The article investigates the synergistic effects of SiO2 nanoparticles and welan gum in enhancing oil recovery, focusing on their surface interactions and rheological properties. The study highlights the potential of natural polymers in improving the efficiency of enhanced oil recovery (EOR) techniques, particularly in high-salinity and high-temperature environments. Experimental results demonstrate that the addition of SiO2 nanoparticles significantly enhances the viscosity and stability of welan gum solutions, even under challenging reservoir conditions. The research provides a detailed analysis of the adsorption behavior of the SiO2/welan gum mixture on limestone surfaces, revealing the formation of a compact, cross-linked network that improves oil displacement efficiency. The study also explores the impact of salinity and temperature on the rheological properties of the polymer-nanoparticle solution, offering insights into the mechanisms that drive the enhanced oil recovery process. Core flooding experiments further validate the effectiveness of the SiO2/welan gum mixture, showing a notable increase in oil recovery compared to conventional methods. The findings suggest that the use of natural polymers, such as welan gum, in combination with nanoparticles, represents a promising avenue for developing environmentally friendly and efficient EOR techniques.
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Abstract
This study explored the synergistic interaction of SiO2 NPs and Welan gum for offshore application. Interaction characteristics, rheological behavior, Fluid-rock interaction and oil displacement were evaluated. FTIR, SEM and H-NMR were employed to characterize the SiO2-Welan gum solution. The results showed that SiO2 interacts with Welan gum through chemical bonds and possible hydrogen bonds due to hydroxyl group formation, increasing polymer roughness and causing network disruption. The incorporation of SiO2 NPs into welan gum resulted in a viscosity enhancement of up to 50% at 1.5 wt% salinity, demonstrating significant stability improvement. SiO2 NPs contribute to the thermal resistance of the polymer at elevated temperatures. Their synergistic effect was to mitigate salinity-induced viscosity reduction, maintaining fluid consistency more effectively than in the absence of nanoparticles. With SiO2, the adsorption between polymer and limestone decreased over time, showing an average reduction of 19%. The adsorption was homogeneous and was best described by the Langmuir isotherm model. The oil recovery improved by 15% more efficiently than the welan gum alone. The study suggests that SiO2 interacts with Welan gum in the ionic solution creating a three-dimensional network that resists conformational collapse and controls the polymer diffusion in limestone porous media.
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Abbreviations
q
Adsorbed amount (g/kg)
qmax
Maximum adsorbed amount (g/kg)
KL
Langmuir equilibrium constant (L/g)
Ce
Equilibrium aqueous concentration (g/L)
Kf
Freundlich constant related to adsorption capacity
nf
Freundlich intensity constant
nh
Halsey exponent
Kh
Halsey constant
PV
Pore Volume
η
Apparent viscosity (cP)
γ
Shear rate (s⁻1)
k
Flow consistency index
n
Flow behavior index
Introduction
The challenge with EOR is to identify chemicals that meet technical, economic, and environmental needs. EOR mechanisms are centralized around two main objectives: reducing the mobility ratio or increasing the capillary number. However, this is usually not achieved effectively. Several studies have proposed nanoparticles (NPs) to improve conventional EOR mechanisms. By virtue of their properties, NPs can impact oil recovery by either altering the wettability of rock surfaces [47, 48], reducing the viscosity of even heavy oils [68] or improving the tolerance of conventional polymers and surfactants [17, 73]. NPs can withstand high salinity, temperature [66], and pressure environments (H-STPs) in reservoirs without degradation. Therefore, nanomaterials have been widely used in several EOR techniques [8, 53].
Polymer flooding is of centralized importance because it is the largest chemical flooding practice in oil fields [26, 28, 35]. The ongoing research advancements in oil recovery suggest the application of NPs in polymer flooding for their ability to alter several properties of polymers. When NPs are introduced into polymer solutions, they employ various interaction mechanisms that typically involve multiple binding mechanisms, including electrostatic repulsion, electrostatic and van der Waals forces, steric repulsion, and hydrogen and hydrophobic bonding [72]. The specific interaction mechanism influences the development and behavior of the resulting nanofluid polymer. For instance, NPs in suspension with polymers effectively mitigate the adverse effects of salinity on the polymer [16]. However, the salinity tolerance benefit may not be observed when NPs are integrated without forming a suspension. Additionally, NPs can create films that alter oil-wet reservoirs to more water-wet conditions, thereby enhancing the oil recovery. Moreover, hybrid interactions between NPs and polymers can improve the resistance of polymers to high temperatures [23]. This leads to improved rheological properties under high shear stresses, stabilizing viscosity, and improving performance in porous media. Therefore, it is evident that the interaction between polymers and NPs cannot be generalized into a single category or mechanism. Identifying the optimal polymer and NPs combinations for specific reservoir conditions remains crucial [10, 20, 37, 64, 70, 77, 86]. However, existing literature has predominantly focused on synthetic polymer improvement [73]. This is because these synthetic polymers offer initial performance advantages, they suffer from degradation at high temperatures and salinity, significantly limiting their efficiency in many oil reservoirs [31, 32, 58]. On the other hand, natural polymers offer an environmentally friendly, biodegradable alternative, yet their application in polymer-nanoparticle flooding has received limited attention.
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Some studies on natural polymer interaction with NPs reported significant promising results. For instance, adding nickel NPs to the xanthan gum solution led to the highest oil recovery (5.98%), compared to 4.48% flooding caused by xanthan gum alone [70]. In another study on xanthan gum but with various NPs (SiO2, Al2O3, TiO2), all polymer-nanoparticle mixtures demonstrated enhanced rheological behavior and lower oil/brine interfacial tension [37].). In water, incremental oil recovery was achieved by solo NPs: 28.4% (Al2O3-XG), 27.6% (TiO2-XG), and 25.2% (SiO2-XG) [37]. Nevertheless, the most extensively studied nanoparticles are SiO2 due to their remarkable properties when introduced to chemicals. Numerous studies have supported the application of SiO2, as it enhances the temperature, shear resistance, viscoelasticity, and oil recovery of polymers [20, 59, 86]. The promising features of SiO2 also stem from its ease of synthesis and the ability to manipulate its properties by modifying or functionalising its surface for the desired application, such as oil recovery. Furthermore, the large surface area and surface charges facilitate various interactions in terms of fluid–fluid and rock-fluid interfaces [27]. SiO2 NPs provide a foundation for more favourable conditions for oil recovery [15, 25]. The majority of studies have reported the reduction of interfacial tension (IFT) and wettability alteration [12, 87]; however, the direct surface interaction between SiO2 and the rock components is not commonly addressed. Moreover, when it comes to salinity, SiO2 is reported to offer benefits. Studies show that using SiO2 nanoparticles in concentrations of 0.5 to 2 wt% with PAM in high salinity provides better oil recovery results. However, there is limited information about how SiO2 interacts with the polymer but strong evidence on wettability effect [76].
