Experimental study on injectability and plugging performance of fuzzy-ball fluid in porous media

Fuzzy-ball fluid has been widely used in the field of petroleum engineering due to its excellent temperature resistance, salt tolerance, low damage, and environmental friendliness. However, its good plugging performance leads to poor injectability. How to balance injectability and plugging performance has become an urgent problem to be solved. The injectability and plugging performance of fuzzy-ball fluid in cores under conditions of different matching factors and different injection rates were studied through core displacement experiments. First, 4 pressure measuring points were arranged on the core holder, and then simulated formation water, fuzzy-ball fluid, and simulated formation water were injected sequentially. During the above three injection processes, the average resistance coefficient and average residual resistance coefficient of each core segment were calculated respectively, so as to evaluate the effects of different matching factors and different injection rates on injectability and plugging performance. The results show that when the average resistance coefficient and average residual resistance coefficient are used as evaluation criteria, the optimal matching factor range is 1.08–4.43, and at this time, the fuzzy-ball fluid exhibits good injection performance and excellent deep plugging capacity. By adjusting the average particle size of fuzzy-ball vesicles, the fuzzy-ball fluid can be matched with formations with a permeability of 500mD–2000mD. The experimental results provide fundamental insights into the injection parameters of fuzzy-ball fluid that may inform future oil displacement and production enhancement studies.

Fuzzy-ball fluid, invented and named by Zheng Lihui, is a foam-like two-phase fluid composed of fuzzy-ball base fluid and fuzzy-ball vesicles (Zheng et al., 2010). Compared with ordinary foam and micro-foam, there is no Plateau border between fuzzy-ball vesicles. The particle size of the vesicles ranges from 10 to 100 μm, and they have an inner structure of “one core, two layers and three membranes” as well as an outer structure with villi, endowing the fuzzy-ball fluid with excellent stability.

Through PVT experiments, Wang Jinfeng et al. found that within the ranges of pressure from 1 to 20 MPa and temperature from 30 °C to 130 °C, the density variation of the fuzzy-ball fluid was less than 10% (Wang et al., 2013), (Wang et al., 2012). Fu (2018) found that the variation ranges of rheological parameters such as plastic viscosity and apparent viscosity of fuzzy-ball fluid were all less than 17% at three high temperatures of 150 °C, 180 °C and 200 °C. The most obvious change occurred at 200 °C, where the plastic viscosity decreased from 31.5 mPa ·s to 26 mPa ·s and the apparent viscosity dropped from 57 mPa s to 51 mPa s, revealing its excellent temperature resistance. (Fu, 2018). Zhu et al., 2016 found that the fuzzy-ball fluid exhibited good oil stabilization and water control functions in formation water with two salinities (8 × 10 4 mg/L and 4 × 10 4 mg/L), indicating that the fuzzy-ball fluid has excellent salt-tolerant stability (Zhu et al., 2016). Zeng et al. (2018) used fuzzy-ball fluid as the base fluid and compounded it with a desulfurizer to obtain a hydrogen sulfide (H 2 S)-resistant fuzzy-ball fluid. After aging for 48 h at 130 °C and with an H 2 S partial pressure of 2.6 MPa, the properties of the fuzzy-ball fluid, such as apparent viscosity, plastic viscosity, and dynamic shear force, changed slightly (Zeng et al., 2018).

It is evident that the fuzzy-ball fluid has excellent stability against temperature, mineralization and hydrogen sulfide. Based on the above-mentioned stability, the plugging performance of the fuzzy-ball fluid is far superior to that of ordinary foam and micro-foam. Through a large number of experimental studies, Zheng et al. (2018) inferred that there may be three mechanisms for the fuzzy-ball fluid to plug leakage channels, namely, the partial pressure mechanism, the pressure consumption mechanism and the pressure support mechanism (Zheng et al., 2018).

