Synthesis of Polyacrylamide Nanomicrospheres Modified with a Reactive Carbamate Surfactant for Efficient Profile Control and Plugging

doi:10.21203/rs.3.rs-4819480/v1

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Synthesis of Polyacrylamide Nanomicrospheres Modified with a Reactive Carbamate Surfactant for Efficient Profile Control and Plugging

https://doi.org/10.21203/rs.3.rs-4819480/v1

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A carbamate surfactant was synthesized using octadecanol polyoxyethylene ether(AEO) and isocyanatoethyl methacrylate(IEM) as functional monomers. Subsequently, active-carbamate-surfactant-modified PER nanomicrospheres were prepared through two-phase aqueous dispersion polymerization, utilizing acrylamide (AM), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), and ethylene glycol dimethacrylate (EGDMA) as raw materials. The microstructures and properties of the nanomicrospheres were characterized and examined using infrared spectroscopy, nanolaser particle size analysis, thermogravimetric analysis, scanning electron microscopy, rheometry, rotating drop ultra-low interfacial tensiometry, and core-driven experiments. The results revealed that the synthesized PER nanomicrospheres exhibit a uniform particle size distribution, with an average particle size of 336 nm. Furthermore, the nanomicrospheres exhibit a thermal decomposition temperature of 278℃, demonstrating good thermal stability. The nanomicrospheres also exhibit favorable expansion and viscoelastic properties. Upon the injection of 3wt.% of the PER nanomicrospheres into a non-homogeneous core, the blocking rate (η) reaches 90.32%, while the recovery rate increases by 30.2%. This improvement is attributed to the unique structural design of the nanomicrospheres, allowing them to form a thin film at the three-phase oil-water-rock interface and promoting oil emulsification and stripping. Overall, the PER nanomicrospheres effectively control fluid dynamics within reservoirs, mitigate the loss of oil and gas resources, enhance the economic benefits of oil and gas fields, and thus demonstrate good application prospects.

Nanomicrosphere

Moderator

Blocking rate

Recovery rate

Chinese oil reservoirs are typically formed through terrestrial sedimentation processes, leading to highly heterogeneous geological structures. Consequently, even after extensive water drive operations, approximately 60% of the crude oil remains trapped within the low-permeability regions of these reservoirs. Hence, to improve the overall recovery rate of the oil field, enhancing the recovery rate of crude oil from such low-permeability regions is critical [1, 2]. Low-permeability reservoirs encounter several developmental challenges owing to their unique geological characteristics. For instance, these reservoirs often lack natural energy, which results in rapid pressure drops and poses challenges for primary-recovery-rate enhancements. Furthermore, fluid seepage in low-permeability reservoirs follows a non-Darcy seepage pattern [3], governed by complex mechanisms involving pore structures, fluid properties, and their interactions. In such reservoirs, fluid flow behavior deviates significantly from Darcy's law, primarily owing to small pore size, excessively low permeability, strong intermolecular forces acting at phase surfaces, and the Jarman effect. The Jarman effect, in particular, intensifies as the reservoir deepens, resulting in a decrease in crude oil saturation, an increase in water content, and enhancement of the capillary drag force. Consequently, numerous oil droplets accumulate at pore throats [4, 5, 6], causing blockages that obstruct water-driven channels. These blockages severely impact fluid seepage, reduce the effective permeability of reservoirs, and complicate oil extraction. Furthermore, the pore throats of low-permeability reservoirs are typically excessively small and exhibit low permeability, further restricting fluid flow capacity. To overcome these challenges and improve oil recovery from low-permeability reservoirs, new technologies and mobilizers must be developed and implemented. These techniques and driving agents must be designed to improve fluid flow within reservoirs and enhance driving efficiency, ultimately optimizing hydrocarbon recovery [7, 8]. Acrylamide-based microspheres are known for their effectiveness in enhanced oil recovery owing to their tunable particle sizes, excellent swelling characteristics, and robust migration abilities. However, traditional polyacrylamide microspheres are currently inadequate for satisfying the complex requirements of low-permeability reservoirs, primarily owing to their harsh field conditions, increased structural heterogeneity, and growing numbers. Furthermore, single-function drive modifiers are unable to significantly enhance reservoir recovery, particularly in reservoirs with multiple major channels and fractures. Their limited ability to adjust flow and effectively manage fluid short-flow and scuttling phenomena [9, 10] can significantly reduce the final recovery efficiencies of reservoirs. Thus, to address the challenges encountered during low-permeability reservoir development, innovative driving agents must be developed promptly. These agents must be endowed with water-absorbing and swelling functionalities to effectively seal fine pores and throats within reservoirs, along with adequate deformation capacity to penetrate these barriers. When traversing the formation, driving agents must be uniformly distributed across the three-phase oil-water-rock interface, generating strong adsorption and stripping forces. These forces can not only help emulsify and strip crude oil but also significantly improve the overall recovery rate of the reservoir. In the context of oil repulsion mechanisms, most studies on the functionalization of polymer microspheres predominantly focus on enhancing their blocking strengths [11–15]. However, research focusing on enhancing their interfacial activity and structural separation pressure is scarce. Currently, suspension, emulsion, and dispersion polymerization are among the prevalent techniques adopted for polymer microsphere preparation. Among these, dispersion polymerization is an environmentally friendly and low-energy method. In this approach, the particle sizes of microspheres can be reduced by increasing the initiator amount, decreasing the monomer concentration, increasing the AM content, and augmenting the crosslinking agent concentration. Based on this background, we designed and synthesized a surface-active functional monomer,named REQ, to meet the practical application requirements of reduced particle sizes and increased hydrophilicity, as well as to enhance the structural separation pressure for improved emulsion stripping of crude oil. This monomer was then copolymerized with AM, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), and ethylene glycol dimethacrylate (EGDMA) to produce nanomicrospheres named PER, through dispersion polymerization. Subsequently, the performance of these nanomicrospheres was evaluated based on their microstructure, interfacial tension, swelling properties, viscoelasticity, shear resistance, and effectiveness in indoor core replacement experiments. The results demonstrated that the PER nanomicrospheres, exhibiting a narrow particle size distribution, excellent swelling properties, good viscoelasticity, and remarkable blocking and recovery rates, could effectively control fluid dynamics within the reservoir. Furthermore, they helped mitigate oil and gas resource loss, enhanced economic benefits, and offered significant strategic value for advancing petroleum engineering technology.