Natural polymers extend beyond xanthan gum, with newly developed alternatives such as okra, cassava, jute, welan, guar, and arabic gum [11]. Despite these options, xanthan gum remains the dominant subject of improvement studies. A broader evaluation of natural polymers, particularly their thermal stability, viscosity retention, and performance when combined with nanoparticles is needed. For instance, welan gum, a bacterial biopolymer, exhibits effectiveness comparable to xanthan gum [92]. However, there is limited information about its applicability. Xu et al. [85] performed a comparative study that showed welan gum has higher displacement and sweep efficiency than xanthan gum. Wang et al. [80] studied the fluid of SiO2 and welan gum as a thickener for drilling fluid. The improved drilling fluid was excellently resistant to rheological sedimentation, mechanical shear and high temperature up to 220℃. The modified welan gum with poly(2-oxazoline) improved the oil recovery by 9.7% compared to pure welan in saline conditons [43]. In Kazakhstan, the study conducted by Serikov et al. [74] examined welan gum using rheology and core flooding tests. The core flooding results showed an increasing oil recovery factor of 20%, whereas rheology tests characterize welan gum only as temperature resistant. In an environment of high salinity and drastic viscosity drop, welan gum loses its rheological properties. The research team in Kazakhstan investigated Beta vulgaris and low concentration of Welan gum for offshore applications proven that the polymer blend exhibited improved resilience to thermal and saline conditions, achieving a viscosity range between 10 and 30 cP, suitable for offshore EOR operations [75].
The current study hypothesizes that NPs can improve natural polymer tolerance to salinity. The study specifically considers adding SiO2 nanoparticles to enhance the structural and rheological properties of welan gum under high salinity conditions (seawater). Thus, a series of experiments were conducted to evaluate the potential of SiO2 NPs to enhance welan polymer. The investigation provides chemical and physical insights into the interaction mechanisms between the nanoparticle and natural polysaccharide, followed by an analysis of rheological characteristics under conditions of high salinity, temperature, and their combination. The research offers an analysis of SiO2/Welan behavior up to the interaction on limestone surfaces, quantifying the dynamic and static adsorption and morphological changes. A comparative oil recovery study was conducted to assess the oil recovery potential under high-salinity conditions. This research represents a step forward in utilizing environmentally friendly fluids for offshore practices.
Methodology
Materials
The Shanghai Yizong Biotechnology Company produced and delivered the welan gum from China (99% purity). Amorphous silica NPs (SiO2) in the form of nano powder, which is 99.9% pure and has a surface area of 175–225 m2/g, were used to make the solution. Sodium chloride (NaCl) (99%) was used during the experiments, which was obtained from Sigma-Aldrich (USA). Samples of limestone reservoir rock were obtained from Texas, USA.
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Nanofluid preparation
A total of 0.1 g of silica nanopowder and 100 mL were stirred to make a 0.1 wt% NPs stock solution. The solution was thoroughly stirred using a magnetic stirrer at room temperature, followed by homogenization using an ultrasonic homogenizer (60 °C at 65% power for 35 min). The solution of welan gum and silica NPs was prepared by diluting the stock solution at different concentrations of the polymer (0.3 wt%, 0.35 wt%, 0.4 wt%, and 0.5 wt%) and stirred again. This procedure produced the nanofluid used in the subsequent experiments. For salinity tests, varying amounts of NaCl (0.1 wt%, 0.5 wt%, 1.5 wt%, and 3.5 wt%) were added to the nanofluid. The selected range represents different conditions. The low range of 0.1 wt% represents fresh formation water, the 0.5 wt% is medium range [42], the Caspian sea is in range of 1.5 wt% [94] and the seawater is 3.5 wt% [7].The adapted method is reported as a steps method which recognize widely in the nanofluid science [14].
Polymer/NPs characterization
The Fourier Transform Infrared (FTIR, Nicolet iS10) Spectrometer was used to determine the functional groups in Welan gum that interact with the SiO2 nanoparticles. Initially, the background spectrum was recorded, followed by the spectra of each sample compared against this background. The analyzed samples included: a) 0.5 wt% Welan polymer, b) 0.1 wt% SiO2, and c) Welan gum in the NP solution at respective concentrations. Morphology and polymer shape was investigated via Scanning electron microscopy (SEM, Crossbeam540, Zeiss). The study examined welan gum’s reaction to salinity with and without SiO2 NPs, using samples containing (1) 0.1 wt% SiO2, 0.5 wt% welan, 3.5 wt% NaCl, and (2) 0.5 wt% welan, 3.5 wt% NaCl. Each 100 mL solution was placed overnight in a vacuum oven at 60 °C, and morphologies were examined at various magnifications. Proton Nuclear Magnetic Resonance 1H-NMR spectroscopy (JMN-ECA 500 spectrometer, Japan) was used to study how the composition of welan gum changed when silica nanoparticles were added. In the first sample, 10 mg of welan gum was mixed with 1 mL of deuterium oxide (D2O). In the second sample, 0.5 wt% welan gum and 0.1 wt% SiO2 were mixed with 1 mL of solvent. Measurements were recorded with 8 scans in glass NMR tubes, and Mnova Software Version 15.0.1 was used for analysis.
Rheology test
All solutions were subjected to rheological tests using the Physica Modular Compact Rheometer 302, manufactured by the Austrian company Anton Paar. For these tests, the CP 50–1 cone plate model was used. On the measuring plate, approximately 1–1.5 mL of each sample was evenly distributed. The CP 50–1 cone plate was lowered, and any excess liquid was carefully trimmed. Measurements were recorded using the Anton Paar RheoCompass 1.30 software. The shear stress profile was set to a Ramp logarithmic scale for viscosity measurements, ranging from 1 s⁻1 to 1000 s⁻1. The same procedure was employed in the earlier study using the same equipment [2].
