The solid phase was centrifuged from AL-1 and OL-1, then rinsed with ethanol three times, and dried at 105 °C for 6 h. The dried solid was mixed with KBr and then pressed into tablets. The tablets were further tested using a Fourier transform infrared spectroscopy (AVATAR 320 Madison, Wisconsin) in the range of 500-4000 cm⁻¹.

An appropriate amount of the dried solid powder was taken after IR spectroscopy test and placed in a test pot. The thermogravimetric loss of the specimen was measured at 30-800 °C by a thermogravimetric analyzer under nitrogen atmosphere at temperature-increasing rate of 10 °C/min.

1 g AL-1 and 1 g OL-1 emulsions were added to 100 mL ethanol and sonicated for 30 min. Then the suspension was dropped onto conducting resin. The resin was dried and coated with gold. Finally, the specimen was characterized using a scanning electron microscopy (EM-30AX, COEXM).

OL-1, AL-1 and SA were diluted 100 times with ethanol, and then sonicated, and dropped onto the surface of single-crystal silicon. After being dried, the sample was tested using a high-resolution atomic force microscope (SPM-8100FM, Shimadzu, Japan) in a contact mode.

5 g of SA, AL-1 and OL-1 emulsions each, with effective solid content (mass fraction) of 20%, were added into 95 g ethanol. After sonicated, the ethanol suspensions with 1% solid content were prepared. The core slice was soaked into the ethanol suspension for 30 min. Then the core slice was taken out and dried at 70 °C for 10 min. The surface free energy of the core slice was calculated by testing the contact angles of deionized water and n-hexadecane on the core surface before and after modification using the Owens two-liquid method according to Eqs. (1) and (2) ^[26]:

The dispersion force, polarity force and surface energy of deionized water are 21.8, 51.0 and 72.8 mN/m, respectively. The dispersion force, polarity force, and surface energy of n-hexadecane are 27.6, 0 and 27.6 mN/m, respectively.

The core slice was soaked in the ethanol suspensions with 1% mass fraction and containing SA, AL-1 and OL-1 with different solid contents for 30 min. Then the sample was taken out and dried. The contact angle of #3 white oil on the core surface before and after modification was tested by the pendant drop method using a contact angle measuring instrument (OCA200, Bruker Alicona).

The core pieces before and after immersion in 1% OL-1 ethanol suspension were dried and sprayed with gold, and then characterized using a scanning electron microscopy (EM-30AX, COEXM).

The capillary glass tube was used to simulate the pores and fractures in shale. The tube was immersed in the ethanol suspension with 1% OL-1, 1% SA and 1% AL-1 for 1 h. Then the tube was taken out and dried naturally. The treated capillary tube was then immersed in #3 white oil and stabilized for 2 h. The imbibing height of the oil phase inside the capillary tube was monitored with the liquid level outside the capillary tube as the basis.

The cores were suspended above an electronic balance. 1/2 volume of the core was immersed in the white oil and the white oil suspension containing 1% SA, 1% AL-1 and 1% OL-1, respectively, which were placed in the balance. After the core was immersed, the electronic balance was cleared to zero. When the oil phase intruded into the core, the balance reading decreased. The decreased value is the mass of the oil phase intruding into the core, by which the amount of oil intruding the core was monitored.

An OBDF was prepared according to the following formula: White oil + 2% primary emulsion (DR-EM) + 2% secondary emulsion (DR-CO) + 1% organic bentonite + 2.5% filter loss reduction agent (FLRA) + 1% calcium oxide + 20% calcium chloride aqueous solution + barite. The oil-to-water ratio is 90 to 10. The density is 1.95 g/cm³. Different mass fractions of oleophobic agent OL-1 were added to the OBDF and stirred at 11 000 r/min for 20 min to obtain an oleophobic OBDF. The rheological property (65 °C), emulsion breaking voltage, high-temperature and high-pressure filtration loss (120 °C, 3.5 MPa) of the drilling fluids after aging treatment (120 °C, 16 h) were tested according to the national standard ^[27].

