Cen Chena,
Seqiang Zhuob,
Songze Li*a,
Nanxin Yina,
Chao Luoa,
Hong Renc,
Min Jiad,
Xinyue Wanga and
Qun Chenge
aChongqing University of Science and Technology, China. E-mail: songze83@cqust.edu.cn
bGuangxi Shale Gas Exploration and Development Co., Ltd, China
cExploration and Development Research Institute, Zhongyuan Oilfield Company, China
dPetroleum Engineering Technology Research Institute of Jiangsu Oilfield, China
eJinFeng Laboratory, China
First published on 17th February 2025
In the process of drilling and development of tight sandstone reservoirs, a large number of external fluids can invade the pore structure of the reservoir due to its strong hydrophilicity, resulting in blockage of the pore structure and a decrease in oil and gas production capacity. To reduce the aqueous phase trapping damage of tight sandstone reservoirs, the SiO2@KH550/FC-A nano-composite system was prepared to construct hydrophobic surfaces in core samples. First, the surface of nano-SiO2 was modified by KH550 to prepare nanoparticles with good dispersion. FTIR, XRD, SEM, and TG were used to characterize the nanoparticles before and after modification. The fluorosurfactant FC-A was prepared in the laboratory and combined with SiO2@KH550 to produce the SiO2@KH550/FC-A composite system. The system increases the contact angle of the hydrophilic surface from approximately 20° to 130°, achieving wetting modification. Excellent stability of hydrophobicity was obtained, and the contact angle did not significantly decrease within 5 minutes. In contrast, using FC-A and SiO2@KH550 individually, the contact angle of the hydrophilic surface could not be increased to over 90°. The SEM results showed that after treatment with the composite system, a layer of micro–nanoscale particles was attached to the hydrophilic surface. It was proved that SiO2@KH550 and FC-A were adsorbed on the surface, forming a low surface free energy solid interface at the micro and nano scales, which greatly improved the surface hydrophobicity. Furthermore, after the composite system was used to treat tight sandstone cores aged at 100° for 16 hours, the amount of imbibition of the core samples treated with SiO2@KH550/FC-A significantly decreased from 2.6 mL of brine to only 0.5 mL after 8 hours. The core spontaneous imbibition rate was also reduced to 0.0004 g min−1 within 5 minutes, while the maximum brine water spontaneous imbibition rate was 0.27 g min−1. The core displacement experiment further showed that the fluid in the core pores can be more easily flowed back under gas displacement after wetting modification. The water saturation of the core samples decreased to 16.3% after displacement, and the core permeability recovered to 88.4%, indicating that the SiO2@KH550/FC-A composite system can significantly improve the liquid phase flowback ability.
At present, the method of adding surfactants to fluids is widely used.7–12 Surfactants can reduce the surface tension of fluids and improve the flowback efficiency, thereby reducing possible liquid phase trapping damage. However, the effect of surfactants is insufficient. In addition, surfactants are prone to foaming, resulting in the formation of a Jamin effect in the formation pores, which further reduces the permeability.8,9,11
Wetting inversion technology has been gradually proposed and applied in oil and gas exploitation.13–15 At present, wetting inversion is mainly achieved by chemical agents such as surfactants, nanofluids, and polymers. Cationic surfactants, cationic and non-ionic surfactant combined systems, and fluorinated surfactants can modify water-wet surfaces to neutral wetting surfaces.7–10,12–15 In addition to traditional surfactants, researchers continue to develop new surfactants for increased wettability and adaptability. For example, there is considerable temperature resistance and salt resistance exhibited by zwitterionic surfactants and Gemini surfactants,16,17 and they can play a role in complex reservoir environments.
