measure of Kw and construction curves of Kr and Np 16. Injection 50 vp of oil Core flooding test Results: Changes in oil effective permeability show the alteration of permeability by asphaltenes precipitation. After n-heptane injection, without inhibition asphaltenes precipitation creates skin damage above 99% if it is compared with the original permeability. After clean-out with the injection of DAX (Diesel, Alcohol, Xylene) the skin damage is reduced to 37% and after application of the nano-fluid containing nano-particles for asphaltenes inhibition an additional skin damage reduction was achieved until 34%. Inhibition is proven after application of nano particles, the oil effective permeability is maintained for a longer time after 50 pore volumes of injected oil. And also the relative permeability to oil visibly increases after application of the fluid containing alumina nano-particles. Table 1 resumes Effective permeability to Oil and Effective permeability to water in a porous media after each flooding test stages. Stage After Asphaltenes precipitation with n-heptane (Induced skin without inhibition) After Clean out with DAX (Diesel, Alcohol, Xylene) After treatment with nano-particles After second Asphaltenes precipitation with n-heptane (Induced Skin after inhibition) Effective permeability to Oil (Ko) Effective permeability to water (Kw) 0.09 md md 5.79 md md md md 2.54 md 6.48 md Table 1 - Effective permeability to Oil and Effective permeability to water in a porous media after each flooding test stages.

8 8 OTC In the core flooding tests, the relative permeability to oil visibly increases after application of the fluid containing alumina nanoparticles. Fig.5 shows the positive changes in relative permeability after the injection of 0.5 vp of nano fluid containing nanoparticles. Fig. 5 Relative Permeability Curves before and after Nano particles injection FIELD APPLICATION Well candidate selection CPSXL4 Well was the first well selected to test the new stimulation technology on the basis of inhibition work with nano-particles. This well was considered a candidate because of the following reasons: Precipitation of asphaltenes occurs in CPSXL4 reservoir, traditional methods of inhibiting asphaltenes at reservoir level were not effective, and, because asphaltenes inhibitors and asphaltenes dispersants have similar chemical composition, those are mixtures of organic acids and derivatives aromatic hydrocarbons, so when those are squeezed into the reservoir they can disperse deposits diluted in the crude, but with a little chance to maintain a residual in the reservoir. Past interventions in the Cupiagua Sur well CPSXL4 have shown significant benefits from these works. It is possible to obtain good information related to the formation damage mechanism, and this is achieved with clear interpretations, analysis of production data and laboratory testing of the well fluid. CPSXL4 Well is one of the best wells in Cupiagua Sur wells. Since the beginning of Cupiagua Sur field an adequate pressure management has been accomplished.

9 OTC Axis 1 CPSURXL4 Oil ( bbl/d ) Water ( bbl/d ) Axis 2 Gas ( Kcf /d ) CPSURXL Date Fig. 6 CPSXL4 Historical Production 0 CPSXL4 was completed as a Mirador and Barco oil producer in May 2002 with 7 tubing and 4 ½ production liner. Following this, two frac jobs were pumped in Mirador. In summary, the main well bore damages are: organic deposits (asphaltenes), presence of inorganic deposits are mostly possibly barium sulfate (BaSO4) in Barco formation and calcium carbonate (CaCO3) in Mirador formation, blocking fluid problems (condensate water, completion fluid, etc.) and finally, fines migration, as shown in the skin characterization Diagram: Mirador Barco 7 % 30 % MSP (Mineral Scale Parameter) 21 % 17 % FBP (Fines Blockage Parameter) 31 % 14 % OSP (Organic Scale Parameter) 24 % 30 % KrP (Relative Perm. Parameter) 12 % 1 % IDP (Induced Damage Parameter) 5 % 7 % GDP (Geomechanical Dam. Parameter) Table 2 Damage Parameters for CPSXL4

10 10 OTC Fig. 7 - Skin Characterization Diagram CPSXL4 Developing a new technology to inhibit Asphaltenes depositation with nano-particles Four Chemical Stimulations were performed in the well; the first one in January 2004 to attack inorganic deposits delivers IOR of 800 bopd, in April 2006 an inorganic-organic stimulation was conducted with a benefit of 530 bopd. In January 2011 performed was a selective stimulation in Barco and Mirador Formations the instantaneous oil rate increase was 152 bopd. The Chemical Stimulation with inhibition developed with nano-particles was designed to overcome the main problems associated with the old inhibition treatments. The main differences are listed below: Pumping in different stages to remove the formation damage present in the well; Pickling job, organic and inorganic remotion, and finally, inhibition treatment. Usually organic stimulations are performed for the purpose of periodical cleaning or removing asphaltenes deposits that precipitate in these wells. The management of pressure drop is needed in the field to avoid large pressure draw downs which accelerate flocculation and precipitation of asphaltenes. The management of pressure drop is needed in the field to avoid large pressure draw downs which accelerate flocculation and precipitation of asphaltenes. High flow rates and high pressure drops prevent good retention of conventional inhibitors asphaltenes formation. It is expected that the adsorption of asphaltenes inhibitors based on nano-particles such as nano-alumina, have a better affinity with the mineral structure of the reservoir rock, and can be retained for extended periods. Stimulation and inhibition job strategy in CPSXL4 Sur Well The overall stimulation job was performed in several stages to ensure the best reservoir conditions for the inhibition treatment (last stimulation stage).