Adsorption characterization
Analytical techniques, such as UV–vis spectroscopy, quantify analyte absorption in the ultraviolet–visible region. Adsorption data were obtained using an Evolution 300 UV–vis Spectrophotometer (Thermo Fisher Scientific) with VISIONpro software. For the test, polymer solutions were prepared (0.1 wt%- 1.5 wt%). Absorbance was measured from 190 to 700 nm, repeated three times for accuracy. To start the adsorption test, 5 g of limestone was added to each solution. The solutions were then mixed in a DLAB SK-O180-S LED Digital Orbital Shaker at 180 rpm, and readings were taken at 15, 30, 60, 120, 240, and 600 min. We used a stock solution of 0.1 wt% SiO2 and welan gum at 0.05 wt%, 0.1 wt%, 0.3 wt%, and 0.5 wt% concentrations to investigate how NPs changed the adsorption trends [3]. The same process was followed to obtain adsorption data. The adsorption analysis is dependent on identifying the new equivalent concentration (Ce concentration) of the welan gum solution. The adsorption of new concentrations was determined by back-calculating new absorbance readings using the generated standard curve. The adsorption capacity was calculated using the following equation:
\(\Gamma\)adsorption capacity of limestone to Welan gum, g/kg or (mg/g)
\({C}_{i}\)concentration of Welan gum solution before adsorption, wt%
\({C}_{e}\)concentration of Welan gum solution after adsorption, wt%
\(V\)volume of Welan gum solution, mL
\(m\)mass of limestone, g.
The adsorption models were used in this study to analyze the experimental adsorption data. The adsorption mechanisms and trends can be understood based on the information provided by these equations. A thorough analysis of the adsorption isotherms is essential to understanding the adsorption mechanism [1]. The static adsorption model is described in Table 1.
Interfacial tension (IFT) for interaction of water-SiO2 NPs solution with dodecane was obtained using the IFT 700-HPHT (Vinci Technologies, France) device. The pendant/rising drop optical methods were employed for IFT measurements. Experiments were performed with 4 solutions: 1) DIW, 2) 0.05 wt% SiO2-DIW 3) 0.1 wt% SiO2-DIW 4) 0.25 wt% SiO2-DIW. Experiments were also repeated with the same samples in presence of 3.5 wt% NaCl. For base measurement of DIW-dodecane IFT the needle was located so that the dodecane bubble rises in the bulk DIW environment through the needle. For that dodecane was coloured to red to differentiate it from water. However when nanoparticle-containing solutions were used, the transparent dodecane acted as the bulk while white SiO2 fluid drops went through the needle. The same needle with diameter of 0.64 mm was used in each experiment.
Core flooding
The core flooding experiment was conducted using Vinci Technologies ACA- 700 Aging Cell device(France). The study used Indiana limestone as the core material to represent the oil wet systems. Therefore, the experiment comprised three primary stages: core preparation, initial saturation, and core flooding. The characteristic of the core porosity was determined using the Vinci Technologies porosimeter apparatus (France).
The significance of reservoir rock wettability is widely recognized, although it remains incompletely characterized. Considering several reservoir rocks display non-uniform wettability characteristics [67]. To establish the oil wet, the dry weight of the cores was recorded, then the core was injected by synthetic oil (dodecane) followed by an immersing period of 24 h to attain its oil saturation.. Two cores of Indiana limestone exhibited an average porosity of 20%. The absolute permeability was 116 mD. These results are consistent with the specifications provided by Kocurek Industries, the supplier of the limestone core [93].
To establish the residual oil saturation, the core was injected by the brine (3.5 wt%) until there was no more oil production. The final step was the re-injection of dodecane up to five pore volumes to displace the brine and establish a continuous oil-saturated condition. These saturation steps were conducted under a confining pressure of 1000 psi and a back pressure of 400 psi.
The main core flooding experiment was divided into three stages: pre-flush, main flush, and post-flush. In the pre-flush stage, 2 pore volumes of brine were injected at a flow rate of 1 cc/min until dodecane production stopped. This was followed by the main flush phase, in which a solution containing welan gum and SiO2 nanoparticles (1 pore volume) was injected to evaluate its effect on fluid displacement and recovery. In the post-flush phase, 2 pore volumes of brine were injected again to displace the remaining solution and finish the flooding process.
The confining pressure was maintained constant at 1000 psi and the back pressure at 400 psi throughout the experiment. The designed controlled environment therefore provides a close approximation to the reservoir conditions and, hence, valid experimental results (Fig. 1).
Fig. 1
Aging cell apparatus and schematic
Results and discussion
The current investigation focused on the interaction between welan gum and SiO2 NPs to determine their combined impact on rheological properties and oil displacement efficiency in reservoir conditions. The results provide insights into fluid characterization and adsorption quantification.
Characterization analysis
From FTIR spectra, several peaks were observed over the range of 4000–400 cm−1, as shown in Fig. 2. Welan gum displayed a prominent peak at 3290 cm⁻1, associated with the hydroxyl (O–H) stretching vibrations within its sugar ring, consistent with findings in [19] and other studies on polysaccharides. This peak highlights the hydrophilic nature of Welan gum and its abundant hydroxyl groups, which play a critical role in hydrogen bonding. Peaks at 1630–1640 cm⁻1 correspond to C = O stretching vibrations of carboxylic or ester groups, which are typically present in polysaccharides. Additionally, peaks around 1018–1030 cm⁻1 signify C–O–C or C-O–H stretching, attributed to the glycosidic linkages inherent in polysaccharides [39].
Fig. 2
FTIR spectra of Welan gum, SiO2, and SiO2 NP—Welan gum
The FTIR spectra of SiO₂ nanoparticles demonstrated characteristic peaks, with a broad band at 3400–3500 cm⁻1 attributed to O–H stretching of surface silanol (Si–OH) groups. This is a marked feature of silica nanoparticles and aligns with the findings of [29]. Peaks at 1070–1100 cm⁻1 represent Si–O-Si asymmetric stretching, while those at 800 cm⁻1 are indicative of symmetric stretching. The distinct band at 460–480 cm⁻1 corresponds to Si–O-Si bending vibrations, a signature of the silica network. The band around 800 cm⁻1, corresponding to Si–O bending vibrations, signifies the movement of oxygen at a right angle to the Si–Si lines within the Si–O-Si plane, as described in [60, 65]. This distinct peak confirms the presence of a robust silica network structure, consistent with other studies investigating silica’s structural vibrations.