Nanoparticles can form an adsorption layer on a rock surface and change the physical and chemical properties of the surface, so as to achieve altered wettability. For example, some metal oxide nanoparticles, such as silica and alumina, can adsorb on the reservoir rock surface and change its wettability.11,18,19 A common procedure researchers have used is the optimization of the material composition and properties of nanofluids. The wetting inversion effect and stability of nanofluids can be improved by adjusting the type, size, concentration, and other parameters of nanoparticles, as well as compounding with other chemical agents. New nanomaterials and nanofluid systems have also been developed to meet the application requirements under different reservoir conditions.20–24
A synergistic effect is obtained when a surfactant is mixed with other chemical agents or nanomaterials to improve the wetting alteration effect. For example, fluids mixed with surfactants and nanomaterials employ the wettability of surfactants, and also exhibit the structural and interface characteristics of nanomaterials, so as to better realize the wettability alteration of the reservoir. In this study, nano-silica powder was modified to improve the dispersion of nano-particles, and after it was mixed with fluorine-containing anionic surfactants, the wetting alteration ability of the composite system was then studied. Furthermore, by means of advanced microscopic characterization techniques, including scanning electron microscopy (SEM), the adsorption behavior and wettability change process of the wetting alteration agent on the rock surface were revealed, which provided strong support for the study of wetting inversion technology.
We selected typical tight sandstone for the wettability and core flow tests. The core characteristics are shown in the table below. A mica plate was used for the wettability evaluation and SEM test. Because mica is similar to the mineral component and wettability of the actual core samples, and its surface is slippery, it is easier to study the adsorption characteristics. Mica chips with the length and width of 1.5 cm × 1.5 cm were used after fresh stripping. The physical properties of core samples are described in Table 1.
Core sample | Porosity [φ]/% | Permeability [k]/md | Length [L]/cm | Diameter [D]/cm | Imbibition fluid used |
---|---|---|---|---|---|
1-1# | 6.64 | 0.0236 | 5.148 | 2.494 | SI in brine water |
1-2# | 7.28 | 0.0523 | 5.192 | 2.506 | SI in FC-A |
2-1# | 6.62 | 0.0285 | 4.496 | 2.502 | SI in SiO2@KH550 |
2-2# | 6.71 | 0.0332 | 5.190 | 2.504 | SI in SiO2@KH550/FC-A |
3-1# | 5.88 | 0.0765 | 5.044 | 2.502 | CD in brine water |
3-2# | 6.72 | 0.0662 | 5.124 | 2.506 | CD in FC-A |
4-1# | 7.13 | 0.0512 | 5.142 | 2.496 | CD in SiO2@KH550 |
4-2# | 5.97 | 0.0327 | 5.060 | 2.508 | CD in SiO2@KH550/FC-A |
SiO2@KH550 can be obtained by the surface modification method using the KH550 silane coupling agent. This method can significantly improve the dispersibility and stability of nano-silica in water-based media, and enhance its compatibility with organic polymer materials, so as to improve the performance of composite materials. KH550 is an organosilicon compound. By reacting the KH550 silane coupling agent with the hydroxyl group on the surface of nano-silica, the surface of nano-silica can be converted to alkoxy silane, thus achieving surface modification.
SiO2 is activated by drying at 40 °C for 48 h, which ensures the activation of necessary functional groups. The dried SiO2 was then mixed with deionized water and anhydrous ethanol in a flask, and a satisfactory dispersion was reached by ultrasonic stirring for 2 hours (as shown in Fig. 1). Then, KH550 (3 wt%) was added, and the solution was stirred for 30 minutes. After the system was evenly mixed, the reactor was heated in a water bath kettle at 80–90 °C for 4 hours to promote the required chemical interaction between SiO2 and KH550. Finally, the nano-powder was washed with anhydrous ethanol three times, and then the product was dried at 100 °C for 3 hours to obtain the sample, which was recorded as SiO2@KH550.
After SiO2@KH550 was thoroughly stirred and ultrasonically dispersed, lab-synthesized FC-A was added to the suspension to form the SiO2@KH550/FC-A composite system. A schematic diagram of the entire process for preparing SiO2@KH550/FC-A is shown in Fig. 2.
