Using Zeolite-Zirconia-Copper Nanocomposites as a New Asphaltene Inhibitor for Improving Permeability Reduction during CO 2 Flooding

Using nanoparticles for adsorbing asphaltene was known as an efficient method among researchers for crude oil upgrading and in this study, Zeolite-zirconia-copper nanocomposites (NCs) has been synthesized and characterized with SEM, XRD, BET, and EDX for asphaltene precipitation inhibition in the static phase and solving asphaltene deposition problems of dynamic CO 2 flooding in low permeability carbonate reservoir. CO 2 -oil IFT tests, isotherm models, natural depletion tests at static phase were performed in the presence of NCs and the results were compared with zeolite nanoparticles. Then, CO 2 core flooding tests at dynamic phase were designed in the presence of NCs at obtained static conditions for surveying permeability/porosity reduction in porous media. After adding NCs and zeolite nanoparticles, the 2 nd to 1 st slope ratio in CO 2 -oil IFT tests increased from 19.697 % to 20.895 % and 29.851 %, respectively which shows NCs adsorbed more asphaltene in comparison to zeolite nanoparticles which confirmed UV-Vis results. NCs was decreased asphaltene precipitation more than zeolite at same points during natural depletion tests and it was selected for dynamic CO 2 tests. After adding NCs, asphaltene depositions which occurs after CO 2 injection was decreased and permeability/porosity reduction parameters were improved.


Introduction
Asphaltene precipitation is one of the main concern during oil production, and changing thermodynamic conditions such as pressure, composition and temperature were known as important factors for asphaltene precipitation [1][2]. Asphaltenes are insoluble in paraffins (heptane and pentane) and soluble in aromatics (toluene) [3][4][5]. During oil production, problem occurs in the presence of asphaltene particles such as plugging due to precipitating of this solid phase in crude oil [6][7]. Researchers introduced many methods for solving asphaltene precipitation problems such as treatments with mechanical approaches or adding solvents and surfactants while due to specific characteristics of nanoparticles, using nanoparticles especially nanocopmosite was introduced as an efficient method among different researchers [8][9][10][11][12][13][14][15]. The main parameter in their study is asphaltene adsorption on nanoparticles surface which is depends on many factors such as carboxlic, pyrrolic, Pyridinic existence in asphaltene, type and nanoparticles characteristics, and nanoparticles-asphaltene interactions [16][17][18][19][20][21][22][23][24][25]. Various nanoparticles such as zeolite and metal oxides and zirconia find applications in different fields such as catalysis and adsorption [26][27][28][29][30][31]. Zeolites are known as a reliable adsorbent of asphaltene, and hydrated aluminosilicate is the most important parts of zeloites [26][27], and these nanoparticles used as support materials for stabilizing nano particles to produce high efficacy nanocomposites [32][33][34]. Hosseinpour et al., shows that zirconia samples with surface acid sites has high BET specific surface area values in compared to other nanoparticles such as Fe2O3, NiO, and WO3 which shows strength of interactions between the asphaltenes and this sample [28]. Another nanoparticles which are widely used due to their availability and low cost is copper oxide nanoparticles. Copper oxide has a wide variety of applications including efficient adsorbent [35 40], foam stabilization [41], EOR [42], and environmental remediation [43].
Changing composition due to adding CO2 gas was known as a main concern in porous media, and it was found that asphaltene deposition especially at inlet part of core caused formation damage which should be fully considered [44]. Moreover, it was found that agglomeration problem relevant to nanoparticles can be solved with using nanocomposites [45]. In this study, Zeolite-zirconia-copper oxide nanocopmosites (NCs) was used for surveying asphaltene adsorption on the surface and removal of asphaltene as an asphaltene inhibitor and the results compared with zeolite nanoparticles as a reference. As NCs had better results in the static phase and was selected for performing dynamic CO2 tests. Although the application of zeolite, zirconia, copper oxide at different cases were surveyed in previous studies such as asphaltene adsorption, introducing NCs as a possible asphaltene adsorbent and dynamic CO2 tests were not covered according to our knowledge. At first phase, adsorption behavior during CO2-oil IFT tests was observed and results was matched with real natural depletion tests in the presence of NCs and zeolite nanoparticles. Natural depletion was used as the main sources of making asphaltene precipitation by changing pressures and the main points in natural depletion tests were selected based on the CO2-oil interfacial tension (IFT) tests (higher adsorption potential points). Two tests of CO2-oil interfacial tension (IFT) and Ultravioletvisible spectroscopy (UV-Vis) spectrophotometer were used for describing asphaltene adsorption on the nanoparticles surface, and two different isotherm models of Langmuir and Freundlich were completely surveyed in the presence of NCs and zeolite nanoparticles.
Finally, based on the static phase results, NCs was selected for performing dynamic CO2 tests and improving permeability reduction/porosity reduction parameters.