In addition, the fuzzy-ball fluid also has advantages such as low reservoir damage and good environmental friendliness. Therefore, it has been widely applied in engineering fields including drilling (Hu et al., 2013); (Cui et al., 2013), well workover (Liu et al., 2021); (Li et al., 2011), fracturing (Zheng and Weng, 2015); (Jiang et al., 2019), acidizing (Wen et al., 2015), and gas stabilization and water control (Meng, 2021), and has achieved favorable results. (Wei et al., 2020) evaluated the flow characteristics of fuzzy-ball fluid in heterogeneous reservoirs. (Al-Ibad and Civan, 2013) analyzed the plugging performance of gel particles in the near-wellbore zone.

Nevertheless, compared with conventional foam and micro-foam, fuzzy-ball fluid possesses distinctive vesicular microstructure, elastic deformability and adaptive pore-throat plugging characteristics. It exhibits prominent advantages in deep profile control, high-permeability channel plugging, water channeling inhibition and reservoir heterogeneity improvement, serving as a promising profile control and flooding agent in mature high water-cut oilfields. Most existing studies mainly focus on its plugging capacity as well as temperature and salinity resistance, while ignoring its practical engineering defect. Its strong pore-throat blocking effect greatly increases seepage resistance, resulting in poorer injectivity than traditional foam systems. Laboratory core flooding tests demonstrate that the maximum injection pressure of fuzzy-ball fluid can reach 20 MPa, which exceeds the rated working pressure of conventional field injection equipment (Li and Xia, 2011). This easily leads to difficult injection and failure of deep migration of the agent, greatly restricting the large-scale field application of this technology.

At present, few studies have systematically revealed the coupling mechanisms among vesicle particle size, injection rate, injectivity and plugging performance. The optimal matching relationship between fuzzy-ball size and reservoir pore-throat medium, as well as the supporting injection parameter optimization system have not been established. Consequently, excellent laboratory flooding performance cannot be well applied in field practices, forming a technical gap between theoretical research and field application.

In view of the above problems, this paper systematically investigates the injectivity and plugging performance of fuzzy-ball fluid under different particle sizes and injection rates. The influencing mechanisms of key parameters on fluid seepage law and pore-throat plugging principle are clarified, and the optimal matching mode between fuzzy-ball microstructure and reservoir medium is determined. This study can effectively solve the problem of high field injection pressure, provide theoretical basis and technical guidance for balancing injectivity and plugging performance, optimizing construction parameters. It is of important academic significance and engineering value for promoting the large-scale and efficient application of this novel profile control technology in high water-cut oil reservoirs.

Material preparation

1. Preparation of simulated formation water

The water used in the experiment was simulated formation water, and the composition of inorganic salt ions contained is shown in Table 1. 2. Preparation of fuzzy-ball fluid

The fuzzy-ball fluid consisted of four types of treatment agents, namely, fuzzy-ball coating agent, fuzzy-ball floss agen, fuzzy-ball membrane agent, and fuzzy-ball core agent, as shown in Figure 1. The fuzzy-ball coating agent was composed of hydroxypropyl guar gum and carboxymethyl starch, serving to increase the viscosity of the system. The fuzzy-ball floss agen was made up of xanthan gum and polyanionic cellulose, which functioned to enhance the system viscosity and form a fuzzy structure. The fuzzy-ball membrane agent consisted of dodecanol, ethanol, and triethanolamine, playing a role in forming a stable “membrane” structure. The fuzzy-ball core agent was composed of lauryl dimethyl amine oxide, sodium dodecyl sulfonate, and polyoxyethylene octylphenol ether, which was used to form a “gas core” structure.

Five levels were set for each treatment agent, and 25 groups of experimental formulations were optimized and designed. Each group of fuzzy-ball fluid was photographed under an optical microscope, and the particle sizes of 100 vesicles were measured for each group. Then, the number of fuzzy-ball in different particle size ranges was counted, as shown in Figure 2. The particle size range of fuzzy-ball was 0–450 μm. Among them, the proportion of fuzzy-ball with a particle size of 0–200 μm exceeded 80%. The particle size distribution of fuzzy-ball conformed to the Gauss Amp distribution.