The present investigation revealed significant shifts in FTIR spectra when Welan gum was combined with SiO2 nanoparticles, indicating robust interactions between these components. The O–H stretching peak moved from 3290 cm⁻1 (Welan) and 3400 cm⁻1 (SiO2) to 3265 cm⁻1 in the composite, suggesting hydrogen bonding between Welan gum’s hydroxyl groups and SiO2’s surface silanol groups. This observation aligns with findings by [80], which noted similar shifts in polysaccharide-silica composites, attributing them to hydrogen bond formation and hybrid network integration. Peaks at 1080–1100 cm⁻1, representing Si–O-Si stretching, exhibited slight broadening, indicating SiO2 encapsulation within the polymer matrix. Additionally, a new band emerged in the 450–500 cm−1 range, likely corresponding to Si–O bond bending vibrations, which is enhanced by the silica-Welan gum interaction. The similarity of these peaks to previous findings by [80] corroborates the chemical bonds facilitating the Welan/SiO2 interaction. Notably, this investigation identifies additional peaks in the polysaccharide range (2100–1639 cm⁻1), emphasising interactions between Welan gum and silica nanoparticles.
By using 1H-NMR, the compositional data obtained from IR spectra were confirmed. Figure 3 illustrates the chemical content of the polymer (a) and polymer/NP solution (b). The 1H-NMR (500 MHz, D2O) for Welan gum showed peaks of 3.51 (d, J = 14.3 Hz, 0H), 4.69 (s, 39H), and 5.26 (s, 0H). For the sample of SiO2 NP – Welan gum, some of the chemical shifts include δ 5.25 (s, 3H), 5.18 (s, 3H), 3.80 (t, J = 9.3 Hz, 2H), 3.73–3.60 (m, 6H), 3.57–3.41 (m, 6H), 2.09 (s, 1H), 1.90 (s, 4H), 1.43 (s, 1H), 1.17 (s, 11H), 1.13 (s, 1H), and 0.74 (s, 2H). Due to impurities, some of the peaks were neglected by the software. Welan gum exhibited signals at 5.26 ppm, indicating the presence of α-L-mannosyl and α-L-rhamnosyl units. The peak observed at 4.69 confirmed the existence of β-D-glucuronosyl and β-D-glucosyl residues. These findings align with those reported by Zhao et al. [90]. When it comes to the polymer/NP solution, a peak at 0.74 ppm is indicative of the -Si-CH2- bond which implies the bonding between silica and polymer molecules. Chemical shifts in the range of 1.13–1.90 show the methylene groups and δ 2.09 is the signal of -CH3-. Multiplets in the range 3.73–3.60 ppm belong to the oxygen groups of Si–O which were also established from IR spectra. Peaks of 5.25 ppm and 5.18 ppm correspond to the -RCH2 = CH2- groups which reference the strong interaction between the Welan gum and NP molecules. These peaks align with those proposed by Lai et al. [55]. The molecular structure schematic inferred from the FTIR and H-NMR is presented in Fig. 3b and c.
Fig. 3
1H-NMR spectra for a) Welan gum, b) SiO2 NP – Welan gum c) inferred molecular structure of welan gum and d) inferred SiO2 NP-Welan gum molecular structure attachment
In Fig. 4, the presence of SiO2 in welan gum is indicated by well-defined, and shaped particles observed at larger scales, confirming its crystalline nature. In contrast, the welan gum appears as a fibrous or network-like structure, suggesting its role in binding or modifying the surface characteristics of the composite. The same block-like structure has been reported in other works [21]. Comparing the two SEM images of welan gum with and without SiO2 (see Fig. 5) reveals significant structural differences in the welan gum samples. The sample with SiO2 exhibited a much more interconnected, fibrous network structure with smaller, more uniform features compared to the flaky, fragmented appearance of the sample without SiO2. The SiO2 nanoparticles appeared to create a network with improved structural integrity. Initially, the high salt concentration (3.5 wt%) significantly impacted the polymer's behavior through ionic interactions, causing charge screening effects that reduce electrostatic repulsion between polymer chains and promote aggregation. While the sample at this salinity without SiO2 resulted in sharp, plate-like fragments, the SiO2 containing sample maintained a more porous, web-like architecture with finer structural elements. This suggests the silica nanoparticles effectively prevent the extensive aggregation and collapse seen in Fig. 5a. Interactions between the negatively charged silica surfaces and welan gum polymer chains likely improved the structural stability. The interconnected structure is depicted at lower magnification, implying that SiO2 potentially enhances welan gum’s functionality for targeted applications. This structural arrangement supports the chemical analysis, which indicates the formation of bonds between silica and the polymer, enhancing stability and dispersion.
Fig. 4
SEM image of 0.1wt% SiO2 + 0.5 wt% welan gum
Fig. 5
SEM image 0.5 wt% welan gum with salinity of 3.5 wt% a) without SiO2 b) with SiO2 0.1wt%
Rheology analysis
Effect of salinity on SiO2/welan gum rheology
Viscosity decreases as salinity increases, as demonstrated by various research. In high salinity the polymer performance degraded by reducing viscosity and hydration, limiting their efficiency. Therefore, addressing polymer degradation in various-salinity conditions ensures better stability and functionality. As offshore conditions feature high seawater salinity, their exact impact on various polymer solutions has been comprehensively studied and showed different tolerance behavior for polymers [46, 56, 71, 83]. The studies delineate the polymer saline tolerance range and extend to modifying polymer structures, such as incorporating hydrophobic groups, enhancing salt resistance by reducing electrostatic shielding. Therefore, the study explores the saline tolerance of natural polymers and evaluates SiO2 NP’s role in preserving polymer stability under saline conditions. The current study shows the rheological behavior of welan polymer with and without SiO2 NPs. The experiments were limited to economically viable concentrations. Consequently, the investigation examined only 0.1 wt% SiO2 and 0.5 wt% of welan, as illustrated in Fig. 6.
Fig. 6
Effect of salinity on rheological performance of (a) 0.5 wt% welan solution (b) 0.5 wt% welan + 0.1 wt% SiO₂ solution
Increasing the salinity reduced the viscosity of welan gum, whereas the introduction of SiO2 NPs led to an observed improvement. For example, at a shear rate of 10 (1/s), the viscosity decreased from 116.7 to 50, 26, 13 and 6 cp when the salinity increased to 0.1, 0.5, 1.5 and 3.5 wt%, respectively. Conversely, the viscosity for solutions containing SiO2 NPs was 129, 60, 27 and 7.2 cp for salinities of 0.1, 0.5, 1.5 and 3.5 wt%, respectively. It can be deduced that the addition of SiO2 nanoparticles provides viscosity enhancement at higher salinities, with a maximum improvement of approximately 50% at 1.5 wt% salinity and 44.6% at 3.5 wt% salinity. However, at 0.1 wt%, the SiO2 marginally increases the viscosity, demonstrating no significant improvement.