2.1 Materials

AM and AMPS (both of analytical grade) were obtained from Tianjin CITIC Kaitei Chemical Co., Ltd. Additionally, octadecanol polyoxyethylene ether (AEO), polyvinylpyrrolidone, isocyanatoethyl methacrylate (IEM), and acetone (all of analytical grade) were sourced from Shanghai McLean Biochemistry Technology Co., Ltd. Azodiisobutylamidine hydrochloride (V50), ammonium sulfate, sodium chloride, sodium hydroxide, and sodium formate were provided by Sinopharm Group Chemical Reagent Co.

2.2 Equipment

The following equipment was used: A HAAKE Rheometer HAAKE PolyLab QC (Mars) from Guangdong Shengze Technology Co., Ltd was used for rheological measurements. An Scanning Electron Microscope (Quattro SEM) from Beijing OPTO Optical Technology Co., Ltd was utilized for imaging. A thermogravimetric analyzer (TGA550) from Cloud Spectrum Instruments (Shanghai) Co., Ltd was employed for thermal analysis. A Fourier infrared spectrometer (AIM-8800) from Shimadzu was used for structural analysis. A multi-functional core filtration loss replacement device (DLS-5) from Haian Petroleum Research Instrument Co., Ltd was utilized for filtration studies. A laser particle sizing instrument (Bettersize2000) from Laiheng Technology (Beijing) Co., Ltd was employed for particle size analysis. Finally, an electrothermal constant temperature digital display water bath (KHW-D-1) from Beijing Kepray Medical Instrument Co., Ltd and an electromagnetic stirrer (B11-1) from Zhejiang Nade Science and Technology Co., were used during sample preparation.

2.3. Synthesis of the REQ monomer and PER nanomicrospheres

Initially, AEO was dried in a vacuum oven at 50℃ for 10 h to eliminate residual moisture. Subsequently, AEO, IEM, and acetone were mixed in a dry three-necked flask in the ratio of m(AEO):m(IEM):m(acetone) = 1:1.1:10. This mixture was subjected to electric stirring at 40℃ for 4 h. The resulting product was washed, precipitated, and filtered using ether under reduced pressure. The obtained product was then dried under vacuum for 24 h at room temperature, yielding the surface-active functional monomer REQ, as indicated in the reaction formula depicted in Fig. 1(a).