11 OTC Fig. 8 Stages for the stimulation job in CPSXL4 A pickling job was set to clean the production tubing, an EDTA begun to dissolve carbonate scale; and organic treatment stage was intended to dissolve organic scale. The inhibition job in CPSXL4 was carried out in December 2012 pumping 220 bls of nano fluid containing alumina nano particles and 411 bbls of displacing fluid to reach the desired penetration radius of 7.2 ft. As displacing fluid (Over flush) a mixture of DAX (diesel, Alcohol, Xylene) was used. A coiled tubing unit was used and a selective packer was set between Mirador and Barco formations, the job was performed pumping fluid at a very low rate and pressures below fracture gradient. After 12 hours of soaking time the well was opened for production at controlled flow rates. RESULTS AND DISCUSSION A production well test was performed between each one of the stages to verify the post stages well performance (Fig. 9). The following performance was observed: The net initial incremental OIL rate (IIOR) was 1280 Bopd. The API performance increase from 40 at the beginning of the Stimulation job to 41.5 at the end of the inhibition with nano particles stage. Nodal system analysis showed the improvement obtained in IPR (Skin reduction) and also the VLP performance was altered because of the increase in oil production. The post inhibition stimulation production performance has been monitored for almost 8 months where the last three months of production has remained a constant behavior of 300 BLS above the baseline. The best performance in terms of oil increasing was observed after EDTA and organic treatment stage 1200 BOPD with respect to base case. Post nano-particles inhibition gain 80 BOPD. This incremental was not expected because the inhibition was intended to extend the life of the stimulation process.

12 12 OTC Fig. 9 Well Performance after each stage Well Treatment following up The post inhibition stimulation production performance has been monitored for almost 8 months where the last three months of production has remained a constant behavior of 300 BLS above the baseline. Fig. 10 Incremental Production after CPSXL4 Job

13 OTC Figure 11 shows the Asphaltenes concentration in produced oil vs. Nano-particles residual in water. Starting the treatment the asphaltenes concentration in oil is lower than 3% for the first 40 days. After the asphaltenes concentration increases up to 4%. Figure 12 shows Oil Production gains in well CPSXL4with the nano-fluid containing nano-particles. After 213 days of the application in well CPSXL4, more than 149,200 barrels of additional cumulative production has been produced in the well CPSXL4, and remains inhibitor residual values close to 0.2 mg / l in the produced water, which indicates that there is still residual inhibitor which remains adsorbed on the formation. Fig Asphaltenes concentration in produced oil vs. Nano-particles residual in water. Fig. 12 Concentration of nano-particles in produced water.

14 14 OTC CONCLUSIONS Incremental Production of 1200 bls after organic and inorganic remotion stage indicated that those formation damage mechanism are the main factor that affect the productivity of CPSXL4 as Characterization and lab showed. Post nano-particles inhibition gain 80 BOPD. This incremental was not expected because the inhibition was intended to extend the life of the stimulation process. The concentration of nano particles in water produced showed that up to date the inhibition process still works which is observed in the production performance above baseline. Experimental study permitted to evaluate the effectiveness of nano-fluids containing alumina nano-particles for asphaltenes inhibition. Well stabilized nano-fluid containing alumina nano-particles have a good performance into the reservoir, even at very low permeability conditions. Asphaltenes content measured in the produced oil increased after the well treatment with the nano fluid containing alumina nano-particles. Nano fluid containing alumina nano-particles had good retention into the formation for longer than six months. REFERENCES 1. Thou S, Ruthammer G and Potsch K, 2002, Detection of Asphaltenes Flocculation Onset in a Gas Condensate System, SPE Acevedo S., Castillo J., Fernandez A., Goncalves S., Ranaudo M., Energy & Fuels 12 (1998) Mousavi- Dehghani S., Riazi M., Vafaie-Sefti M., Mansoori G., J. Pet. Sci. Eng. 42 (2004) Gonzalez, G., Moreira, M.B.C. Colloids Surf. 1991, 58, Langmuir, I. Journal of the American Chemical Society 38(1961) Pernyeszi, A., et al Journal of colloids and surfaces A 137 (1998) Cortés F.B., Mejía J.M., Ruiz M.A., Benjumea P., Riffel D.B. Sorption of Asphaltenes onto Nanoparticles of Nickel Oxide Supported on Nanoparticulated Silica Gel. Energy Fuels. 2012; 26: Franco C, Patino E., Benjumea P., Ruiz M.A., Cortes F.B. Kinetic and thermodynamic equilibrium of asphaltenes sorption onto nanoparticles of nickel oxide supported on nanoparticulated alumina. Fuel (2012), 9. Goual, L., Sedghi, M., Zeng, H., Mostowfi. F.; McFarlane, R. & Mullins, O.C. (2011). On the Formation and Properties of Asphaltene Nanoaggregates and Clusters by DC Conductivity and Centrifugation. Fuel, Vol.90, No.7, pp Franco C., Zabala R., Botero O., Zapata J., Mora E., Candela C., Castillo A., Inhibited Gas Stimulation to Mitigate Condensate Banking and Maximize Recovery in Cupiagua Field. SPE PP, presented at the SPE Formation Damage Conference, Lafayette, Louisiana, US February 15-17, 2012.