In the subsequent injectivity and plugging performance experiments, the fuzzy-ball fluid formulation was selected as 1.4wt% fuzzy-ball coating agent +0.9wt% fuzzy-ball floss agent +0.4wt% fuzzy-ball membrane agent +0.1wt% fuzzy-ball core agent, with an average particle size of 131.3 μm for the fuzzy-ball in this formulation. 3. Core preparation

Seven pieces of one-dimensional artificial homogeneous sandstone with a specification of Φ25 mm × 300 mm were used as experimental samples for the fuzzy-ball fluid core injection experiment. The porosity and permeability of seven core samples are shown in Table 2.

Experimental methods

Fluid injectivity and plugging capacity were characterized by resistance coefficient (RC) and residual resistance coefficient (RRC) respectively. Multi-point pressure measurement core flooding tests were carried out to screen optimal injection parameters, with test device and procedure illustrated in Figures 3, 4.

Four pressure monitoring ports divide the core into three segments. Real-time pressure data was recorded to calculate sectional RC and RRC, and further evaluate fluid seepage and plugging behaviors. Detailed test procedures are listed below: Measure core gas permeability, then vacuum and saturate core with simulated formation water. Pore volume and porosity are calculated based on core mass variation and water density. (2) Load core into the holder and preserve samples at 57 °C constant temperature for 2 h. (3) Inject 1 PV formation water at fixed flow velocity, keep confining pressure 2.5 MPa above injection pressure. Record pressure and outlet liquid yield every 5 min (4) Inject 2 PV fuzzy-ball fluid following the same operating parameters. (5) Conduct subsequent water flooding of 2 PV with identical injection conditions. (6) Adjust injection velocity under fixed matching factor, repeat above tests to analyze velocity influence on fluid performance.

Matching factor refers to the ratio of average fuzzy-ball size to core average pore diameter. This parameter was adopted to reveal the size-pore matching law of fuzzy-ball fluid in porous media. The expression of the matching factor is shown in Equation 1. where δ represents the matching factor, dimensionless; d f represents the average particle size of fuzzy-ball, μm; d c represents the average pore diameter of the core, μm.

Injectivity and plugging performance under different matching factors

With the injection rate kept constant at 1.5 mL/min and the average particle size of fuzzy-ball fixed at 131.3 μm, the matching factor was adjusted by changing the permeability of the artificial sandstone core. The injection pressure curves of fuzzy-ball fluid under different matching factor conditions are shown in Figure 5.

The injection pressure curve of the fuzzy-ball fluid with a matching factor of 6.61 is shown in Figure 5a. During the injection of the fuzzy-ball fluid, the pressure variation at Pressure Measuring Point 1 exhibits significant fluctuation characteristics. This indicates that the fuzzy-ball presented a behavior of alternating plugging and migration during the injection process. The observed pressure fluctuation behavior is consistent with the alternating plugging and migration mechanism reported for deformable particle systems in porous media (Li et al., 2016). Similar pressure oscillation phenomena have been observed in polymer gel particle transport studies, where particles periodically accumulate at pore throats and then are forced through under continuous injection pressure. This indicates that the fuzzy-ball fluid behaves similarly to other deformable particulate systems, where the competition between particle deformation capacity and pore-throat dimensions governs the macroscopic injection pressure response. In contrast, the pressures at Pressure Measuring Points 2 and 3 were relatively low with small fluctuation amplitudes. These results suggest that during the injection of the fuzzy-ball fluid, the pressure difference was mainly concentrated in the front section of the core, meaning the fuzzy-ball fluid primarily plugged the front section and was difficult to migrate deep into the core to form effective plugging.

In the subsequent water flooding process, the pressure at Pressure Measuring Point 1 began to decrease after a brief increase, while the pressures at Pressure Measuring Points 2 and 3 decreased slightly with small fluctuation amplitudes. Specifically, the pressure difference in the front section of the core decreased significantly, whereas the pressure differences in the middle and rear sections showed no obvious changes. This implies that under the displacement effect of subsequent water injection, the fuzzy-ball migrated deep into the core, but the migration distance did not reach Pressure Measuring Point 2. It is thus indicated that when the matching factor is 6.61, the migration performance of the fuzzy-ball in the core is poor.