For the synthesis of PER nanomicrospheres, two-phase aqueous dispersion polymerization was adopted. Herein, a monomer composition mass ratio of m(AM):m(H₂O):m(AMPS):m(REQ):m(EGDMA) = 20:42:20:17:0.3 was utilized. Specifically, AM was dissolved in distilled water, following which AMPS and REQ were added to the solution and homogeneously mixed to produce the monomer phase. The pH of this phase was adjusted to a value of 5.5 while maintaining its temperature within 25℃. Subsequently, a saturated brine phase was prepared using distilled water mixed with predefined proportions of ammonium sulfate, sodium chloride, and sodium formate. This brine phase was then added to an open glass reactor and subjected to inert gas deoxygenation. After 30 min, the monomer phase and polyvinylpyrrolidone were added to the system, and an initiator solution (0.2% wt. V50) was introduced through a catheter. The injection rate of the microsampler was set to 0.5 mL/h, while the temperature of the system was adjusted to 40℃. Once the color of the emulsion changed from white to off-white, indicating phase transition, the temperature of the system was increased to 70℃. The reaction was allowed to continue for 1 h until the emulsion turned milky white, yielding PER nanomicrospheres. The primary reaction mechanisms are illustrated in Fig. 1(b), while Fig. 1(c) presents the molecular formula for the PER nanomicrospheres.

2.4. Characterization and Performance Testing

2.4.1 Characterization

Structural characterizations of dry powders of the monomer and PER nanomicrospheres were performed using infrared spectroscopy with a potassium bromide press. In addition, the surface morphology of the freeze-dried nanorods was imaged using a Quattro SEM (Scanning Electron Microscope) from Beijing OPTO Optical Technology Co. Before imaging, the sample surfaces were coated with a thin layer of gold [16]. Next, the heat resistance of the viscoelastic nanomicrospheres was assessed using a thermogravimetric analyzer under a nitrogen atmosphere. During this analysis, the temperature was increased at a rate of 10℃/min from 20℃ to 600℃.

The average particle size and distribution of the dry powder of PER nanomicrospheres were quantified in anhydrous ethanol using a laser-diffraction-based dry laser particle sizer (Bettersize2000) [17]. The average particle size and distribution of the nanomicrosphere emulsions were evaluated in 0.3 wt% aqueous solution. Before particle size measurements at room temperature, all dispersions were subjected to mild ultrasonication to ensure proper dispersion. The particle concentration in the solutions was 0.05%, and samples were aged at 60℃ for 30, 50, 70, and 90 days. To evaluate the blocking ability of the synthesized nanomicrospheres, solutions were prepared using two different solvents: pure water and 5000 ppm brine, each at a mass fraction of 0.3%. Subsequently, the expansion multiplicity and rate of these solutions at various time points were measured and analyzed using the laser particle sizer.

2.4.2 Viscoelasticity and interfacial tension testing

To assess the viscoelastic properties of the PER nanomicrospheres, a 0.1% solution of the PER nanomicrospheres in water was prepared. After allowing the nanomicrospheres to swell for 48 h, the water-absorbed, swollen nanomicrospheres were collected through filtration. The viscoelasticity of these swollen nanomicrospheres was then evaluated using a rheometer. The rheological tests, conducted over an oscillation frequency range of 0-100 rad/s, measured both the energy storage modulus (G′) and energy dissipation modulus (G″). Notably, polymer mobilizers must exhibit low interfacial tension to successfully drive crude oil through the formation. Herein, for interfacial tension testing, nanomicrospheres, referred to as PER, were prepared with a suitable concentration. Their interfacial tension values were tested using a rotating drop ultra-low interfacial tensiometer under different temperature and mineralization conditions.

