Introductions

In recent years nanoparticles (NPs) have found great applications in the oil, gas and other industries1,2. Since the last decade, numerous studies are trying to recover more oil by innovating newly methods using the nanotechnology in this industry3. In order to implement the application of nanotechnology in the Enhanced Oil Recovery (EOR) field of the oil upstream sector, different types of NPs such as metal oxide, magnetic, organic, inorganic, and non-silica NPs were investigated4. Ahmadi et al.5,6 synthesized ZnO/SiO2/xanthan nanocomposites to adsorb asphaltene and reduce its effect on shale and carbonate rocks. As the researches show, various mechanisms are involved in EOR application of these NPs. These mechanisms are interfacial tension (IFT) reduction that was explained in the works of Ju et al.7, Hendraningrat et al.8 and Zaid et al.9, wettability alteration in the studies of Saien and Gorji10 and Al-Anssari et al.11, heavy oil swelling in the work of Kazemzadeh et al.12, asphaltene stabilization that was explained by El-Diasty et al.13, oil viscosity reduction in the works of Mohammadi et al.14, Li et al.15 and Taborda et al.16, injection fluid viscosity increase is another mechanism that was studied by Ehtesabi et al.16,17,18, nano-emulsion creation was investigated by Bobbo et al.19, Angannaei et al.20 and Hashemi et al.21 studied pore channel plugging, and disjoining pressure was described by Mcelfresh et al.22 and Aveyard et al.23. A study by Mansouri and Ahmadi24 showed that in the dynamic phase after the addition of nanocomposites, asphaltene precipitation that occurs after carbon dioxide injection is reduced and permeability/porosity reduction parameters are improved. Ahmadi et al.25 showed that in the presence of calcium oxide during WAG experiments, the recovery coefficient with the help of nanocomposites increased from 42.9 to 73% and the residual oil saturation decreased from 40.9 to 19.4%.

Among the metal oxide NPs, Al2O3 is useful for viscosity reduction26,27, CuO and Ni2O3 are effective in oil viscosity reduction and increased the viscosity of injected fluid26,28,29, MgO improve the fine migration without any effects on oil recovery26,30, and ZnO, TiO2, CuO, Fe3O4 and Polyacrylamide hybrid very effective in wettability alteration and IFT reduction17,31,32,33,34,35. In the inorganic NPs, SiO2 and illite is very effective in wettability alteration and IFT reduction31,36,37. Maghzi et al.38 investigated the effects of SiO2 NPs in a micromodel resulted in 8.7% increase in the oil recovery factor by addition of 0.1 wt% SiO2 NPs. Fumed SiO2 only altered the wettability39, saline SiO2 is effective in increasing oil recovery26,40, HLP is very effective in modifying IFT and wettability41,42, and LHP increased oil recovery by wettability modification41,43. The effect of a polymeric nanofluid in oil-wet and water-wet fractured micromodels was studied by Zhang et al.44. The effect of wettability on two-phase flow and the resulting residual oil saturation was investigated by Rostami et al.45 with micromodels. Hendraningrat et al.8 and Ogolo et al.26 concentrated on finding the best NP and base fluid to reach the ultimate oil recovery factor.

Soleimani and Dehaghani46, showed that use of NPs in high concentration (Conc) is highly effective and 20-time low salinity are accountable in low concentration, high temperatures however, combining these two solutions increases their effectiveness and even at ambient temperatures and low concentrations of silica NPs, acceptable production is observed. Dehkaee et al.47 investigated the effect of nanofluids based on NiO, SiO2 and NiO/SiO2 NPs in different concentrations, on IFT for improving oil recovery. It was demonstrated that 30 wt% NiO/SiO2 nanocomposite had the lowest IFT. In 2022, the effects of modified graphene oxide on wettability and IFT was investigated by Jafarbeigi et al., and a comparative study was performed using IFT and contact angle (CA) measurements. Based on the results of this study, the CA decreased from the initial value of 161° (oil wet) to 35° (water wet). Additionally, IFT decreased from 18.45 dyne/cm to 8.8 dyne/cm48.