Materials
Crude oil was collected from one of Iranian oil reservoir in the west with oil density of 0.864 g/cm 3 and viscosity of 9.9 cP at 40 °C. Table 1 shows reservoir oil composition and the results of saturate-aromatic-resin-asphaltene (SARA) test was shown in Figure 1. 8. Performing filteration and cacination process at 300°C for three hours.

Interfacial tension (IFT) tests
CO2-Oil interfacial tension tests were performed with experimental set up as Figure 2. This set up can measured interfacial tension at specific pressure and temperature.
IFT set up contained different parts such as high pressure dioxide carbon cylinder, high pressure and high temperature cells, pumps for injecting the crude oil, pressure gauges, temperature sensor, and data acquisition software. Four steps were used for obtaining CO2oil IFT as: 1. Using Nitrogen for flushing the lines and main cell.
2. Injecting dioxide carbon gas through the main cell.

Natural depletion tests
Natural depletion tests were performed with static apparatus as Figure  4. Obtaining asphaltene content percent with standard IP143 method.
5. Calculation of the asphaltene precipitation with subtracting asphaltene content from step 4 from asphaltene content at initial case.

Batch adsorption experiment
Surveying adsorption of asphaltene on NCs or zeolite nanoparticles surface was performed with Batch adsorption tests with toluene solution. Below five steps were used during this as below: The main items in Equation 1 and the relevant units are Initial asphaltene concentration ( ), mg/L, asphaltene equilibrium concentration( ) , mg/L, volume of solution (V), L, NCs mass (m), mg.

Dynamic experimental CO2 procedure
A dynamic apparatus schematic of set up that used to perform dynamic CO2 tests is shown in The dynamic set up was contained three different vessels of brine, nanofluids, and crude oil, hydraulic pump, core holder for holding carbonate core plug, pressure and temperature sensors, overburden pump, DP sensors, and acquisition data. Below five steps were used for performing dynamic CO2 tests in the presence of NCs: 1. Injecting formation water (0.1 cc/min) and calculate water permeability 2. Obtaining residual water saturation with performing oil injection 3. Calculation of effective oil permeability at residual water saturation 4. Injecting simultaneous oil and CO2

Obtaining permeability reduction
For nanocopmosite, the procedure was same except step 4, which oil contains NCs 30 ppm at 1700 Psi and 40 ℃. Figure 5 shows the experimental approach which was used in this study: Figure 5. Schematic of experimental procedures. Figure 6 shows X-ray analysis for detecting the structure of NCs and Zeolite nanoparticles.
τ, k, λ, β, and θ are crystalline size (nm), shape factor (0.9), x-rays wavelength (0.154 nm), line broadening, and Bragg angle. NCs calculated average size was 30.21 nm.  The surface area, pore size, and pore volume of the pure zeolite and zeolite-zirconia-copper nanocomposites (NCs) were measured by BET. The results are summarized in Table 3. The specific surface areas of the NCs (327.82 m 2 /g) is less than the pure zeolite (369.48 m 2 /g) as a result of introducing of ZrO2 and CuO. An increase in the ZrO2 and CuO contents decreases the specific surface area due to increasing the crystal size and the pore blockage of the support. As compared to NCs, pure zeolite has a higher specific surface area and higher pore volume, but lower average pore diameter.  Figure 8 and Table 4 show EDX results for zeolite nanoparticles and NCs.