2.4.3 Core replacement and oil repellent performance testing

(1) A 3% nanomicrosphere dispersion was prepared according to experimental requirements. For assessments, the prepared core was first vacuumed and saturated with water. Subsequently, its porosity (ø) was calculated based on its length, diameter, dry weight, and wet weight. Thereafter, water was passed through the core at a flow rate of 0.5 mL/min. Following this, its permeability was calculated using Darcy's law (1–1) once the injection pressure stabilized:

${K_1}=\frac{{{{10}^6}Q\mu L}}{{A\Delta P}}$ , (1–1)

where K₁ denotes the hydrometric permeability of the homogeneous core (10^− 3 µm²), Q represents the fluid flow rate (mL/min), µ represents the viscosity of water (mPa·s), L denotes the length of the core (cm), A represents the cross-sectional area of the core (cm²), and ∆P denotes the pressure difference across the core (kPa).

The pore throat diameter was estimated using the Kozeny–Carman Eq. (1–2):

${d_P}=2\tau \sqrt {{\raise0.7ex\hbox{${8000k}$} \!\mathord{\left/ {\vphantom {{8000k} \phi }}\right.\kern-0pt}\!\lower0.7ex\hbox{$\phi $}}}$ , (1–2)

where dp denotes the pore throat diameter (µm), and τ represents tortuosity (dimensionless, with a value of 2.5, as reported in a previous study) [18]. Once the water drive pressure stabilized under the given flow rate, the nanomicrosphere dispersion was injected into the core with an injection volume of 10 times the pore volume. The pressure at each point was recorded, and the permeability of the core after nanomicrosphere injection (K₂) was measured. The blocking rate (η) was then calculated as follows:

$\eta =\frac{{{K_1} - {K_2}}}{{{K_1}}} \times 100\%$ . (1–3)

(2) In the core replacement experiment, the core was first dried in an oven and fixed in a core holder. It was then saturated with distilled water until the pressure stabilized. Subsequently, the core was filled with crude oil and allowed to saturate until the pressure stabilized again. In this process, the amount of liquid discharged during crude oil injection was monitored to estimate the oil saturation of the core [18]. To determine the recovery rate, the oil within the core was replaced by distilled water until the volume of the discharged liquid stabilized, indicating successful replacement. Subsequently, the core was injected with a 3% nanomicrosphere solution at an injection volume of 2.5 times the core pore volume. After injection, the change in fluid volume was closely monitored to check for further fluid displacement. Once the system pressure stabilized, indicating uniform distribution of the nanomicrosphere solution within the core, distilled water was injected into the core. During this injection, the volume and water content of the outgoing fluid were measured. Once the water content of the total fluid, containing both oil and water, exceeded 98%, indicating that most of the residual oil had been displaced by water, the experiment was terminated. The recovery rate was then calculated by comparing the original volume of oil with its final volume.

3.1 Characterization of Polymers

3.1.1 Fourier transform infrared (FTIR) spectroscopy

Figure 2(a) depicts the FTIR spectra of AM, AMPS, the REQ monomer, and PER nanomicrospheres. Here, curve a reveals several important absorption peaks. For instance, peaks corresponding to the stretching vibrations of the N-H bonds in the amino group (-NH₂) are located at 3355 cm^− 1 and 3195 cm^− 1. The stretching vibration of the carbonyl group in the amide moiety (C = O) presents a strong absorption peak at 1678 cm^− 1. Furthermore, the characteristic absorption peak of the stretching vibration of the C = C double bond appears at 1608 cm^− 1. Meanwhile, the characteristic absorption peaks of the out-of-plane bending vibrations of the unsaturated double bond appear at 971 cm^− 1 and 820 cm^− 1. Moving to curve b, the absorption peak at 3038 cm^− 1 corresponds to the stretching vibration of the C-H bond in the unsaturated C = C double bond. Meanwhile, the characteristic absorption peaks of the out-of-plane bending vibrations of the unsaturated double bond appear at 950 cm^− 1 and 837 cm^− 1. Furthermore, the antisymmetric stretching vibration of the sulfate group (O = S = O) forms two peaks at 1238 cm^− 1 and 1095 cm^− 1. Curve c indicates the absence of the characteristic peak of isocyanate (-NCO) at 2277 cm^− 1, suggesting its complete consumption during the reaction. Meanwhile, the appearance of the characteristic peak of stearyl alcohol-polyoxyethylene ether (C-O-C) at 1103 cm^− 1 confirms the successful synthesis of the functional monomer REQ. Curve d reveals the stretching vibration absorption peaks of the N-H bonds in the amide group at 3354.06 cm^− 1 and 3201.69 cm^− 1. Meanwhile, the stretching vibration absorption peaks of the C-H bonds in methyl (-CH₃) and methylene (-CH₂) appear at 2929.74 cm^− 1. Furthermore, the stretching vibration absorption peak of the carbonyl group (C = O) in the amide moiety is observed at 1658.71 cm^− 1. In-plane bending vibration absorption peaks of methyl and methylene groups are apparent at 1452.33 cm^− 1. The bending vibration absorption peak of the C-O bond appears at 1319.25 cm^− 1, while its telescopic vibration absorption peak appears at 1125.52 cm^− 1. Additionally, the characteristic absorption peak of the C-S bond in the sulfonic acid moiety indicates that the desired molecular structure has been successfully achieved.