The injection pressure curve of the fuzzy-ball fluid with a matching factor of 3.06 is shown in Figure 5b. At the initial stage of fuzzy-ball fluid injection, the pressure at Pressure Measuring Point 1 increased rapidly, first plugging the front section of the core. With the continuous injection of the fuzzy-ball fluid, the fuzzy-ball gradually migrated into the core, leading to pressure buildup at Pressure Measuring Points 2 and 3. Although the pressure difference was still mainly concentrated in the front section of the core, the pressure differences in the middle and rear sections increased significantly compared with those when the matching factor was 6.61. This shows that as the matching factor decreases, the injectivity of the fuzzy-ball fluid is improved. In the subsequent water flooding process, the pressures at all three pressure measuring points showed a downward trend, with a larger decrease amplitude at Pressure Measuring Point 1 and smaller decrease amplitudes at Pressure Measuring Points 2 and 3. That is, the pressure difference in the front section of the core decreased significantly, while the pressure differences in the middle and rear sections showed no obvious changes.

The injection pressure curve of the fuzzy-ball fluid with a matching factor of 1.60 is shown in Figure 5c. During the injection of the fuzzy-ball fluid, the pressures at all three pressure measuring points showed a gradual increasing trend, with the maximum pressure at Pressure Measuring Point 1 reaching 10.9 MPa. Although the pressure also exhibited fluctuations, both the fluctuation amplitude and frequency were reduced. Compared with the cases of matching factors 6.61 and 3.06, the pressure difference in the front section of the core decreased, while the pressure differences in the middle and rear sections increased. This indicates that the fuzzy-ball fluid not only had good plugging performance but also further improved injectivity. In the subsequent water flooding process, driven by the subsequent water, the fuzzy-ball migrated deep into the core. The pressure fluctuations at all three pressure measuring points decreased, and the pressure differences in the front, middle, and rear sections of the core all decreased gradually.

The injection pressure curve of the fuzzy-ball fluid with a matching factor of 1.08 is shown in Figure 5d. During the injection of the fuzzy-ball fluid, the pressures at all three pressure measuring points showed a gradual increasing trend, with the maximum pressure at Pressure Measuring Point 1 reaching 8.3 MPa. Compared with the case of a matching factor of 1.60, both the fluctuation amplitude and frequency were further reduced. Meanwhile, the pressure difference in the front section of the core further decreased, and the pressure differences in the middle and rear sections further increased, indicating a further improvement in the injectivity of the fuzzy-ball fluid in the core. Under the effect of subsequent water flooding, the fuzzy-ball fluid migrated deeper into the core. The pressure fluctuations at all three pressure measuring points decreased, and the pressure differences in the front, middle, and rear sections of the core all decreased gradually.

The above experimental results indicate that the matching factor is an important factor affecting the injectivity and plugging performance of the fuzzy-ball fluid. Only when the matching factor is within a certain range can the fuzzy-ball fluid simultaneously possess good injectivity and plugging performance. During the injection process, fuzzy balls undergo alternating plugging and migration, causing fluctuating injection pressure and hindering the accurate determination of matching factor parameters. Therefore, the average Resistance coefficient (ARC) during the injection of the fuzzy-ball fluid and the average residual Resistance coefficient (ARRC) during the subsequent water injection are used to further evaluate the effect of the matching factor on the injectivity and plugging performance of the fuzzy-ball fluid. The ARC and ARRC are defined by Equations 2,3, as follows: where: ARC—average Resistance coefficient; ARRC—average residual Resistance coefficient; Δ P i —pressure difference between two adjacent pressure measuring points along the core holder during the injection of fuzzy-ball fluid, MPa; Δ P w 1 j —pressure difference between two adjacent pressure measuring points along the core holder during water injection before the injection of fuzzy-ball fluid, MPa; Δ P w 2 k —pressure difference between two adjacent pressure measuring points along the core holder during subsequent water injection after the injection of fuzzy-ball fluid, MPa; n—number of pressure records during the injection of fuzzy-ball fluid; m—number of pressure records during the initial water injection; t—number of pressure records during the subsequent water injection. Based on Equations 2, 3, the ARCs and ARRCs of the front, middle, and rear sections of the core during the injection of fuzzy-ball fluid and subsequent water injection under different matching factor conditions were calculated respectively, and the results are shown in Figure 6.