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Fig. 2 Schematic diagram of the reaction process of modification of SiO2@KH550 and the SiO2@KH550/FC-A system. |
After that, the initial contact angles of the core samples were tested. Each sample was measured three times, and the average value was taken. A suspension of 1% nano SiO2@KH550, 0.1% FC-A solution, and 1% SiO2@KH550 + 0.1% FC-A composite system was prepared. Then, three fresh mica sheets were placed on the systems for adsorption for 24 hours. After the mica sheets treated with the various systems were dried, they were employed to measure the contact angles using deionized water droplets, and contact angle images were recorded.
(1) The rock sample to be tested was dried so that the initial saturation of all samples remained at a consistent level. The drying temperature of the rock sample was not higher than 80 °C (with fluctuation less than ±5 °C), and the core mass was weighed at intervals until the difference between the two weighings was less than 10 mg.
(2) The length, diameter, and weight of the dried rock sample were measured.
(3) The core was hung on a clamp at the top end of the balance, and the end face was maintained parallel to the liquid surface in the container. Then, the platform was slowly raised until it contacted the end face of the core sample. At this time, the spontaneous imbibition dynamic process began, and the data generated by the balance was recorded over time. During the course of imbibition, the side surfaces of the core plugs were sealed with epoxy to minimize the experimental error caused by evaporation.
Displacement tests were carried out under different pressure differences. Under each pressure difference, gas flooding tests were conducted to reach the unchanged flooding volume. Fig. 3 shows a diagram of the gas displacement device. First, the core samples were dried in a vacuum, and the size and quality were measured. The core samples were then soaked in various systems and aged at 120 °C for 24 hours to achieve adsorption equilibrium. The treated core samples were taken out to dry and saturated with brine. The core was placed in a core holder, and was driven forward to stability under different pressure differences.
Gas permeabilities were measured before and after the test, and weighed and recorded after the displacement. When the inlet pressure changed, it was ensured that the confining pressure was 1.5 times that of the inlet pressure to prevent the gas from leaking out from the side and causing errors in the permeability test. The quality of the fluid loss flowing out of the outlet and permeability were tested and recorded.
The brine water was collected from the formation in an oil reservoir in China. The detailed ionic composition of the water appears in Table 2.
Cl− (mg L−1) | CO32− (mg L−1) | HCO3− (mg L−1) | SO42− (mg L−1) | Br− | Ca2+ | Mg2+ |
---|---|---|---|---|---|---|
9839.09 | 45.34 | 125.39 | 1843.56 | 10.43 | 642.94 | 459.41 |
Brine water was used to maintain the same salinity as the formation, thus preventing degradation of the pore structure of the core due to salinity sensitivity. This ensures that the core samples will not be affected by sensitivity during imbibition and core displacement tests.
First, the effect of the concentration of FC-A on the contact angle was studied. Fig. 7 shows that with increasing concentration, the contact angle of the solid surface after FC-A treatment initially increases and then decreases, and the maximum value occurs when the concentration is approximately 0.1%, which is approximately 68°. The error bar indicates that the experimental error is small, and the results can be regarded as a statistical regularity. This is mainly because the adsorption of FC-A on the surface of mica as a surfactant conforms to the Langmuir isothermal adsorption model. At low concentrations, FC-A molecules are adsorbed as a monolayer, and the adsorption amount on the solid surface is relatively low, which indicates that the hydrophilic head group of FC-A molecules can be adsorbed on the solid surface, and the entire molecule lies sparsely on the solid surface due to the low distribution density. When the concentration increases, the adsorption capacity of FC-A molecules increases, resulting in the vertical establishment of FC-A molecules on the solid surface, exposing the hydrophobic chain on the solid surface and increasing the surface hydrophobicity. However, because FC-A is an anionic surfactant, the adsorption capacity of FC-A is relatively lower due to the electrostatic repulsion on the surface of the molecules and the solid matrix.