Cores drilled from two adjacent locations of the same outcrop were scanned using a nano-CT instrument (Skyscan 2214, Bruker) to characterize the pore structure of the original cores. The porosity of the cores was also calculated according to the test images ^[28]. The above cores were then immersed into the OBDF and the oleophobic OBDF containing 1% OL-1, respectively. Then they were taken out after standing in oven at 150 ℃ for 7 d. The OBDF adhering to the surface of the cores was wiped off, and then the cores were scanned again.

The cores were placed in two aging cans. 300 mL of the OBDF and 300 mL of the oleophobic OBDF containing 1% OL-1 were then added into cans, respectively. The cans were sealed and stood at 150 °C for 7 d. After cooling naturally to ambient temperature, the cores were taken out. The OBDF adhering to the surface was wiped off. The compressive strengths of the cores were tested under the confining pressure of 0, 5, and 10 MPa, respectively, using a microcomputer triaxial creep tester ^[10]. Then according to the Mohr-Coulomb criterion ^[29], the cohesion C and internal friction angle β of the cores before and after immersion were calculated based on Eq. (3).

Fig. 1 shows the infrared spectra of the oleophobic agents OL-1 and the intermediate product AL-1. Both OL-1 and AL-1 have strong absorption peaks at 1095 cm⁻¹, which are attributed to Si—O—Si stretching vibration, and prove that the products contain nano-silica. The absorption peaks at 1500 cm⁻¹ and 1452 cm⁻¹ were originated from the stretching vibration of benzene ring. The infrared characteristic peaks at 2930, 2850 cm⁻¹ were caused by the stretching vibration of methyl group and methylene group. The simultaneous presence of methyl group, methylene group and benzene ring group, and the absence of characteristic peaks corresponding to double bonds indicate that styrene has been polymerized to produce polystyrene. The absorption peaks at 1210 cm⁻¹ and 658 cm⁻¹ emerged on infrared spectra of OL-1 corresponding to the stretching vibration of —CF₂— and —Si—C—, respectively, which are the distinct functional groups of PFT, demonstrating that PFT has been successfully introduced on the nano-silica surface.

The thermogravimetric analysis (TGA) was used to characterize AL-1 and OL-1, and the results were shown in Fig. 2. With temperature increasing from room temperature up to 200 °C, only a small mass loss occurred for AL-1 and OL-1, which was caused by high-temperature desorption of the originally adsorbed water. The masses of both materials were basically constant from 200 °C to 300 °C, reflecting good thermal stability. The mass fractions of AL-1 and OL-1 in the TGA curves were decreased by 27% and 23%, respectively, with the temperature increasing from 300 °C to 500 °C, which was caused by thermal cracking of polystyrene molecular chains. With the further increase of the temperature from 500 °C to 650 °C, the residual mass of AL-1 basically did not change. In contrast, the residual mass fraction of OL-1 decreased from 73% to 63%. Besides, a peak emerged at 530 °C on the differential thermogravimetric curve (DTG) of OL-1, which was originated from the high-temperature desorption of PFT from the surface of OL-1, meanwhile indicating the PFT content in the oleophobic agent was about 10%.

The microscopic morphology and element composition of AL-1 and OL-1 were characterized by scanning electron microscopy and energy spectroscopy (Fig. 3). Both AL-1 and OL-1 were micron-sized ellipsoidal particles with a large number of tightly packed spherical nano-silica on the surface. The analysis shows that the elements in AL-1 include silicon, oxygen, carbon, and a small amount of nitrogen, among which silicon and oxygen were mainly derived from the nano-silica embedded on the surface, carbon was derived from polystyrene, and a small amount of nitrogen may be derived from the initiator azobisisobutyronitrile. In addition to silicon, oxygen and carbon, fluorine was also present in the composition of OL-1, which again proved the successful introduction of PFT.

Surface roughness refers to the unevenness of tiny peaks and valleys on solid surface ^[30]. When the surface energy of the solid is lower than that of the oil phase, the solid surface shows gas wetting. At this time, increasing the roughness can build more "wave valley" structures on the solid surface. This is beneficial to trap gas in the wave valleys and inhibit liquid phase from penetration, and keeping liquid droplets in the Cassie non-wetting state ^[19], thus to enhance oleophobic effect ^[30-31]. Fig. 4 shows the roughness structures built by SA, AL-1, and OL-1 on the surface of single-crystal silicon, observed by an atomic force microscope. As seen from the figure, SA is distributed in a nanoscale on the surface of single-crystal silicon (burr-like in Fig. 4a), and a small number of SA particles formed large agglomerates. AL-1 and OL-1 particles are microscale, and nano-humps appeared on the surface.