The ARC values of the core front, middle, and rear segments under different matching factors are presented in Figure 6a. The matching factor exhibits a significant regulatory effect on the injectivity of fuzzy-ball fluid within porous media. As the matching factor increases, the ARC of the front segment increases monotonically, while the middle-segment ARC rises first and subsequently declines, and the rear-segment ARC decreases continuously. This evolutionary trend demonstrates that the increase in matching factor impairs the overall injectivity of fuzzy-ball fluid. At a matching factor of 1.08, the ARC values of the three core segments are basically equivalent, indicating that fuzzy-ball fluid can achieve uniform and effective migration in porous media with favorable injectivity. Within the matching factor range of 1.08–4.43, the front-segment ARC is higher than that of the single middle and rear segments and approximates the sum of the ARCs of the latter two segments. This phenomenon indicates that fuzzy-ball fluid can efficiently propagate toward the deep core zone while maintaining a moderate flow resistance, achieving a favorable balance between injectivity and in-depth plugging performance. When the matching factor exceeds 4.43, the front-segment flow resistance is far greater than the total resistance of the middle and rear segments, which severely restricts the deep migration of fuzzy-ball fluid and results in deteriorated injection performance.

Figure 6b illustrates the ARRC distribution under various matching factors, reflecting the spatial heterogeneity of plugging resistance induced by fuzzy-ball retention. With an increasing matching factor, the front-segment ARRC increases steadily, the middle-segment ARRC presents an initial increase followed by a decrease, and the rear-segment ARRC decreases gradually and eventually stabilizes. Such variation confirms that higher matching factors contribute to enhanced near-well plugging strength. At a matching factor of 1.08, the ARRC values of all core segments are low and nearly identical, suggesting that fuzzy balls can be thoroughly displaced to the deep core by subsequent water flooding but fail to form reliable in-depth plugging. For matching factors ranging from 1.08 to 4.43, the front-segment ARRC is moderately higher than the individual middle and rear-segment ARRCs and is comparable to their sum. This characteristic verifies that fuzzy-ball fluid can realize effective deep migration and establish stable residual flow resistance, thereby achieving efficient in-depth formation plugging. When the matching factor is greater than 4.43, the ARRC is predominantly concentrated in the front core segment, with a negligible resistance response in the middle and rear zones. This indicates that excessive matching factors inhibit the deep propagation of fuzzy balls and prevent the formation of valid in-depth plugging barriers.

Injectivity and plugging performance under different injection rates

With the average particle size of fuzzy-ball fixed at 131.3 μm and the core permeability fixed at 2000 mD, the effect of injection rate on injectivity and plugging performance was studied by changing the injection rate of the fuzzy-ball fluid. The injection pressure curves of the fuzzy-ball fluid under different injection rate conditions are shown in Figure 7.

As shown in Figure 7a, when the injection rate was 1.5 mL/min, the pressure at Pressure Measuring Point 1 increased rapidly. With the continuous injection of the fuzzy-ball fluid, the fuzzy-ball gradually migrated into the core, leading to pressure buildup at Pressure Measuring Points 2 and 3. The pressure difference was mainly concentrated in the front section of the core. During the subsequent water injection process, the pressures at all three pressure measuring points showed a downward trend, with a larger decrease amplitude at Pressure Measuring Point 1 and smaller decrease amplitudes at Pressure Measuring Points 2 and 3. That is, the pressure difference in the front section of the core decreased significantly, while the pressure differences in the middle and rear sections showed no obvious changes.