3.1.2 Environmental SEM microscopy testing

At 25℃, the surface morphologies of the PER nanomicrospheres, as illustrated in Fig. 2(c), display a relatively uniform particle size distribution, with full-sized and regular spheres. Internally, the nanomicrospheres exhibit moderately branched long polymer chains, with an ordered arrangement on the microscopic scale. This ordered structure endows the nanomicrospheres with desirable mechanical properties and stability. However, when brine is used as the solvent, as depicted in Fig. 2(d), the amide groups within the nanomicrospheres undergo hydrolysis, gradually transforming into carboxyl groups. Owing to electrostatic repulsion among the carboxyl groups, the polymer chains within the nanomicrospheres stretch further, resulting in volume expansion of the nanomicrospheres and consequently a fuller appearance. As the temperature increases to 60°C, the thermal movement of water molecules intensifies, facilitating the entry of water molecules into the nanomicrospheres and their subsequent diffusion into the interior. At even higher temperatures, partial stacking bonding occurs between the polymer chains within nanomicrospheres (Fig. 2(e)), likely owing to changes in intermolecular forces resulting from aggravated thermal motion. Owing to this stacking bonding, the nanomicrospheres quickly achieve swelling equilibrium, demonstrating rapid volume expansion and strong water absorption. At the molecular level, the surface morphologies of the PER nanomicrospheres in the saline solution at 60°C change considerably (Fig. 2(f)). For instance, the carboxyl groups on the surfaces of the nanomicrospheres appear more active at elevated temperatures, enhancing their interaction with water molecules. This interaction further promotes the swelling and inflation of the nanomicrospheres. Notably, the improved solubilization and swelling behaviors of the nanomicrospheres likely enhance their water-absorbing properties and swelling capacity. Interestingly, these changes at the microscopic level have a significant impact on the overall properties of the nanomicrospheres [19].

3.1.3 Thermal stability test

As depicted in Fig. 2(d), the thermal decomposition of the PER nanomicrospheres occurs in three distinct stages. In the first stage (at approximately 278 ℃), the sulfonic acid and amide groups within the polymers exhibit strong hydrophilicity. As the temperature increases, water from these groups volatilizes, causing partial decomposition of the amide groups and a consequent loss of weight. The second stage (278 ℃-353 ℃) involves the imidation reaction of amide groups and the thermal decomposition of the hydrophobic side chains. During this period, any unreacted residual monomers are consumed. In the third stage (353 ℃-450 ℃), the primary decomposition of organic matter within the polymer ensues. For instance, the primary and branched chains of the polymer begin to disintegrate, while impurities generated by side reactions decompose. When the thermal decomposition temperature reaches 450 ℃, approximately 8.79% of PER nanomicrospheres remain. Notably, the functional monomer REQ strengthens intermolecular bonding, effectively improving the thermal stability of the nanomicrospheres.