The roughness of SA, AL-1, and OL-1, measured by an atomic force microscope, are 6.08, 8.18, and 18.78 nm, respectively. It is evident that OL-1 and AL-1 both are microscale and can establish nano-scale roughness through the nano-silica adsorbed on their surfaces. When OL-1 and AL-1 are adsorbed on rock surface, micro-nano dual-level roughness structure could be built up, which could improve the rock surface roughness more effectively compared with SA which could only build single-level nano-roughness. In addition, during the preparation of OL-1 using AL-1, PFT can settle and condense on the surface of nano-silica. This increases the size of nanoparticles, and thus more effectively improves the roughness than that of AL-1.

The solid surface energy can reflect the attraction of molecules on the solid surface to adsorbed molecules. The lower the surface energy, the lower the attraction is, and vice versa ^[10,32]. Under ideal conditions, when solid phase surface energy is lower than oil phase surface energy (20 mN/m), oil phase molecules tend to aggregate when contacting the solid surface. The attraction of solid surface to oil phase molecules is less than the attraction among the oil phase molecules. Macroscopically, oil phase aggregates into balls on solid phase surface, which increases the contact angle ^[16]. Therefore, reducing the surface energy of solid phase can improve oleophobic effect. The surface energy test results show that the original surface energy of the core is 47.88 mN/m. After adsorbing SA and AL-1, the surface energy decreased to 28.45 mN/m and 35.63 mN/m, respectively, but still higher than that of the oil phase. Based on the Cassie wetting theory, the surface energy of the solid phase is lower than that of the oil phase, which is necessary to achieve oleophobic modification ^[31]. The surface energy of the rock modified by SA and AL-1 is still higher, so failing to achieve a stable oleophobic state. In contrast, the surface energy of the core modified by OL-1 was significantly decreased to 0.13 mN/m, reaching an ultra-low surface energy state.

Contact angle can most visually characterize the surface wettability of the solid phase ^[33]. Fig. 5 illustrates the contact angles of white oil on the surface of cores before and after modification using 1% SA, 1% AL-1 and 1% OL-1, respectively. The contact angle of white oil on the original core is 16.39°. After adsorbing SA and AL-1, the contact angle did not increase but decreased to 8.52° and 0, respectively. This is because the surface energy of the core treated by SA and AL-1 is still higher than that of oil phase. The surface of the core was oleophilic at that time, but adsorbing SA and AL-1 increased the roughness, and it’s beneficial to enhance the oleophilicity, so the contact angle decreased ^[34]. After the adsorption of OL-1 on the rock surface, it built micro-nano roughness and significantly reduced the surface energy of the rock, so the contact angle of white oil increased to 143.60°, realizing oleophobic modification. Comparing the modification effects of SA and OL-1 indicates that, on the basis of grafting low surface energy materials, micro-nano roughness is more beneficial to keep liquid phase in Cassie non-wetting state than single-level nano-roughness ^[33⇓-35], which in turn enhances the oleophobicity of the solid phase surface.

Fig. 6 presents the effects of oleophobic modification on rock surface by different mass fractions of OL-1. With the increase of OL-1 mass fraction from 0.5% to 2.0%, the white oil contact angles increased from 135.41° to 148.92°. With the further increase of OL-1 mass fraction, the white oil contact angle was stabilized at 150° and reached a superoleophobic state. According to the above variation trend, it can be speculated that when the mass fraction of oleophobic agent is low, the adsorption capacity of OL-1 particles on the core surface will increase gradually with the increase of mass fraction, making the rock surface roughness increase continuously and the surface energy decrease rapidly, so the white oil contact angle increases substantially. When the mass fraction reaches a certain level, the adsorption of OL-1 on the solid surface will be saturated. Further increasing the mass fraction of OL-1 will hardly impose effects on the surface energy and roughness, so the contact angle is basically stable, and the wettability remains unchanged.