Asphaltene adsorption isotherms in the presence of NCs and zeolite nanoparticles
Langmuir and Freundlich isotherm models which were used in this study are expressed in  Figure 9 shows asphaltene adsorption on the NCs nanocomposites and zeolite's surface in the batch experiment tests up to 1000 ppm. As it is clear from the graph, there is two different slope below 100 ppm and after that. At first slope (below 100 ppm), the slope was dramatically increased. Then after, asphaltene adsorption on the NCs nanocomposites and zeolite's surface was reached to around 78.5 mg/g, and 33 mg/g, respectively. Accordingly NCs was adsorbed more asphaltene on its surface in compared to zeolite nanoparticles. It was concluded from previous research that the type of sorbent is affected on the amount and type of adsorption [53][54][55]. According to the isotherm model results in this study, the adsorption data was adapted well with Langmuir isotherm than Freundlich isotherm, and this shows that adsorption surface was monolayer and homogeneous. Freundlich isotherm occurs in the surface that has different energy with heterogeneous surface [55], and from the comparison between Freundlich isotherm between zeolite and NCs, it was observed that the data less adopted with NCs in comparison zeolite.
Langmuir and Freundlich isotherm linear plots were shown in Figures 10 and 11, respectively.  Isotherm parameters of both isotherm models were shown in   Table 5. Langmuir and Freundlich isotherm model constants.

Effect of nanoparticles concentration and pressure on CO2-crude oil interfacial tension
The effects of NCs and zeolite nanoparticles on the CO2-Oil IFT was shown in Figures 12.
According to the results, there are two different ranges from 200-2600 Psi. Interfacial tension of CO2-Oil was decreased due to dissolving CO2 in the crude oil. The slope in this graph was changed due to forming the aggregate of asphaltene particles. According to the Figure 12, NCs was changed the slope in the second region more than Na-ZSM-5 zeolite, and in other words adsorbed higher amounts of asphaltene on its surface. As it was mentioned in the previous section, NCs had the better asphaltene adoption in comparison to zeolite nanoparticles which confirmed with this results. Moreover, one of the other important results from the results was delaying agglomeration in the presence of NCs and zeolite nanoparticles, and it was confirmed that NCs had better results in this aspect as well that zeolite. Previously Kazemzadeh et al was seen same results in the presence of Fe3O4 nanoparticles [56]. Seven pressures from the second region (1700-2600 Psi) were selected for obtaining the relation between these high adsopber points with adsorption in the real crude oil in natural depletion tests.
As it was mentioned, there are two different slope of CO2-Oil in the base and both nanoparticles of NCs and zeolite, and Table 6 summarized different equations in these two region. 2 nd to 1 st slope ratio was 19.697 % at base case, and 2 nd to 1 st ratio increased to 20.895 % and 29.851 % in the presence of NCs and zeolite at 30 ppm, respectively. Thus, NCs had better performance with regards adsorbing asphaltene on its surface in comparison zeolite. Figure 12. Effects of pressure and nanoparticles (NCs and zeolite) on CO2 -oil IFT.

Natural depletion tests in the presence of nanoparticles
As it was mentioned in the previous section, seven pressure of 1700, 1850, 2000, 2150, 2300, 2450, and 2600 Psi have selected as high adsorption points for static phase and performing natural depletion tests in the presence of NCs and zeolite nanoparticles. Asphaltene precipitation content versus pressures at static pressure were shown in Figure 13, and effects of both NCs and zeolite nanoparticles were observed according to this graph. It was seen asphaltene precipitation of NCs and zeolite nanoparticles decreased from (5.10 wt%, 14.34 wt %) to (3.78 wt%, 8.40 wt %) and (2.8 wt%, 6.25 wt %), respectively during pressure reduction from 2600 Psi to 1700 Psi. Accordingly, based on the results, NCs had better results for decreasing asphaltene precipitation in the static phase in comparison to zeolite, and it can be in a direct relation with higher adsorption potential in the previous steps. Figure 13. Effects of NCs and zeolite nanoparticles on asphaltene precipitation during natural depletion tests. Asphaltene precipitation ( wt%)