The abovementioned NPs have been applied in many researches, however the best NP has not been investigated yet. One of the primary obstacles in field implementation of NPs is finding cost-effective NPs with strong EOR characteristics. Finding low-cost solutions for large-scale NP generation for field applications will be an issue. Furthermore, some NPs are not environmentally friendly. Considering all aspects of the NPs application in EOR processes, one of the most applicable and economic NPs is nanoclay that is available in the nature and environmentally friendly. Clay minerals are hydrous aluminosilicates with a sheet-like structure that absorb water on their surface. Although most of the nanoclay applications are in drilling sector of oil industry, its capabilities to be used as the water based EOR method assistant was not studied comprehensively by the researchers. Cheraghian et al.49 investigated the polymer absorption on reservoir rock and the role of silica and clay NPs in small concentrations on this phenomenon. Based on the results, Polymer suspensions containing nanosilica had lower absorption than samples containing nanoclay49. Cheraghian et al.50 investigated the effect of nanoclay in heavy oil recovery during polymer injection. Based on their investigations, the viscosity of injected liquid is optimized with high Conc of polymer and also to displace heavy oil by polymer solution. Nanoclay Conc in polymer solution should not exceed the threshold value (0.9 wt%). The increase in oil recovery by co-injection of polymer (polyacrylamide) and nanoclay was about 5% more than the injection of polymer solution alone and the best sample for heavy oil recovery is the polymer suspension containing 0.9 wt% of nanoclay50. Cheraghian51 investigated nanoclay effects in surfactant polymer solution for heavy oil recovery experimentally, using glass model. According to the experimental results, polyacrylamide has a good dispersion and the particle size distribution was uniform when nanoclay is added. Moreover, with co-injection of surfactant and polymer, heavy oil recovery is increased and when nanoclay is added, the recovery is increased by 6.6%51. Cheraghian and Hendraningrat52 evaluated nanoclay and silica foam effect on the absorption of polymeric surfactant during oil recovery. The results demonstrated that increase in Conc of nanoclay leads to the decrease in absorption of solution on sandstone surface. All polymers containing nanoclay have lower absorption than the polymeric surfactant significantly. The final oil recovery by nanoclay containing polymeric surfactant flooding increases compared to the flood of polymer surfactant by 10.8%52. Blends of linear low-density polyethylene with 1, 3, and 5 wt% organoclay melt-inhibiting properties with improved barrier properties can be ideal alternatives to flexible thermoplastic composites as packaging materials53.

Rahimi et al.54 by adding 0.1 wt% nanoclay to a 0.12 wt% surfactant solution, increased the foam decay time from 30 to 76 min. However, despite the nanoclay’s ability to increase foam stability and enhanced oil recovery than empty nanoparticle foam, it cannot be a substitute for frequent silica nanoparticles54. Zheng et al.55 studied the process of nanoclay-surfactant-stabilized foam in offshore heavy oil reservoirs to enhance the oil recovery of steam flooding. According to the investigations, the foam stability and thermal resistance experiments proved that with nanoclay addition in an alpha-olefin surfactant system, the foam stability can improve significantly55. Ahmadi et al.56 at the lowest level in hot water, the recovery coefficient has the best conditions for obtaining the highest recovery. Rezaei et al.57 proposed integration of surfactant, alkali and nano-fluid flooding as an EOR method. Four anionic surfactants were selected and alkalis and NPs (SiO2, ZnO and nano-clay) were combined with them. All of the additives (in concentrations less than 5 wt%) leaded to further reduce the IFT and CA of the samples in comparison to the pure surfactants. Lu et al.58 investigated the effects of surfactants on stability of nanoclay emulsions. Surfactants adsorption onto the nanoclay particle surface prior to the formation of emulsion was measured via Zeta potential in water. As oil in water emulsions were produced by nanoclay, its stability was improved by addition of surfactants58.

Most of the published articles about nanoclay application in EOR process focus on the rheological flow behavior modification that take place when nanoclay used as an EOR method assistant and the other mechanism such as wettability alteration, IFT reduction, disjoining pressure and pore plugging are not considered thoroughly. In order to use nanoclay as a water-based EOR assistant, there are two main issues that should be considered comprehensively. The first issue is nanoclay stability that is not comprehensively investigated in the recent publications. Soleimani and Sadeghi59 published an article in this category. The main results of that publication will be used in this study. On the other hand, the literature review showed nearly nothing about the capabilities of nanoclay as an EOR agent to modify IFT and alter wettability mechanisms. This issue is considered thoroughly in this study and turns the light for further investigations.

The nanoclay stability results done by Soleimani and Sadeghi in corporation with IFT and CA are analyzed in different situations to select the optimum EOR fluid of nanoclay assisted water-based EOR method21.

In the following sections, nanoclay preparation methods and the other used materials are briefly described in the Material Preparation section. In the Experimental Methodology part, the method of nanofluid preparation, nanoclay stability, nanoclay morphology, clay swelling, IFT, CA, static and dynamic flowing characteristics measurement is explained. Moreover, the results of the above-mentioned tests are discussed in the Results and Discussion section.