The effect of SiO2@KH550 concentration on the contact angle was studied, and the results are shown in Fig. 7. When the concentration of SiO2@KH550 increases, the contact angle clearly decreases, and all of samples are strongly hydrophilic (less than 20°). This is mainly due to the construction of micro–nano structures on the solid surface by nanoparticles, resulting in an increase in the contact area of the solid and liquid. Moreover, with the increase in the concentration, the nanoparticles deposited and accumulated, resulting in more obvious surface roughness and decreased contact angles. This also reveals that micro–nano rough structures can be formed that support the subsequent construction of low-surface energy solid surfaces based on the micro–nano rough structures.
Furthermore, the effect of composite system concentration on the contact angle was studied. The results are shown in Fig. 7 with a fixed concentration of 0.05% for SiO2@KH550. The contact angle of the mica sheet treated by the composite system without FC-A remained at 22°, and with increasing concentration of FC-A, the contact angle of the mica treated by the composite system rapidly increases. When the concentration of FC-A reaches 0.2%, the contact angle increases to 128°. As the concentration continues to increase, the contact angle of the system gradually becomes stable. The results showed that the SiO2@KH550/FC-A composite system can significantly increase the contact angle and modify the wettability from hydrophilicity to hydrophobicity.11,12,19
According to studies, the adsorption of surfactants is the reason for the change in wettability. However, because FC-A is an anionic surfactant, there is electrostatic repulsion between the anionic group of its head group and the negatively charged sandstone surface, resulting in low adsorption of FC-A. Therefore, the surface of the substrate treated with FC-A continued to show hydrophilicity. Finally, after the mica was infiltrated by the SiO2@KH550/FC-A composite system for 24 h, the contact angle of the mica surface significantly increased, reaching up to 132°, and showing a hydrophobic surface. The wettability of mica treated by the SiO2@KH550/FC-A composite system showed excellent durability with no changes after 5 min.
According to the results of the contact angle change over time, the contact angle did not change with time, and the hydrophobic surface was stable. The results showed that the hydrophilic surface can be modified to a hydrophobic surface by the composite system.
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Fig. 9 The surface morphology of a hydrophilic solid treated with (a) SiO2, (b) SiO2@KH550, and (c and d) SiO2@KH550/FC-A. (e) SEM mapping images. |
After modifying SiO2, organic groups were grafted onto the surface of SiO2, which enhanced its dispersion and hydrophobicity, and altered the morphology of the surface composition with increased uniformity and the formation of micro and nanostructures. The gap between these micro and nanoparticles trapped air to form an air cushion where water droplets were suspended at the gas–solid–liquid interface, while the FC-A surfactant reduced the surface free energy. The mapping results in Fig. 9(e) show that Si, F, Al, and O are the main elements of the nanomaterial loaded on the matrix surface, which proves that the fluorine surfactant and nanomaterial were simultaneously adsorbed on the matrix surface.
![]() | (1) |
Fig. 10 shows that surface tensions are related to surfactant concentrations at different concentrations of FC-A and the SiO2@KH550/FC-A composite system. The results showed that surface tensions gradually decreased with increasing concentration. Until the concentration reached the critical micelle concentration (CMC), the surface tension remain stable. The surface tension of FC-A is lower than that of the SiO2@KH550/FC-A system because the FC-A is adsorbed on the nanoparticle surface, reducing its effective concentration in solution.
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Fig. 10 Surface tensions with different concentrations of FC-A and the SiO2@KH550/FC-A composite system. |
According to the capillary force equation, it is assumed that for the same reservoir, the capillary pore diameter can be regarded as the same, and therefore, σcos
θ jointly determines the capillary force. According to the surface tension and contact angle measurements, the σ
cos
θ of FC-A and the SiO2@KH550/FC-A composite system at different concentrations was obtained, so as to determine the effect of the treatment agent on the capillary force. Fig. 11 shows that the σ
cos
θ of water is approximately 65 without a treatment agent. However, the capillary force significantly decreases with increasing concentrations of FC-A or the SiO2@KH550/FC-A composite system. When the concentration is greater than 0.1%, the σ
cos
θ value of the SiO2@KH550/FC-A composite system was even less than 0, which denotes the change in the capillary force in the reservoir from the driving force to the resistance force after treatment of the system. The external fluid did not enter the pore space due to the capillary spontaneous imbibition.