Fig. 7 depicts the changes in the core surface morphology before and after the adsorption of the oleophobic agent OL-1. The pores and microfractures observed is abundant on the surface of the original core, which are easily invaded by oil phase (Fig. 7b). After the adsorption of OL-1, a large quantity of OL-1 particles are densely adsorbed on the surface of the core, forming an oleophobic film (Fig. 7c, 7d).

When the capillary tube is inserted into a wetting liquid phase, the attraction of the capillary tube to the liquid phase raises the liquid level in the tube. However, when inserted into a non-wetting phase liquid, the capillary force hinders liquid phase invasion, and the liquid level in the tube is lower than that outside the tube. The above phenomenon is called a capillary effect ^[36], which is crucial for oil phase imbibing into wellbore ^[37]. In this study, a capillary tube was modified with SA, AL-1 and OL-1, respectively, and then inserted into #3 white oil, and the imbibition height of oil phase before and after modification was tested. The results show that the original capillary tube is lipophilic before modification, and the oil phase imbibition height is 31 mm when inserted into white oil. After modification by SA and AL-1, respectively, the oil phase imbibition height is 27 mm and 35 mm inside the capillary tube over the outside oil level, indicating that the surface of the capillary tube is still lipophilic after modification. After using OL-1 to modify the capillary tube, the oil-phase imbibition height is 33 mm below the outside liquid level, demonstrating that the capillary tube is oleophobic after OL-1 modification that effectively inhibits oil-phase imbibition. Based on the Young-Laplace equation, the capillary pressure generated by capillary force on the oil phase was calculated before and after modification ^[38]. The capillary pressure is 273.76 Pa before modification. After SA and AL-1 modification, the capillary pressure became 235.45 Pa and 306.95 Pa, respectively. Whereas, after OL-1 modification, the capillary pressure became −297.71 Pa, which changed from promoting oil phase imbibition to inhibiting oil phase imbibition.

Usually, the smaller the pore size is, the greater the capillary pressure is. When the pore size in shale is as small as 100 nm, the capillary pressure may be more than 1 MPa ^[37]. With the gradual increase of reservoir protection awareness and the promotion of underbalanced drilling operation ^[39-40], the positive pressure difference between drilling fluids column pressure and formation pore pressure has been dramatically reduced, making capillary pressure become an essential factor affecting fluid phase imbibition into shale pores. Reversing the direction of capillary pressure is expected to slow down or even block oil phase intrusion into well wall.

When the shale is immersed in OBDF, the oil phase may imbibe through its pores and fractures into the shale, affecting the mechanical stability of the shale gas well wall ^[41]. The cores were immersed in white oil and white oil dispersions containing 1% OL-1, 1% AL-1 and 1% SA, respectively. The variation of oil phase imbibition with time was monitored (Fig. 8). When the core just contacted the oil phase, the pores and fractures were not invaded by fluids, and the porosity was large. Then fast oil phase invasion occurred due to the strong lipophilicity of the shale and strong capillary action. With the extension of time, as the oil content inside the shale was close to saturation, the oil imbibition became stable. The amount of oil imbibition into the core soaked in #3 white oil reached 0.14 g after 150 min. The amount into the core in white oil dispersions containing SA and AL-1 increased faster within 30 min, then the oil invasion speed decreased significantly after 30 min, and the final amount was 0.10 g and 0.11 g respectively after 150 min. Because of the plugging effects of SA and AL-1 adsorbed on the core surface, the oil imbibition amount decreased compared with that of the untreated core. The oil imbibition amount in white oil dispersion containing OL-1 reached saturation after 20 min, but the mass is only 0.05 g. It decreased by 64.29%, compared with the core immersed in pure white oil, indicating that OL-1 could effectively inhibit oil phase intrusion.