Materials preparation

In this research, montmorillonite is used as nanoclay to investigate its effect on IFT and CA modification in different fluid solutions and rock-fluid interaction as an EOR assisted fluid with the mechanical and physical properties mentioned in Table 159. The X-ray fluorescence (XRF) spectrometry analysis of the nanoclay is exhibited in Fig. 1.

Table 1 Mechanical and physical properties of nanoclay.
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Fig. 1
Fig. 1
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Weight percentage of elements of nanoclay59.

Cetyl trimethyl ammonium bromide (CTAB) is used as the surfactant this study. The main goals of using CTAB is to stabilize the nanoclay and also as an EOR fluid with the characteristics and molecular structure in the research59.

Experimental methodology

The experimental methodology of the study is shown in Fig. 2. The first four steps that focus on the stability in flow diagram Fig. 2 were performed in the study59. In this research, three main EOR fluids were investigated. One is nanoclay in distilled water suspension (called nanoclay), the other is nanoclay in CTAB suspension (called nanofluid) and the last one is CTAB alone. Nano suspensions were prepared using a two-step method59. In order to measure the stability, imaging with camera, Dynamic Light Scattering (DLS) and zeta potential were considered. The results of the stability tests are demonstrated in Table 2. The best stability was for the sample with 0.1% wt nanoclay in presence of 0.5% wt CTAB.

Fig. 2
Fig. 2
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Diesel and nanoclay drop CA, (A) Immediately (132°), (B) After 2 h (132°).

Table 2 Results of nano-fluids stability after 24 h from preparation59.
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In order to evaluate the desired EOR fluid, IFT, CA, static imbibition and core flood tests are designed and explained in more details in the following sections.

Interfacial tension measurement

In a multi-phase system, there is force unbalance in contact area of the phases that is called surface tension (liquid-gas phase) or IFT (liquid-liquid phase). In this study, a model of KSV SIGMA setup is used to measures the IFT with standard ring method.

Contact angle measurement

One of the important mechanisms in water-based EOR method is wettability alteration in case the rock is oil wet in nature as in this study. In order to accurately detect this mechanism, CA measurement is recommended that is measured with a setup using pendant drop method.

Static imbibition

After preparing the desired fluids (nanoclay, CTAB and nanofluid) and selecting the plugs with nearly the same specific characteristics in Table 3, In order to measure the efficiency of the selected EOR fluid, its displacement capabilities were measured in imbibition test using Amott cell. All of the core plugs were aged to be surely oil-wet, therefore to investigate the efficiency of oil recovery from the cores, the selected EOR fluid should alter the wetting nature of the sample through wettability alteration mechanisms.

Table 3 Core plug characteristics.
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Core flood test

The final test in this study is the core flooding. Two different tests were performed to understand the effects of the nanoclay and nanofluid (EOR fluids) in oil recovery from the cores with the specification in Table 4. All of the core plugs were aged to be surely oil-wet.

Table 4 Core plug characteristics.
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Results and discussions

Interfacial tension measurement results

The results of IFT measurements in different fluid environment (oil in presence of distilled water, nanoclay and CTAB) are described in Table 5. CTAB has the greatest effect on IFT reduction. Nanoclay also has positive effect on IFT reduction, since it reduces the water-oil IFT from 24.99 to 20.01 dyne/cm.

Table 5 IFT measurement (at the moment of preparation) (mean ± SD N = 3).
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The results of IFT measurement with respect to time, for nanoclay-CTAB are described in Table 6. The largest value is for 48 h, and the lowest is for the time of preparation (0 h).

Table 6 IFT Measurement of oil in presence of nanoclay-CTAB suspension (at the moment of preparation until 48 h) (mean ± SD N = 3).
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Table 6, the IFT value for nanoclay-CTAB suspension and oil at initial time was 0.157 dyne/cm. It should be noted that this value is lower than the CTAB-oil IFT value and this shows the influence of nanoclay in IFT reduction. Moreover, the IFT became larger and this is due to the CTAB rule as nanoclay stabilizer, sine the Critical Micelle Concentration (CMC) value for CTAB is going to be lower as time passes.

Contact angle measurement results

The results of CA measurement when diesel is the bulk fluid and the drop of nanoclay is placed on the dolomite thin section are described in Fig. 4. There are two main measurements, one is immediately after preparation and the other one after two hours of time passing. It is obvious from Fig. 2 that the CA remains constant at the value of 132°. It should be noted that the thin sections (rock surface) were aged for nearly 15 days in contact with oil so it is completely oil wet. This indicates that nanoclay has no effect on wettability alteration.