![]() | ||
Fig. 11 Variation of σ![]() ![]() |
Fig. 12 shows the change in the amount of spontaneous imbibition liquid with time in the process of core self-imbibition for 4 types of test fluids. The core self-imbibition process is divided into two stages, namely, the rapid self-imbibition stage that initially occurs, and the slow self-imbibition stable stage that follows. Self-imbibition water saturation with simulated formation water is the highest at any imbibition time, indicating that the core pore surface is strongly hydrophilic, and water molecules easily enter the core pore through capillary force self-imbibition.
![]() | ||
Fig. 12 The curves of spontaneous imbibition of brine water, FC-A, SiO2@KH550, and the SiO2@KH550/FC-A composite system. |
The self-imbibition of FC-A decreased, because FC-A not only reduced the surface tension of the system, but also increased the core wettability, resulting in a decrease in capillary force. For the SiO2@KH550 suspension, although SiO2@KH550 will further increase the core hydrophilicity, the particle size of SiO2@KH550 may cause a certain blockage at the throat of the core surface, resulting in a degree of imbibition that is slightly lower than that of the simulated saline imbibition. The reddish brown curve is the self-imbibition curve of the core in the SiO2@KH550/FC-A composite system. According to the results, the self-imbibition amount for the core in the composite system is very small, only 0.5 mL after 8 hours of self-imbibition. In addition, during the self-priming process, the self-priming amount quickly becomes stable. The self-priming rate is relatively high in the first hour, and then, the curve becomes flat.
Based on the spontaneous imbibition data, the spontaneous imbibition rates of the core samples in different liquids were calculated, and are shown in Fig. 13. The spontaneous imbibition rates of the core in brine and SiO2@KH550 are very high, and they are maintained at a high level within the first 100 min. Compared with FC-A and the SiO2@KH550/FC-A system, the imbibition rate of the core significantly decreases. In FC-A solution, the maximum imbibition rate is only 0.09 g min−1, and rapidly decreases to 0.01 g min−1 within 10 min, and then is maintained at a low value. In the SiO2@KH550/FC-A system, the maximum imbibition rate is only 0.04 g min−1, and also rapidly decreases to 0.0004 g min−1 within 5 minutes.
These results showed that the SiO2@KH550/FC-A system significantly reduces the core imbibition rate. The SiO2@KH550/FC-A system subsequently forms a stable hydrophobic layer on the pore surface of the core, which significantly inhibits water entry into the pore. It is worth noting that the spontaneous imbibition rate of FC-A solution also drastically decreased. According to the capillary force equation, the capillary force is determined by surface tension and wettability. As a highly effective surfactant, FC-A can significantly reduce the surface tension and thus greatly reduce the capillary force, although it has no obvious effect on the wettability. According to Fig. 10, σcos
θ can be significantly decreased from 66.1 without FC-A to 9.4 with 0.1% FC-A. Therefore, the spontaneous imbibition rate and imbibition amount of FC-A solution are significantly lower than that of brine solution.
Compared with the cores treated with SiO2@KH550, the water saturation of the cores decreased to 77.5% at 1 MPa, indicating that stronger hydrophilicity leads to more severe liquid phase trapping. However, the water saturation of the core treated with FC-A decreased to 64.2%, and the water saturation of the core treated with the SiO2@KH550/FC-A composite system rapidly decreased to 48.2%, with the gas phase permeability also recovering from 41.5% to 70.4%. After 1.5 MPa flooding, the water saturation of untreated and SiO2@KH550-treated cores significantly decreased, and the permeability recovery degree increased, but the decrease in FC-A- and SiO2@KH550/FC-A-treated cores slowed (because the absolute water saturation dropped to a relatively low value). The permeability recovery curve was still above curves 1 and 2. When the displacement pressure difference increased to 2 MPa, the water saturation of the core slowly decreased as the displacement progressed to stability, and the recovery curve for all core permeabilities entered into a stable stage. This indicated that the pores involved in the flow at the core at this stage no longer increase, the fluid that can flow no longer changes, the displacement has tended to be stable, and the remaining fluid is basically an immobile residual liquid.