Salinity affects the interaction between polymer chains and NPs, significantly altering the mixture’s flow behavior and structural stability. Viscosity and salinity were inversely related even with and without NPs. Polymer's relation to salinity has been discussed in many research studies. Kakati et al. [49] found the influence of 10%, 25%, 50%, and 100% seawater salinities on HPAM. Results depict that the highest concentration of salts resulted in the lowest viscosity. Moreover, Gao and Technology [36] report the damaging effects of NaCl on the rheological properties of welan gum solution. For example, the drop of 440 cP was indicated when the concentration of salt was increased from 10 g/L to 150 g/L. Additionally, Xu et al. [84] reported the same viscosity-decreasing impact in xanthan polysaccharides. Such observations back up our results. The trend of the viscosity-decreasing effect of NaCl is explained by the interaction between the salt ions in the solution and the charges on the polymer chain, which lowers the repulsive forces between them when the salinity is high. The polymer molecules shrink as a result of this contact, and their size decreases. The interaction and binding of polymer chain parts with each other results from coiling up. These interactions form a more compact macromolecule structure [84]. Additionally, cations create bridges between charged groups on the polymer backbone in a high-salinity environment. This leads to a decrease in viscosity owing to intrachain complexation or collapse of the polymer chains [49]. A previous study on chitosan composite solutions showed that silane nanoparticles strengthen the structure by forming a rigid 3D network, making it more stable against temperature and salinity changes [51]. Regarding SiO2 interaction with salinity, Hutin et al. [45] reported that SiO2 nanoparticles undergo a change in stability due to reduced electrostatic repulsion in high-salinity conditions. The effect is more pronounced at low NaCl concentrations (up to 1 wt%), where 0.1 wt% silica nanoparticles maintain a stable size because the electrostatic repulsion between negatively charged particles remains sufficient. This repulsion prevents aggregation, ensuring the particles remain dispersed without notable size changes. However, at higher salinities, the attraction of Na⁺ ions to the negatively charged silica surface disrupts this stability, weakening electrostatic repulsion, which leads to aggregation. This phenomenon appears to be consistent with our findings [45].
The viscous and elastic moduli were also obtained employing Cox-Merz rule, which is empirical relation. The empirical nature of the Cox-Merz rule means that its applicability can vary depending on the specific characteristics of the fluid being analyzed. The Cox-Merz rule states that the complex viscosity can approximate the steady shear viscosity. Using \(\omega =\gamma\) and setting \(f = 2\pi /\omega\), where \(\omega\) is angular velocity in rad/s and \(f\) is frequency in Hz, we approximated the values of for complex viscosity to subsequently obtain viscous modulus (\({G}^{{\prime}{\prime}}\)):
where, n is flow behavior index. The same procedure was repeated to identify the effect of temperature.
The Fig. 7 shows the constant increase of moduli with the increase in frequency and the reduction of G’ and G” values with an increase in salinity. At the presence of 0.1 wt% NaCl the relatively high values of viscous and elastic moduli was observed, suggesting that even at low salinity, polymer-nanoparticle interactions contribute significantly to the viscoelastic structure. Increasing salinity to 0.5 wt% saw a declining trend, as at the lowest frequency G’ and G” values were 0.097 Pa and 0.073 Pa, compared to 0.175 Pa and 0.231 Pa at 0.1 wt% salinity. This means that the polymer network becomes weaker at low frequencies with increased NaCl concentration.
Fig. 7
Elastic and viscous moduli of 0.5 wt% welan + 0.1 wt% SiO₂ solution at different salinities
The reduction of both moduli and therefore weaker network in polymer-nanoparticle system in saline conditions was reported by [82]. Therefore our findings agree that adding silica nanoparticles, both moduli values enhanced due to ionic bridging between salt ions, polymer chains and nanoparticles [82].
However, the value of elastic modulus experiences growth, and viscous modulus declines at the highest frequency when compared to lower salinity values. For example, elastic moduli at the highest frequency increased from 5.616 Pa to 7.589 Pa when salinity increased from 0.1 wt% to 0.5 wt%. Conversely, the viscous modulus reduced from 7.407 Pa to 5.754 Pa. The same phenomena are observed at a much higher salinity of 3.5 wt%, where minimum G’ and G” values were 0.039 Pa and 0.012 Pa, while at the highest frequency, they reach 11.721 Pa and 3.626 Pa, respectively. Findings illustrate that the solution becomes more solid-like at lower salinity and lower frequencies and has a stronger elastic network. This could suggest that the polymer chains and nanoparticles form a more structured, interconnected network that can store more mechanical energy when deformed. The study by [81] also observed the transition of the behavior from liquid-like to solid-like due to creation of certain structure between nano-silica and PEB.
At the same time, the solutions with higher NaCl content seem to have stronger structures at higher frequencies. On the other hand, a decrease of viscous modulus at higher salinity and frequency shows that the solution resists flow less and loses less energy as heat when deformed, suggesting its less fluid-like behavior. Notably, when examining the zero salinity condition (Fig. 9), the system exhibits a higher G''and lower G'value, indicating that the polymer is in a more dispersed state and lacks robust intermolecular forces.
Effect of temperature on SiO2/welan gum rheology
The impact of temperature on the viscosity of the welan polymer solution and the welan/SiO₂ nanoparticle solution was studied at three different temperatures: 25 °C, 50 °C, and 75 °C. The temperature selection was based on the temperatures of the Kazakhstan oil field.
As observed in Fig. 8, at a shear rate of 10 (1/s), the viscosity of welan polymer decreased from 118 to 107 cp and 78 cp when the temperature elevated from 25 °C to 50 °C and 75 °C, respectively. Upon the introduction of SiO2 nanoparticles, the viscosity reduced from 122 to 110 cp and 87 cp as the temperature increased from 25 °C to 50 °C and 75 °C, respectively. The incorporation of SiO2 NPs sustained higher viscosity as temperature rises, with increases of 3.4%, 3%, and 11.8% observed across the aforementioned temperature range. It appears that the most significant contribution of the SiO2 NPs occurs at the highest temperature (observed at 75 °C), with minimal impact at moderate temperatures of 50 °C.