Good compatibility is a prerequisite for applying the oleophobic agent in OBDF ^[10]. Table 1 lists the rheological properties, emulsion breaking voltage and high-temperature and high-pressure filtration loss (120 °C, 3.5 MPa) of OBDF with different mass fractions of oleophobic agent after aging treatment. The plastic viscosity remains unchanged with the increase of OL-1 mass fraction. From the microscopic point of view, plastic viscosity reflects the internal friction among additive particles, liquidmolecules, and additive particles with liquid molecules in drilling fluids. Since the surface energy of OL-1 is extremely low, its intermolecular force with other components is small, that is to say the intermolecular internal friction is low, so the influence of OL-1 on the plastic viscosity of the system is little. The yield point and gel strength decreases slightly with the increase of OL-1 mass fraction. Solid particles, such as organic bentonite and cuttings in drilling fluids, attract emulsified water droplets to form a frame structure. When added into drilling fluids, OL-1 particles will be involved in constructing the grid structure. But due to the low surface energy of OL-1, the force between OL-1 and different particles is relatively small, resulting in the existence of “weak joints” in the framework, which decreases the framework strength. Therefore, the system strength is reduced. The emulsion breaking voltage maintains at 1300-1400 V, indicating that OL-1 imposes no effect on the emulsion breaking voltage. The high-temperature and high-pressure filtration loss of OBDF first decreases and then increases with the increasing mass fraction of the oleophobic agent. After adding 1.0% OL-1, the high-temperature and high- pressure filtration loss decreases from 2.6 mL to 1.3 mL, with a 50% reduction. The gradual deposition of OL-1 during the mud cake formation could enhance the oleophobicity of the mud cake and generate a capillary resistance in the pores of the mud cake to inhibit the penetration of the oil phase, effectively reducing the filtration loss. However, with a further increase of the mass fraction of oleophobic agent, the high-temperature and high-pressure filtration loss increases significantly. The filtration loss even increases to 3.5 mL when the mass fraction of OL-1 is 3.0%, probably due to the high mass fraction of OL-1 making the mud cake too oleophobic to cause its surface and internal unable to adsorb the oil phase effectively. The failure of adsorbing oil further leads to high oil phase permeability and weak blocking performance of the mud cake, resulting in increasing filtration loss. Therefore, for this system, the mass fraction of the oleophobic agent should be controlled within 2.0% to achieve good compatibility.

Add 1% oleophobic agent to the OBDF to prepare the oleophobic OBDF. Then the effects of oleophobic OBDF and conventional OBDF on the pore-fracture structure and mechanical stability of the rocks were further evaluated. The oil phase intrusion could reduce the porosity of the rock. Therefore, the changes in rock porosity before and after soaking in the OBDF could characterize the oil phase intrusion degree. The oil phase intrusion into the pores and fractures of the shale core before and after soaking in non-oleophobic OBDF and oleophobic OBDF were observed by a nano-CT instrument (Fig. 9), where the dark part represents the distribution of pores and fractures inside the shale ^[42]. Fewer pores and fractures were detected and lower porosity was measured after oil phase intrusion. It could be seen from the figure that after the core was soaked in the non-oleophobic OBDF, the oil phase entered the pores and fractures, which were mainly concentrated in the peripheral part of the core. The porosity decreased by 37.4% from 0.99% to 0.62% after soaking in the non-oleophobic OBDF. In contrast, after the core was soaked in the oleophobic OBDF, the pores and fractures distribution identified by scanning did not change apparently. The porosity decreased by only 4.5% from 0.89% to 0.85% after soaking, indicating that the oleophobic OBDF could effectively inhibit oil phase intrusion.

Rock cohesion reflects the cementation strength of its internal structure and can characterize the mechanical stability of the rock ^[43]. The cores were placed in the non-oleophobic OBDF and the oleophobic OBDF, respectively. The core cohesion and internal friction angle were calculated according to the triaxial stress test results and Mohr-Coulomb criterion (Table 2). The cohesion of the core before soaking is 15.15 MPa. After soaking in the non-oleophobic OBDF, it decreased to 11.30 MPa. While the core cohesion after soaking in the oleophobic OBDF is 14.11 MPa, which increased by 24.9% compared with that soaking in the non-oleophobic system, proving that the oleophobic agent OL-1 could effectively improve the mechanical stability of the rock. The internal friction angle can reflect the friction coefficient when rock slides along the fracturing surface. The larger the internal friction angle, the higher the friction coefficient, and the higher friction resistance during shear sliding ^[29]. Previous studies pointed out that the lubricating effect of oil phase intruding into rock would reduce the frictional resistance and internal friction angle ^[7]. However, our test results show that the difference between the internal friction angles after soaking and before soaking is tiny and stable at about 61°, indicating that the effect of oil- phase lubrication on wellbore stability is relatively small.