Finally, after displacement, the water saturation of untreated and SiO2@KH550-treated cores decreased to 52.6% and 58.3%, respectively, and the permeability recovered to 43.80% and 38.2%. However, the water saturation of the cores treated with FC-A and the SiO2@KH550/FC-A composite system decreased to 34.84% and 16.3%, respectively, and the permeability recovered to 59.78% and 88.4%, indicating that the SiO2@KH550/FC-A composite system can significantly improve the liquid phase flowback ability.
The hydroxyl group on the surface of SiO2 reacted with the organic groups on the modifier to form a three-dimensional network structure centered on Si–O–Si (composed of hydrophobic organic groups). Therefore, the core surface contact angle significantly increased after SiO2@KH550/FC-A treatment. These results are consistent with those of surface roughness, morphology, and chemical composition. Fig. 13 shows the wetting behavior of a hydrophobic coating prepared using SiO2@KH550/FC-A. As mentioned earlier, water tends to diffuse on smooth hydrophilic surfaces, resulting in a relatively low water contact angle in sandstone, as shown in Fig. 13. In contrast, surface roughness enhances wetting behavior. As shown in Fig. 13, the presence of SiO2 enhanced the surface roughness, while the FC-A on the surface reduced the surface energy, resulting in an increase in the water contact angle of the coating to 110°. The protruding structure created trapped air at the interface between the droplets and the coating, impeding the diffusion of the droplets and thus increasing the contact angle. According to the Cassie–Baxter model, when the surface roughness of the material is high and the intrinsic contact angle is large, the liquid will be prevented from penetrating into the rough structure, resulting in the air being clamped and the liquid–gas contact increased. Thus, the following mathematical expression can be derived:12,25,26
cos![]() ![]() ![]() ![]() ![]() | (2) |
cos![]() ![]() ![]() | (3) |
Fig. 15 illustrates the wettability modification induced by various fluids. After the solid surface was treated with SiO2@KH550, rough surfaces of micro–nanostructures were constructed on the surface of the hydrophilic matrix, and the contact area between water molecules and the solid surface increased over 120°.11 At the same time, due to the high surface energy of the nanoparticles, there was a greater attraction to water molecules.12 Therefore, the micro–nano rough structure with a high surface energy resulted in stronger hydrophilicity. For the solid surface treated with the FC-A surfactant, the surface free energy can be significantly reduced because the hydrophobic chain of FC-A is exposed to the outside, and therefore, the hydrophilicity decreases. However, because the surface lacks a micro–nano rough structure, the wettability still cannot reach the hydrophobic FC-A.11,18 Combined with the micro–nano rough structure of the nanomaterials and the low surface energy properties of the fluorosurfactants, a hydrophobic surface can be constructed with a contact angle over 120° on the strongly hydrophilic quartz surface.19–21
In oil/gas reservoirs, after the core pore structure is modified by the composite system, the hydrophobicity of the pore surface is greatly increased, resulting in a sharp decrease in capillary force, and even the capillary driving force can be transformed into a repulsive force. This would result in difficulty for external fluids to spontaneously absorb into formation pores through formation capillary mechanics, thus greatly reducing fluid imbibition, increasing flowback efficiency, and effectively reducing aqueous phase trap damage in tight sandstone.27
The results of core self-imbibition experiments confirmed that the construction of a hydrophobic surface can significantly reduce the self-imbibition of core pores to external fluids. In the SiO2@KH550/FC-A system, the imbibition amount of the core was very small, at only 0.5 mL after 8 hours. Additionally, the maximum imbibition rate was only 0.04 g min−1, and also rapidly decreased to 0.0004 g min−1 within 5 minutes. The water saturation of the core treated with the SiO2@KH550/FC-A composite system decreased to 16.3%, respectively, and the permeability recovered to 88.4%, indicating that the SiO2@KH550/FC-A composite system can significantly improve the liquid phase flowback ability.
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