Dynamic CO2 tests in the presence of NCs
According to the static phase results, NCs had better results than zeolite in aspect of asphaltene adsorption and asphaltene precipitation reduction. Thus it was selected for surveying permeability/porosity reduction parameters. As it is clear from the Table 7, NCs had better results in lower pressure for asphaltene precipitation at 1700 Psi, 1700 Psi, 40 ℃, and 30 ppm were selected for performing CO2 flooding at dynamic phase. One of the important factor in porous media is considering deposition condition. As it was shown in the static phase, NCs had better performance in comparison to zeolite. Accordingly it was selected for surveying its effect on the deposition rate in porous media. Dynamic displacement tests were used for CO2 flooding in the presence of NCs. As it is clear from the Figure 14, asphaltene deposition in the presence of gas was increased during CO2 flooding.
By measuring the asphaltene content of injected and produced oil samples, one can estimate the amount of deposition in porous media. The asphaltene of produced oil was measured by IP143 standard technique. After adding NCs in crude oil, asphaltene precipitation was decreased. As CO2 gas volume was increased from 1 to 6 pore volume, asphaltene precipitation (Wt %) was decreased from (15.12, 18.12) to (11.85, 14.44), respectively.
. flooding 2 on the asphaltene deposition during CO s Effect of NC . 14 Figure As it was mentioned before, after saturation of carbonate core sample with brine, recombined oil and CO2 [25 Mole % which was more than initial onset value (20 Mole %) at static phase] displaced through the low permeability carbonate core and initial permeability reduction was recorded. Then, same procedure was performed in the presence of NCs [dispersed in oil] at 30 ppm, 1700 Psi, and 40 ℃ as Figure 15 and 16. Many other cases and laboratory studies have been reported on precipitation and deposition of asphaltenes in porous media during immiscible or miscible gas flooding operations [57][58][59][60][61][62][63]. Figure 15 shows permeability There is direct relation between porosity reduction and asphaltene deposition rate. Figure 18 shows the porosity variations of low permeability carbonate reservoir versus the amounts of pore volume. As it was observed in the previous steps, NCs was decreased asphaltene precipitation in static phase at 30 ppm, and at same concentration asphaltene deposition was decreased in carbonate reservoir. Moreover, higher porosity reduction was seen at higher asphaltene deposition points. Porosity reduction (-)

Pore volume injection (cc)
Base NCs

Conclusions
Zeolite-zirconia-copper oxide nanocomposites (NCs) has been synthesized successfully with average size of 30 nm and it was used for asphaltene adsorption and solving asphaltene precipitation problems and the results compared with zeolite nanoparticles. Results show that NCs nanoparticles adsorbed higher amount of asphaltene and asphaltene precipitation decreased more in the presence of NCs nanoparticles than zeolite nanoparticles which reveals this point that this nanocomosites can be used efficiently as an asphaltene inhibitor and the agglomeration process was delayed efficiently. Adsorption data fitted well with the Langmuir model in comparison to the Freundlich model, which shows that the adsorption occurs in a homogeneous surface with monolayer coverage in the presence of both nanoparticles. Based on the BET results, NCs has lower surface area, higher pore volume but higher diameter in comparison to zeolite nanoparticles. EDX analysis confirmed that NCs synthesized successfully. There is two different slope in CO2-oil IFT readings as pressure increased (200 Psi-2600 Psi), and second slope is slower than the first one which is due to aggregation of asphaltene. Seven pressures of 1700, 1850, 2000, 2150, 2300, 2450, 2600 Psi and NCs and zeolite nanoparticles at concentration of 30 ppm were selected for performing natural depletion tests and it was concluded that as pressure decreased from (2600 Psi to 1700 Psi), asphaltene precipitation in the presence of NCs and zeolite decreased from (Base: 5.10 wt%, 14.24 wt%) to (2.80 wt%, 6.25 wt%) and (3.78 wt%, 8.40 wt%), respectively. It was observed that the NCs has higher efficiency with regards adsorbing asphaltene on its surface and decreasing asphaltene precipitation during natural depletion in comparison to zeolite nanoparticles, and and it was selected for performing CO2 dynamic tests. In dynamic phase. It was observed that NCs had high potential material for improving permeability impairment, porosity reduction, and asphaltene deposition rate during CO2 flooding tests.