The results of CA measurement when diesel is the bulk fluid and the drop of nanofluid is placed on the dolomite thin section are depicted in Fig. 3. The CA changed from 135° to 125°. This modification of the CA with time passing is an indication of wettability alteration.

Fig. 3
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Diesel and nanofluid drop CA (A) Immediately (135°), (B) After 2 h (125°).

The results of CA measurement when diesel is the bulk fluid and the drop of CTAB is placed on the dolomite thin section are described in Fig. 4.

Fig. 4
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Diesel and CTAB-drop CA (a) Immediately (72°), (b) After 2 h (69°).

Static imbibition test (Amott cell)

A plug with one of the EOR fluids (nanoclay, nanofluid, CTAB) was placed in each of the Amott cells. As time passed, the EOR fluid enters the plug and consequently the oil is accumulated at the top of the cell. The amount of the depleted oil from the plug is recorded at different times for each EOR fluid.

Amott test-nanoclay results

The plug 4-223H was surrounded by nanoclay as EOR fluid. The amount of produced oil was recorded every 24 h for 384 h (16 days). Based on the observations after 96 h, the amount of produced oil was almost fixed. The results of this test are in the Fig. 5 and Table 7. The recovery curve of this process is depicted in Fig. 6.

Fig. 5
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The amount of oil produced in the Amott cell in presence nanoclay (EOR fluid) over time.

Table 7 Oil accumulation during time (4-223H plug and nanoclay).
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Fig. 6
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Recovery curve for the 4-233H plug and nanoclay as EOR fluid.

Amott test-CTAB results

The plug 4-237H was surrounded by nanofluid as EOR fluid. The amount of produced oil was recorded every 24 h for 384 h (16 days). Based on the observations after 96 h, the amount of produced oil was almost fixed. The results of this test is in the Fig. 7 and Table 8. For further investigations, the recovery curve is depicted in Fig. 8.

Fig. 7
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The amount of oil produced in the Amott cell in presence CTAB (EOR fluid) over time.

Table 8 Oil accumulation during time (4-241H plug and CTAB).
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Fig. 8
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Recovery curve for the 4-241H plug and CTAB as EOR fluid.

Amott test-nanofluid results

The plug 4-237H was surrounded by CTAB as EOR fluid. The amount of produced oil was recorded every 24 h for 384 h (16 days). Based on the observations after almost 120 h, the amount of produced oil was almost fixed. The results of this test is in the Fig. 9 and Table 9. For further investigations, the recovery curve is depicted in Fig. 10.

Fig. 9
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The amount of oil produced in the Amott cell in presence nanofluid (EOR fluid) over time.

Table 9 Oil accumulation during time (4-237H plug and nanofluid).
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Fig. 10
Fig. 10
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Recovery curve for the 4-237H plug and nanofluid as EOR fluid.

Active mechanisms in Amott test

Static flow test (Amott Cell test) has been investigated to understand the effect of the EOR fluids on the active mechanisms involved in it. This test is in static mode and there is not any viscous forces (external pressure difference is not applied on the system), so possible active mechanisms are wettability alteration and IFT reduction.

The time considered for the tests in all three cases, until the ultimate oil recovery of the oil, was sixteen days. The amount of ultimate oil production for CTAB, nanofluid and nanoclay, was 2.3 mL, 2.1 mL and 1.8 mL respectively. According to the IFT and wettability measurement tests that were performed separately for each of three cases, both mechanisms of IFT reduction and wettability alteration are active with different quantities. From the results of the experiments, it can be seen that the effectiveness of the IFT reduction mechanism is higher for nanoclay compared to the wettability alteration mechanism. The opposite is true for CTAB (effect of wettability alteration mechanism is greater than the IFT reduction). According to the oil-wet nature of the selected core, wettability alteration mechanism will have a greater impact on the oil production. It is worth mentioning that in the case of nanoclay, the effect of IFT reduction through the gravity drainage is more evident than the wettability alteration due to the absorption. According to the efficiency that was observed for the nanofluid case, it can be concluded that the active mechanisms in this case are actually an optimal combination of both wettability alteration and IFT reduction mechanisms.

Core flooding test

Two different core plugs (4-81V, 4-79V) and two different EOR fluids were selected to perform flooding tests.