As shown in Figure 7b, when the injection rate was 1.0 mL/min, the pressures at all three pressure measuring points showed a gradual increasing trend during the injection of the fuzzy-ball fluid. Although the pressure difference was still mainly concentrated in the front section of the core, compared with the case of an injection rate of 1.5 mL/min, the pressure difference in the front section of the core decreased and the pressure difference in the middle section increased. This indicates that as the injection rate decreases, the injectivity of the fuzzy-ball fluid gradually improves. During the subsequent water injection process, driven by the subsequent water, the fuzzy-ball gradually migrated deep into the core. The pressures at the three pressure measuring points showed a characteristic of fluctuating decrease, and the pressure differences in the front, middle, and rear sections of the core all decreased gradually.

As shown in Figure 7c, when the injection rate was 0.5 mL/min, during the injection of the fuzzy-ball fluid, the pressure difference in the front section of the core further decreased, and the pressure differences in the middle and rear sections further increased. The pressure differences in the three sections gradually tended to be equal. This indicates that as the injection rate decreases, the injectivity of the fuzzy-ball fluid is further improved. During the subsequent water injection process, driven by the subsequent water, the fuzzy-ball gradually migrated deep into the core, and the pressure differences in the front, middle, and rear sections of the core all decreased gradually.

Based on Equations 2, 3, the ARCs and ARRCs of the front, middle, and rear sections of the core during the injection of fuzzy-ball fluid and subsequent water injection under different injection rate conditions were calculated respectively, and the results are shown in Figure 8.

As can be seen from Figure 8a, under different injection rate conditions, the injectivity of the fuzzy-ball fluid in the core pore medium varies. With the increase of the injection rate, the ARC of the front section of the core gradually increases, while the ARCs of the middle and rear sections tend to be stable. This indicates that as the injection rate increases, the injectivity of the fuzzy-ball fluid gradually decreases. When the injection rate is 0.5 mL/min, the migration speed of the fuzzy-ball is relatively slow, so the fuzzy-ball have sufficient time to deform and migrate in the core pores until they pass through the core pores. Furthermore, the fuzzy-ball fluid has good injectivity and can effectively migrate to the deep part of the core. When the injection rate is 1.0 mL/min, the time for the fuzzy-ball to deform and migrate in the core pores is shortened, which reduces the ability of the fuzzy-ball to pass through the core pores and thus decreases the injectivity of the fuzzy-ball fluid. Macroscopically, this is manifested as an increase in the ARC of the front section of the core and a decrease in the ARCs of the middle and rear sections. When the injection rate is further increased to 1.5 mL/min, the time for the fuzzy-ball to deform and migrate in the core pores is further shortened, and the ability of the fuzzy-ball to pass through the core pores is further reduced, leading to the retention and accumulation of fuzzy-ball in the pores. At this time, the plugging effect of the fuzzy-ball in the front section of the core is enhanced, and the injectivity is deteriorated, which is characterized by a higher ARC in the front section of the core and lower ARCs in the middle and rear sections.

The effect of injection rate on injectability can be interpreted through the dimensionless capillary number (Ca), which describes the ratio of viscous forces to interfacial tension forces. As injection rate increases, Ca increases, leading to higher viscous drag on the fuzzy-ball vesicles. This increased drag promotes vesicle deformation and passage through pore throats; however, it also enhances the probability of mechanical entrapment in narrower constrictions. Similar trade-offs have been reported for foam and microgel systems (Wang et al., 2021); (Thornhill and Zhu, 2009). The reduction in middle- and rear-section ARC at higher injection rates (Figure 8a) suggests that vesicles preferentially plug the front section under high-rate injection, a behavior attributed to the limited time available for pore-scale redistribution.