Fig. 8
Effect of temperature on rheological performance of (a) 0.5 wt% welan solution (b) 0.5 wt% welan + 0.1 wt% SiO₂ solution
The increase in viscosity observed in polymeric nanofluids can be attributed to the formation of interconnecting bonds between nanoparticles and polymer components. Thus, it contributes to the polymer’s integrity at elevated temperatures. The SiO2 results in a more robust network that resists thermal degradation, enabling the solution to maintain its viscosity more effectively at higher temperatures [37]. The demonstrated thermal stability and performance of the welan polymer-SiO2 nanoparticle system indicate potential applications in high-temperature environments. This is broadening the scope of its practical usage.
The effect of temperature on elastic and viscous moduli was also investigated. Figure 9 illustrates the frequency dependence of G’ and G” at different temperatures. The similar trend of increase of moduli with frequency is seen. Elastic modulus seems to slightly increase over the temperature range both at low and high frequencies. At 25 °C elastic modulus at lowest frequency was 0.205 Pa, while at the highest frequency it was recorded to be 2.256 Pa. When temperature was increased to 75 °C, corresponding values at the lowest and the highest frequencies were 0.254 Pa and 4.364 Pa. The inverse trend is observed regarding viscous modulus at all frequencies. When temperature was increased from 25 °C to 75 °C, the G” at 0.159 Hz decreased from 0.815 Pa to 0.350 Pa, while at 159.15 Hz it declined from 8.961 Pa to 6.027 Pa. Both moduli showed that increase in temperature lead to losing less energy as heat when deformed, meaning it flows more easily and behaves less like a liquid.
Fig. 9
Elastic and viscous moduli of 0.5 wt% welan + 0.1 wt% SiO₂ solution at different temperatures
Dispersion of SiO2 in Welan Gum and Flow behavior
The primary purpose of incorporating polymer into water is to enhance viscosity. However, interactions between the polymer and reservoir rocks, temperature and salinity result in reducing the viscosity [2, 6, 9, 63]. Given these factors, nanoparticles can potentially enhance polymers’ mechanical, electrical, and barrier properties, functioning as thickening agents for low-viscosity Newtonian fluids. Consequently, it is essential that flow behavior is inferred through rheological modelling. The principal challenge lies in ensuring that nanoparticles act as enhancers by maintaining dispersion and preventing agglomeration [38, 52]. For nanoparticles, Van der Waals forces between particles promote agglomeration, thereby affecting flow properties [61]. Thus, the dispersion of nanoparticles remains a critical technical factor [91].
In the results, the incorporation of SiO₂ nanoparticles into Welan gum has been confirmed through FTIR, SEM analysis, and rheological examinations. From a colloidal science perspective, SiO₂ dispersion in Welan gum occurs at multiple scales [80]. At the nanoscale, SiO₂ nanoparticles exhibit a unique combination of high surface area and reactive silanol (Si–OH) groups, facilitating strong hydrogen bonding with polar sides in Welan gum. This is supported by the FTIR data, which demonstrated robust hydrogen bonding between Welan gum and SiO₂, maintaining nanoparticle stability even in high ionic strength conditions. Hydroxyl groups on both SiO₂ and Welan gum may be responsible for creating a three-dimensional structure that provides enhanced properties. The 3D polysaccharide backbone’s carboxyl or hydroxyl groups provide an additional multi-point anchoring system between the SiO₂ and Welan [40]. Rheological analyses indicate that the Welan-SiO₂ mixture maintains stability under saline and high-temperature conditions. Typically, high salinity decreases interactions and leads to nanoparticle aggregation. However, this study demonstrates proper SiO₂ dispersion, as evidenced by the absence of fluctuations across all shear rates. Due to the effective dispersion, the nanoparticles function as cross-linkers within the Welan gum [88].
As seen from Fig. 6 and 8, it is evident that viscosity decreases most significantly at lower shear rates, with the rate of change gradually stabilizing at higher shear rates. This trend indicates a shear-thinning behavior of our solutions that can be represented as Power Law model given by the following equation:
$$\eta ={K\gamma }^{n}$$
(5)
where η is apparent viscosity in cP, K is the flow consistency index, γ is the shear rate in s⁻1, and n is a flow behavior index (n < 1 for shear-thinning behavior).
Viscosity (consistency index, k) and shear-thinning behavior (flow behavior index, n) are influenced by temperature as seen in Table 2. In the absence of SiO₂, k experienced a substantial decrease as temperature rose, dropping from 838 at 25 °C to 207 at 75 °C. Conversely, the inclusion of SiO₂ mitigated this reduction, with k decreased from 820 to 369 over the same temperature range. Statistically the k increased by approximately 13% with SiO₂ at temperature of 50 °C. But it became more significant at 75 °C. k changed by 78.2% with SiO₂, indicating a stronger structural support at higher temperatures. The flow behavior index exhibited an increase with temperature in both scenarios, signifying a shift towards Newtonian behavior. However, the presence of SiO₂ reduced this transition (from 0.131 to 0.555 without SiO₂, compared to 0.157 to 0.399 with SiO₂).
Table 2
Shear model for welan-SiO2 NPs solution
As salinity increased, it also influenced k and n. Higher salinity without SiO₂ decreased k from 194.451 (0.1 wt%) to 8.219 (3.5 wt%).This indicates salinity-induced thinning. However, with SiO₂, reduction of k is less. Yet, it is still sever. For instance, at 0.1 wt% salinity, k rose by 17.1% with SiO₂. Similarly, at 0.5 wt% and 1.5 wt% salinity, k increased by 20.7% and 17.1%, respectively. The evidence indicated that SiO₂ plays a role in mitigating the reduction of viscosity at elevated salinity concentrations. This characteristic implies that SiO₂ moderately improves the fluid’s ability to withstand viscosity decreases caused by high salinity. The addition of nanoparticles resulted in maintaining viscosity levels. A comparable observation regarding the effect of SiO2 nanoparticles on xanthan gum was reported. Nevertheless, the beneficial impacts of SiO2 nanoparticles are less in the presence of salt [18]. Generally, at higher salinities, k is maintained with SiO₂, indicating that SiO₂ disperses effectively in saline conditions, thereby mitigating viscosity loss.