4-81V plug-nanoclay flooding results

Initially, the plug was saturated with oil and brine (22.13% brine saturation, 15.973 mL pore volume). Then, nanoclay was injected at a rate of 39 mL/h into the plug and finally, the flow rate of water and produced oil, as well as the volume of produced oil and the percentage of oil recovery were recorded, the results of which can be seen in Fig. 11.

Fig. 11
Fig. 11
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Recovery curve for the 4-81V plug and nanoclay as EOR fluid.

4-79V plug-nanofluid flooding results

Initially, the plug was saturated with oil and brine (22.86% brine saturation, 16.933 mL pore volume). Then, the selected nanofluid was injected at a rate of 27 mL/hr into the plug and finally, the flow rate of water and produced oil, as well as the volume of produced oil and the percentage of oil recovery were recorded, the results of which can be seen in Fig. 12.

Fig. 12
Fig. 12
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Recovery curve for the 4-79V plug and nanofluid as EOR fluid.

Active mechanisms in flooding test

In the flooding test, in addition to the aforementioned mechanisms in the Amott cell, viscous forces will also be involved due to the applied pressure difference. The time considered for the flooding tests until the ultimate oil recovery, was 200 min (approximately 3.5 h) for nanofluid and 140 min (approximately 2.5 h) for the nanoclay.

The ultimate oil production for nanofluid was approximately 79% and for nanoclay 54.5% of the oil in place. The mechanisms investigated in these tests are IFT reduction, wettability alteration, and disjoining pressure and pore blocking. It should be noted that the mentioned mechanisms may occur simultaneously. The absorption of particles on the surface of the rock to change the wettability occurs simultaneously with the effect of the wedge film. According to the fact that in the both cases, high volume of oil produced in the early times, it can be concluded that the effects of the mentioned mechanisms occurred at a high speed.

In the designed flooding tests, wettability alteration mechanism is active, because the results of the CA measurement show the ability of both cases to change the CA, with the difference that in the case of the nanofluid, this mechanism will work stronger.

The NPs in the suspension form a wedge-shaped film in contact with the discontinuous phase. This film acts as a wedge to separate formation fluids from the rock surface and consequently produces more oil in comparison with the normal additives or fluids. The forces responsible for disjoining pressure mechanism are Brownian motion and electrostatic repulsion between NPs. When the size of the NPs is smaller and the amount of NPs increases, the electrostatic repulsion force between those particles will be larger, and as a result, the effect of the disjoining pressure will be greater. In the case of the flooding tests, disjoining pressure occurs at the interface of oil, rock surface and nanofluid. The more the interface is, the stronger this mechanism will act. Obviously, in the case where nanoclay is used, compared to the nanofluid case, more space is available for the NPs to establish contact in the three-phase region, and this will lead to the formation of a wedge film that acts with greater strength, therefore, this mechanism will work stronger in the case of nanoclay.

In very small pores, the density difference between particles and water slows down the movement of particles and causes them to accumulate. As a result, the pressure in the nearby pores increases and the oil is pushed out. When the oil is released, the surrounding pressure decreases and the blocking materials gradually dissolve and the particles begin to move with the water flow. This effect can be considered as temporary log jamming caused by pore blockage. Regarding the conducted tests, it should be said that according to the size of the selected nanoclay (montmorillonite) and the structure of the core, the pore blocking mechanism has occurred. Evidence for this claim is the increase in core pressure at the beginning of the injection process. This temporary increase in pressure is due to the effect of nanoclay on the mechanism of the pore blocking, which is faced with an increase in pressure and after the oil is discharged from the larger holes, the process of pressure drop begins. It should be noted that due to the appropriate size of NPs, this mechanism occurs at a significant speed after smaller pores and bottlenecks are blocked by nanoclay. Regarding the experiment results, this mechanism is occurred in the case of the nanofluid and CTAB stronger than the nanoclay case.

Conclusions

The main use of nanoclay in the oil industry is in the field of drilling fluids, and cases of nanoclay application in the polymer injection process have been observed in resources, but the design of comprehensive and inclusive laboratory tests in a way that examines the stabilization stage of nanoclay to its operational application in the processes of Amott and flooding in the form of laboratory studies has not been observed in previous research.

The presentation of a new fluid (combined nanofluid) and cost-effective enhanced oil recovery is the main output of the tests designed in this research. Given the high cost of surfactant on the one hand and the limited use of nanoclay alone (test results), the best case is to use a simultaneous combination of these two fluids. The use of the most challenging nanoclay available (montmorillonite) is considered a kind of innovation due to its widespread existence in tests and the investigation of its effect on enhanced oil recovery.