As can be seen from Figure 8b, with the increase of the injection rate, the ARRC of the front section of the core gradually increases and tends to be stable, while the ARRCs of the middle and rear sections gradually decrease and tend to be stable. Under different injection rate conditions, the ARRCs of the front, middle and rear sections of the core are all relatively high, indicating that the fuzzy-ball have good erosion resistance.

Analysis on plugging mechanism of fuzzy-ball fluid

To further clarify the cause of pressure fluctuation during injection, microscopic glass etching experiments were conducted to investigate the plugging mechanism of fuzzy-ball fluid.

As shown in Figure 9, fuzzy balls plug core pores via pressure dissipation when their particle size is no less than the pore diameter. This process consists of four stages. (a) Contact stage: Fuzzy-ball fluid migrates to the pore and makes contact with the inner pore wall. (b) Deformation stage: Fuzzy balls deform under injection pressure. Partial injection pressure is consumed during deformation, leading to pressure rise. (c) Penetration stage: Continuous injection pressure compresses and deforms fuzzy balls until their size matches the pore dimension, allowing them to enter the pore. Additional pressure is required to sustain deformation and overcome friction between fuzzy-ball surface and pore wall, which further increases injection pressure.(d) Separation stage: Fuzzy balls restore their original shape and keep moving forward after passing through the pore. A single fuzzy ball exerts limited pressure rise via pressure dissipation, yet the cumulative effect of numerous passing fuzzy balls remarkably elevates the overall injection pressure.

The pressure dissipation mechanism proposed in Figure 9 can be quantitatively related to the resistance coefficient (RC). The total pressure drop across a core segment ΔP (total) can be expressed as the sum of pressure drops due to: (i) viscous flow through pores ΔP (viscous), (ii) deformation of fuzzy-ball vesicles ΔP (deformation), and (iii) friction between vesicles and pore walls ΔP (friction). From our data, the front-segment ARC under matching factor 6.61 is approximately 3.8 times higher than that under matching factor 1.08, indicating that deformation and friction contributions dominate when vesicle size approaches or exceeds pore diameter. This is consistent with theoretical models of soft particle transport in confined channels (Thornhill and Zhu, 2009)).

As illustrated in Figure 10, fuzzy balls accumulate at pore inlets and block pore throats when their particle size is smaller than the pore diameter. The accumulation plugging mechanism illustrated in Figure 10 can be quantified by the ARRC. When the matching factor is below 1.08, the ARRC values are low and uniform across core segments (Figure 6b), suggesting that vesicles are too small to form stable aggregates. In contrast, when the matching factor exceeds 4.43, ARRC is concentrated in the front segment, indicating that aggregates form quickly but cannot propagate deeply. This intermediate range (1.08–4.43) represents a window where aggregate formation and disaggregation are balanced, enabling both plugging and migration. Such a balance has been similarly observed in preformed particle gel (PPG) systems (Wang et al., 2025).

This study focuses on balancing the injectivity and plugging performance of fuzzy-ball fluid under specific experimental conditions, investigating the matching relationship between fuzzy-ball particle size and core pore dimension, and determining applicable formation parameters and feasible injection schemes. The research findings can offer fundamental theoretical and experimental references for profile control and flooding applications of fuzzy-ball fluid within the tested working conditions. Major conclusions are summarized as follows: Displacement tests with multi-point sectional pressure monitoring verify that flow resistance coefficient and residual resistance coefficient can quantitatively assess the migration behavior, injectivity and pore plugging effect of fuzzy-ball fluid, and characterize the correlation between particle-pore matching degree and fluid seepage properties.

Based on comprehensive evaluation of injectivity and in situ plugging capacity, the optimal matching factor is determined within 1.08–4.43. Fluids in this scope achieve favorable migration ability and effective deep plugging effect under the adopted experimental environment.

Tuning the average particle size of fuzzy-ball vesicles can make the fluid adapt to formations with permeability of 500 mD to 2000 mD in this test system, mitigating the contradiction between favorable plugging performance and restricted injectivity. It should be noted that the conclusions are only applicable to the preset experimental conditions, and further verification is required for complex reservoir environments.