3.1.4 Laser particle size test

The particle sizes of the PER nanomicrospheres were measured at various aging times using a nanoparticle size analyzer. As depicted in Fig. 3(a), the initial particle sizes of the PER nanomicrospheres are normally distributed between 295 and 469 nm, with an average size of 336 nm. This distribution indicates that the nanomicrospheres are uniformly dispersed, with a narrow size range. After 30 days of aging, the particle sizes of the PER nanomicrospheres appear to be normally distributed between 684 and 933 nm, with an average size of 785 nm. This indicates that the sizes of the PER nanomicrospheres increase to more than three times their original sizes after swelling (Fig. 3(b)). After 50 days of aging, the particle sizes of the PER nanomicrospheres are normally distributed between 971 and 1,341 nm, with an average particle size of 1,035 nm, again demonstrating significant swelling to over three times the original size (Fig. 3(c)). Upon reaching 70 days of aging, the particle sizes of the PER nanomicrospheres are normally distributed between 1,296 and 2,389 nm, with an average particle size of 2,117 nm (Fig. 3(d)). The size distribution appears narrow, and the product is still uniformly dispersed. At 90 days of aging (Fig. 3(e)), the particle sizes are normally distributed between 2,370 and 2,771 nm, with an average particle size of 2,510 nm. During this period, the PER nanomicrospheres exhibit less pronounced swelling multiplicity owing to the deformation of their spherical surfaces, along with the rupturing of some long molecular chains. If aging continues beyond this point, the PER nanomicrospheres are anticipated to transform from spherical particles into irregular, shuttle-like molecular fragments, with the solution viscosity approaching that of water.

3.2 PER Nanomicrosphere Performance Testing

3.2.1 Expansion performance test

As depicted in Fig. 3(f), the particle size of the PER emulsion increases gradually over time, reflecting a corresponding increase in its expansion rate. Initially, when exposed to pure water, the emulsion absorbs water more rapidly, resulting in a higher expansion rate. This rapid absorption is attributed to the formation of hydrogen bonds between the water molecules and polymer chains within nanomicrospheres. With increased water absorption, the particle sizes of the nanomicrospheres also increase. However, over time, as the system approaches saturation, the expansion rate of the nanomicrospheres gradually decreases. In contrast, when the emulsion is exposed to salt water, the ionic electrostatic effect inhibits the diffusion of water molecules into the nanomicrospheres. Over time, as the system saturates, the expansion rate of the nanomicrospheres gradually decreases. Overall, the expansion rate of the nanomicrospheres in brine remains lower than that in pure water. The introduction of salt-resistant sulfonic acid groups during the polymerization of PER nanomicrospheres helps mitigate some of the negative effects of the ionic expansion. Over time, these sulfonic acid groups chelate with ionic charges, mitigating the impact of electrostatic repulsion. Consequently, the expansion rate in mineralized water is similar to that in pure water, indicating that the nanomicrospheres possess good salt resistance. The expansion factor and rate are important parameters for evaluating the effectiveness of nanomicrospheres in sealing voids in formations. A high expansion rate implies that nanomicrospheres can significantly expand in volume upon contact with water, effectively filling pore spaces and spreading uniformly and controllably throughout the formation. These properties prevent transport problems associated with excessively rapid expansion or uneven sealing.

3.2.2 Viscoelasticity test

The viscoelastic frequency scanning test of the PER nanomicrospheres, as depicted in Fig. 4(a, b), elucidates their mechanical responses to various oscillation frequencies. The results indicate that the energy storage modulus (reflecting the elasticity) of the PER nanomicrospheres consistently exceeds the loss modulus (reflecting the viscosity of the nanomicrospheres). This suggests that the structures of the PER nanomicrospheres remain stable throughout the testing process, exhibiting more pronounced elastic properties compared to viscous properties. The high elasticity of the PER nanomicrospheres allows them to deform under pressure, enabling them to traverse smaller pore throats without immediately clogging them. However, if the pores are smaller than the nanomicrospheres in their most compressed state, the nanomicrospheres cannot pass through. As depicted in Fig. 4(b), after prolonged oscillation at a relatively low frequency in saline water, the PER nanomicrospheres become easily deformed, primarily exhibiting a viscous effect. At higher frequencies, the molecules become temporarily entangled owing to a longer molecular rearrangement period compared to the oscillation period, establishing a temporary weak cross-linking network. During this phase, G″ dominates. However, as the oscillation period extends, the temporary three-dimensional network disintegrates, and the nanomicrospheres transition from a viscous state to an elastic state, with G′ becoming dominant. This behavior highlights the good deformability and deep transport capability of the nanomicrospheres. Overall, the synthesized PER nanomicrospheres can deform under external forces, effectively sealing pore throats and navigating through them. This capability allows them to be transported to deep parts of formations for effective mobilization. The sealing of the reservoir pore throat is dynamic rather than rigid. These properties make the PER nanomicrospheres potentially useful for subsurface transport and plugging applications, which can be tailored and optimized according to specific geological conditions and engineering requirements [20, 21].