Adsorption isotherm
Adsorption of the injected fluid onto the rock surface is a prevalent issue encountered in CEOR [4‐6]. It is important economically because of the retention of the polymer within the reservoir’s pore throats. The retention reduces porosity and permeability of the porous medium, resulting in reduction of hydrocarbon production and impeding the flow of oil and gas [34]. On the contrary, adsorption can be beneficial for wettability alteration mechanism so changing from oil-wet to water-wet systems enhancing the oil mobility.
When applying the UV–vis spectrometer we witnessed that the highest absorbance results appeared at a wavelength of 193 nm and 264 nm for Welan gum solutions without and with SiO2, respectively. The standard curves (Fig. 10) were constructed to ensure that the R2 value is close to 1 (0.988 and 0.949), meaning the absorbance values for all Welan gum concentrations at the wavelengths of 193 nm and 264 nm are acceptable for future predictions. The increase in concentration increases the absorbance reading; this aligns with natural chemicals studied previously [1]. It is worth noting that the difference in the wavelength is attributed to the new polymer structure network in the presence of SiO2 NPs.
Fig. 10
Absorbance measurements of Welan gum at different concentrations a) without SiO2 193 nm and b) with SiO2 264 nm
Figure 11 illustrates that the adsorption of welan gum was more significant compared to when SiO2 was introduced to the solution. When Welan gum was alone, its adsorption on limestone increased quickly, reaching a peak at around 50 min before gradually desorbing. This drop might happen because of weak initial binding interactions, where some polymer molecules detach before forming a stable adsorption layer.
Fig. 11
Adsorption capacity of Welan gum and SiO2—Welan gum
Conversely, the SiO2 NP-Welan gum adsorption was less than the welan gum but it behaved differently. The solution adsorption increased gradually and steadily indicating a more controlled process. Similarly previous findings show that SiO₂ nanoparticle adsorption influenced the surface fluidity of phospholipid layers. So the initial drop in Welan gum-SiO2 adsorption may resulted from dynamic restructuring at the interface before forming a more stable and controlled adsorbed layer [79]
To discuss the mechanism it is better to look at different sides of this interaction. Firstly the limestone, secondly the welan, thirdly the nature of SiO2 interaction. For limestone, the surface charge of limestone was a result of the hydrolysis of carbonate rocks in water. This also has a known relation to the Zeta-potential, where the studies show increasingly negative values for limestone across all pH levels. Also the release of ions because of the hydrolysis increased concentrations of ions like Mg2 + or Ca2 + shift the zeta potential to positive [50].
Secondly, the interaction mechanism between Welan gum, an ionic polymer, and positively charged limestone molecules involves several steps. This process can be described by double layer formation, where Welan gum releases ions from its structure, leading to ion exchange between the limestone and the polymer. Na + and K + ions from Welan gum are continuously replaced by Ca2 + ions from limestone, contributing to the overall adsorption process. Electrostatic attraction in the ionized water medium results in chemisorption and potential hydrogen bonding between the polymer and limestone molecules [22, 24, 30].
Thirdly the SiO2 nature caused a notable difference when compared to SiO2 NP-Welan gum. Previous studies linked the addition of NPs to the polymer solution with a reduction in polymer particle size, indicating a strong interaction between Welan gum molecules and SiO2 due to the oxygen bonds of Si–O confirmed by FTIR analysis [78]. This interaction likely occurs at several negatively charged SiO2 sites, which interact with positively charged sites on the limestone surface, leaving fewer sites available for Welan gum adsorption. The reduction after 600 min is 44.44%. The SiO2 as believed in this study is incorporated in the welan matrix, yet there is potential that SiO2 is leading the adsorption. This can happen through chemical interactions due to its CaCO₃ composition, limestone's alkaline properties enhance its interaction with silica-based materials, potentially intensifying the binding process [32]. The other secondary interaction could be because of siliceous minerals within the limestone which can possess silanol groups on its surface resulting in the interaction with SiO2 NP [62].
Adsorption kinetics can be characterized by several models, including the pseudo-first-order (PFO) model [54] and the pseudo-second-order (PSO) model [44], which are commonly employed to elucidate the adsorption process [89]. These models assume the rate of site occupation is proportional to the number of unoccupied sites and depend on site availability and adsorbate interactions. The PFO model suggests chemisorption as the rate-limiting step, involving valence forces through electron sharing or exchange between adsorbent and adsorbate. Figure 12 illustrates data plots fitted to these models. The PFO model showed a low R2 for Welan gum, indicating a poor fit and suggesting the adsorption process does not follow this model well. Conversely, the R2 for SiO2-Welan gum was 0.96, indicating a good fit. The PSO model showed an excellent fit for Welan gum, suggesting the adsorption process follows this model. However, the R2 value for SiO2-Welan gum, while reasonable, was not as strong as for the PFO model. The adsorption kinetics in the presence of SiO2 are more complex, potentially involving physical adsorption, diffusion processes, and chemisorption.
Fig. 12
a) Model fit for pseudo-first-order kinetic adsorption, b) Model fit for pseudo-second-order kinetic adsorption
The Langmuir isotherm explains surface coverage by balancing the rates of adsorption and dynamic equilibrium. Adsorption correlates with exposed adsorbent surfaces, while desorption correlates with covered surfaces. This model is suitable for the tested samples, as evidenced by the polymer solution data with and without SiO2, shown in Table 3. The high correlation coefficient (R2) and adjusted correlation coefficient (adj-R2) nearing 1.0 validate the Langmuir model. Despite a high value indicating stronger ion affinity for the adsorbent, the experimental data for both cases show relatively small values. The Freundlich Isotherm model describes adsorption on heterogeneous surfaces, such as rough surfaces or micropores. The data were fitted to a linear equation and the constants were determined by the intercept and slope (Table 3). The model did not obey the theorem without NPs due to the low R2 value but displayed over 90% adaptability with silica addition. The two experimental values significantly differed, confirming the model’s compatibility with only one experimental set. The slope greater than 1 indicates uniform and homogenous adsorption. The Halsey isotherm is used to investigate multilayer adsorption on heterogeneous surfaces. The fitted equations and isotherms revealed poor alignment without silica NPs, with low regression coefficients corresponding to the Freundlich equation suggesting homogeneous adsorption.
Table 3
Isotherms fitting
Solution
Equation
R2
Adj-R2
Langmuir isotherm
without SiO2
y = 0.322x – 0.052
0.995
0.991
with SiO2
y = 0.430x – 0.057
0.993
0.989
Freundlich isotherm
without SiO2
y = 3.007x – 0.205
0.998
0.333
with SiO2
y = 1.128x + 0.418
0.936
0.887
Halsey isotherm
without SiO2
y = 3.008x – 0.472
0.998
0.995
with SiO2
y = 1.128x + 0.962
0.936
0.904
Generally, static adsorption isotherms, representing the equilibrium between adsorbate and adsorbent, strongly correspond to the Langmuir model. Without NPs, Freundlich and Halsey’s theorems don’t do a good job of describing the polymer model. But when NPs are present, high correlation coefficients show that the polymer has both homogeneous and heterogeneous properties, which supports a homogeneous solution.
Interfacial tension
The impact of SiO2 has not been conclusively established. For instance, in our current study, as illustrated in Fig. 13, SiO2 demonstrated a minimal reduction in interfacial tension. It can be seen that there is a slight decreasing trend in IFT with increasing SiO₂ concentration in the absence of salinity. However, at 3.5 wt% salinity, the IFT remained relatively similar and insignificant IFT reduction was noticed.
Fig. 13
IFT measurement results
In one case, where the concentration was low and no salinity effect was observed, a potentially negative impact was noted. Our findings align closely with those previously reported by other researchers. Mohsen and his colleagues studied the effect of SiO2 NPs, their study indicated that SiO₂ nanoparticles failed to reduce the IFT of the oil sample in the absence of surfactant. However, they compared their findings to Esfandyari Bayat et al. [32] who reported that 0.005 wt% (50 ppm) of SiO₂ in 3 wt% NaCl reduced brine IFT from 26.5 to 17 mN/m [32, 69].
Core flooding results
A comparative analysis was conducted to assess the disparities in oil recovery among three primary methods. All procedures were done at room temperature to avoid rock or polymer damage.
Traditionally, the water flooding process, employing a 3.5 wt% NaCl brine, demonstrated that at 2 PV injected, the water cut reached 100%, and no oil was recovered. The overall oil recovery was approximately 38.5%, which remained consistent across all the cores utilized in the experiment as seen in Fig. 14. Upon injection of welan gum in 1 PV after 2 PV of brine, the recovery factor augmented to 40%. It is noteworthy that the post-flush resulted in a 55% recovery. Oil recovery reached 69.2% when SiO2 NPs were present. The total process facilitated by SiO2 NPs was able to enhance recovery by 15.3%. Additionally, it provided an extra 31%. It is evident that SiO2 NPs significantly influenced the entire process.
Fig. 14
Core flooding results
The water flooding process revealed that water bypasses a significant number of oil-bearing zones, reducing oil displacement efficiency. With the application of welan gum, the polymer acts as a pushing agent, resulting in a significant improvement in sweep efficiency. However, the presence of SiO2 NPs has considerably improved the process, although not solely due to sweep displacement. This improvement extends to microscopic changes. This interpretation is supported by the observation that the decline in the solution’s rheological properties under seawater salinity conditions occurs even in the presence of SiO2 NPs.
To assess sweep efficiency, the introduction of SiO2 nanoparticles resulted in consolidating the polymer network.In a second interpretation, the presence of NPs could have indirectly contributed to the mobility ratio by inducing relative permeability changes caused by the nanoparticles. Our previous findings suggest that SiO2 NPs reduce the adsorption of welan by competing for surface space on the limestone. In our finding we are suggesting that the presence of SiO₂ nanoparticles oriented to limestone surfaces offering new controlled interaction and better polymer diffusion.
The adsorption of SiO2 can induce wettability alteration with the rock. However, this aspect was not explored in the current study, which is a limitation. Nevertheless, it was explained by previous studies. It was assumed that the main reason for lower mobility is that NPs are able to alter the wettability of the rock surface, making it more water-wet [13]. This result could also be linked to the nanoparticles adsorbed on the surface of the limestone, preventing excessive welan gum adsorption. In general, this change can improve the displacement efficiency of the oil during the post-flush period, bringing about a higher amount of oil recovery.
Conclusion
This research encapsulates a significant step forward in utilizing natural polysaccharides for EOR and subsurface. The study strategies were tailored to the unique challenges of reservoir conditions in offshore practice.
The surface interaction between SiO₂ nanoparticles and Welan gum occur on a molecular level. The FTIR analysis confirmed hydroxyl groups in Welan gum, with peaks at 3278 cm⁻1 and 3294 cm⁻1 indicating O–H and Si–OH stretching. This suggests chemical bonds mediate the Welan/SiO2 interaction.
Elevated salinity generally reduced the viscosity of Welan gum; however, the addition of SiO₂ nanoparticles enhanced the viscosity. This occur particularly at higher salinity levels, with a maximum enhancement observed at 1.5 wt%. At elevated temperatures, SiO₂ maintained viscosity, with notable increases at 75 °C. The presence of SiO₂ also moderated the transition towards Newtonian flow behavior, as indicated by a lower increase in the flow behavior index. Despite significant reductions in the consistency index (k) due to salinity, SiO₂ mitigated this decrease, maintaining superior viscosity levels. SiO₂ nanoparticles aspire to provide centralized structural support for welan under high salinity and temperature conditions. They offer enhanced stability, unlike welan solution. In contrast, welan tends to devour viscosity stability under such challenging conditions.
The interaction between the SiO₂ nanoparticles/Welan gum mixture and limestone surfaces is primarily driven by electrostatic attraction and hydrogen bonding at the molecular level. This process results in the formation of a compact, cross-linked network between SiO₂, Welan gum, and limestone. The Pseudo-First-Order (PFO) model for the Welan gum-SiO2 system exhibited a high R2 value of 0.96, indicating the significant role of chemisorption. Conversely, the Pseudo-Second-Order (PSO) model for the same system demonstrated less robustness, suggesting complex kinetics involving physical adsorption, diffusion, and chemisorption. The Welan SiO2 NPs model displayed high correlation coefficients (R2 and adj-R2 values approaching 1.0), indicating uniform surface adsorption and a dynamic equilibrium between adsorption and desorption processes. The Freundlich isotherm with Welan SiO2 NPs demonstrated over 90% adaptability, reflecting both homogeneous and heterogeneous properties.
The core flooding results showed that the oil recovery of the welan gum and nanosilica solution made up 69.2%, while the experiment conducted solely on welan gum showed a recovery of 54%. Therefore, SiO2 NPs enhanced the oil recovery factor.
Declarations
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Conflict of interest
The authors declare there is no financial interests/personal relationships which may be considered as potential competing or conflict of interests.
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