3.2.3 Shear performance test

Rheology, the study of the flow and deformation behaviors of substances under stress, is crucial for predicting and optimizing the characteristics of nanomicrospheres in complex stratigraphic environments. Figure 4(c) illustrates the results of the rheological characterization of the PER nanomicrosphere dispersions employing a dynamic rheometer. The results reveal that as the mass fraction of the nanomicrospheres increases, the viscosity of the PER solution also increases. This increase is attributed to enhanced interactions among the nanomicrospheres and the formation of a network structure. However, the viscosity of the solution decreases with increasing shear rate, a phenomenon known as shear thinning. This behavior is common in polymer-conditioned drive systems and occurs owing to the disintegration and rearrangement of polymers. In formation pipelines, nanomicrospheres are subjected to shear forces that lower their viscosity, allowing them to flow more easily and penetrate deeper into the formations.

3.2.4 Interfacial tension test

As depicted in Fig. 4(d), the interfacial tension of the nanohydrogel microspheres decreases as the concentration of the PJO aqueous solution increases, indicating that the nanomicrospheres themselves possess low interfacial tension. Figure 4(e) reveals that at higher temperatures, the particles require longer to reach a low interfacial tension value, with the final interfacial tension potentially falling below 1.0 × 10^− 3 mN/m. This suggests that temperature minimally affects the final interfacial tension value. Figure 4(f) presents the interfacial tension of the nanomicrospheres under different mineralization degrees. As the mineralization degree increases, the time required for the nanomicrospheres to reach a low interfacial tension value also increases. Nonetheless, within 15 min, the interfacial tension of the nanomicrospheres falls below 1.0 × 10^− 3 mN/m, indicating that the mineralization degree also minimally affects the interfacial tension of nanomicrospheres (Fig. 4(f)). Tests reveal that the interfacial tension of the nanomicrosphere solution decreases rapidly upon dilution with water. Overall, although higher mineralization degrees and varying temperatures can slightly affect the time required to reach low interfacial tension, all final products consistently exhibit extremely low interfacial tension. These results demonstrate that the nanomicrospheres perform well in maintaining low interfacial tension values, essential for effective injection, mining, and driving operations.

3.2.5 Core replacement and oil repellent performance test

Figure 5(a) depicts the core flow characteristic curve. Initially, upon stabilization of the injected water pressure, the injection of the nanomicrosphere solution begins, causing the injection pressure to rise. Further, with the continuous injection of nanomicrospheres, the pressure curve gradually stabilizes. During the next water injection operation, the pressure drops abruptly. However, after injecting a certain amount of nanomicrospheres, the pressure stabilizes again. This pattern indicates that the nanomicrosphere solution exhibits good injectability and propagation without causing any clogging during the injection process. Following this emulsion injection, the permeability value of the simulated core decreases from 10.02 mD to 0.97 mD, with its calculated blocking rate (η) reaching 90.32%. This suggests that the injected PER nanomicrospheres effectively block the voids within the core, improving its stability. Figure 5(b) illustrates the oil droplet stripping process. When the nanomicrosphere dispersion is added, the outer edge of the annular contact line of the oil droplet first contracts rapidly and subsequently stabilizes, causing the oil droplet to transition from an irregular spherical shape to a regular spherical form. As the nanomicrosphere dispersion diffuses into the three-phase region, it forms a nanofilm, as illustrated in Fig. 5(c). This nanofilm assists in peeling the oil droplets from the rock surface, further improving the recovery rate. Overall, these results offer valuable insights into the injectivity, propagation, and containment properties of PER nanomicrospheres [22].

Figure 5(d) illustrates the results of experiments assessing the oil-drive-blocking performance of the PER nanomicrospheres. Notably, a significant enhancement in oil drive efficiency is apparent when comparing the primary and secondary water drives. Specifically, the oil drive efficiency of the primary water drive is only 19.1%; however, following the injection of PER nanomicrospheres, the oil drive efficiency of the secondary water drive significantly increases to 49.3%, demonstrating a significant rise of 30.2%. To investigate the mechanism underlying this increase in efficiency, cores injected with nanomicrospheres were carefully examined under a microscope, as depicted in Fig. 5(e, f). The observations reveal that the nanomicrospheres form a stable reticulated structure within the core, creating a distinct retention zone within large pores. This structure increases the resistance of the core to subsequent water drives and directs fluid flow from larger pores into smaller pores within the core. This steering effect significantly extends the reach of the water drive, allowing for the recovery of crude oil from tiny pores that would otherwise be difficult to access.

Using AM and AMPS as the molecular backbone, REQ as the functional monomer, and EGDMA as the cross-linking agent, PER nanomicrospheres are prepared through two-phase aqueous dispersion polymerization. These nanomicrospheres exhibit normally distributed particle sizes between 295 nm and 469 nm, with an average size of 336 nm. Furthermore, the structures and morphologies of these nanomicrospheres are characterized through infrared absorption spectroscopy and SEM analyses. Thermal stability analysis reveals that the nanomicrospheres maintain good thermal stability, with structural changes occurring only at a temperature of 278°C.

Water-absorption and swelling performance tests reveal that the nanomicrospheres exhibit excellent salt resistance, retaining their water-absorption and swelling capabilities even in 5,000 ppm standard brine. Viscoelasticity tests reveal that the G′ value of the nanomicrospheres exceeds their G″ value, indicating good viscoelastic properties. Furthermore, shear resistance tests reveal that the viscosity retention rate of the PER nanomicrospheres in water exceeds 80% across different mass fractions. Moreover, the introduction of the functional monomer changes the polymer structure from an entangled-molecular-chain form to one with improved adhesion. This adjustment helps the nanomicrospheres resist shear in the pore throats of the formation and facilitates their transport to deeper parts of the formation for effective plugging. Finally, indoor simulations of nanomicrosphere transport and plugging behaviors reveal that at a 3 wt.% concentration, the PER nanomicrospheres achieve a plugging rate (η) of 90.32%, increasing the oil recovery rate by 30.2%. This significant improvement is primarily attributed to the ability of functional monomers to lower interfacial tension between oil and water, promote crude oil emulsification, and generate stripping forces at the three-phase oil-water-rock interface. These effects, in turn, improve the mobility of the oil phase and increase oil repulsion efficiency. Consequently, the PER nanomicrosphere plugging technology can significantly enhance oil recovery from reservoirs, offering a new solution for efficient and economic oil extraction.

Funding

This research was supported by the Qin-Chuangyuan "Scientist and Engineer" Team Construction Project (2024QCY-KXJ-052), Key R&D Program of Shaanxi Province (2024GX-YBXM-393), Industrialization Project of Shaanxi Provincial Education Department (23JC008), and Science and Technology Program of Xi’an, China (22GXFW0014). The funding agencies had no role in the study design, collection, analysis, interpretation of data, report writing, and the decision to submit the article for publication.

Author Contributions

Wenwen Yang: Conceptualization; Data curation; Roles/Writing - original draft

Xiaojuan Lai: Funding acquisition; Methodology；

Lei Wang: Investigation; Project administration；

Huaqiang Shi: Software;

Haibin Li: Supervision;

Jiali Chen: Validation; Visualization；

Xin Wen:Formal analysis;

Yulong Li:Data curation;

Xiaojiang Song:Writing;

Wenfei Wang:review & editing.

Data Availability Statement

All of the material is owned by the authors and no permissions are required.

Conflicts of Interest

There are no conflicts of interest to declare.

Institutional Review Board Statement

Not applicable.

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Synthesis of Polyacrylamide Nanomicrospheres Modified with a Reactive Carbamate Surfactant for Efficient Profile Control and Plugging

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1 Materials

2.2 Equipment

2.3. Synthesis of the REQ monomer and PER nanomicrospheres

2.4. Characterization and Performance Testing

2.4.1 Characterization

2.4.2 Viscoelasticity and interfacial tension testing

2.4.3 Core replacement and oil repellent performance testing

3. Results and Discussion

3.1 Characterization of Polymers

3.1.1 Fourier transform infrared (FTIR) spectroscopy

3.1.2 Environmental SEM microscopy testing

3.1.3 Thermal stability test

3.1.4 Laser particle size test

3.2 PER Nanomicrosphere Performance Testing

3.2.1 Expansion performance test

3.2.2 Viscoelasticity test

3.2.3 Shear performance test

3.2.4 Interfacial tension test

3.2.5 Core replacement and oil repellent performance test

4. Conclusions

Declarations

References

Status:

Version 1