Experimental Materials: Ricinoleic acid (95.0%, Shanghai Lisen Chemical Co., Ltd., Shanghai, China); Polyethylenimine (99.0%, Aladdin, Shanghai, China); Tannic acid (98.0%, Aladdin); Acetic anhydride (98.0%, Aladdin); p-Toluenesulfonic acid (98.0%, Aladdin); N, N-Dimethylformamide (99.0%, Macklin, Shanghai, China); Deionized water (Lab-grade, Shanghai, China). As shown in Table 1.

Gas-to-liquid (GTL) base oil (99.0%, synthesized in-house, Sinopec Research Institute, Beijing, China); Calcium oxide (99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China); Emulsifier (Jiangsu Haian Petrochemical Co., Ltd., Nantong, China); Barite (95.0%, Tianjin Yandong Haotian Mineral Products Co., Ltd., Tianjin, China); Sulfonated polyacrylamide (S-PAM, 98.0%, Shandong Noah Biological Technology Co., Ltd., Heze, China).

Experimental Instruments: Fourier-transform infrared (FTIR) spectrometer (Frontier, PerkinElmer, Waltham, MA, USA); Thermogravimetric analyzer (DZ-TGA-101, DZ Instruments, Nanjing, China); Six-speed viscometer (OFITE-900, Fann Instrument Company, Conroe, TX, USA); Electrical stability tester (FANN-23D, Fann Instrument Company, USA); Drilling fluid densimeter (XYM-3, Kensee Instruments, Shanghai, China); Nanoparticle size and zeta potential analyzer (BeNano 90, Dandong Bettersize Instruments, Dandong, China); Laser diffraction particle size analyzer (Mastersizer 3000+, Malvern Panalytical, Worcestershire, UK).

SS-1 was synthesized via multifunctional polycondensation in aqueous solution to construct a crosslinked network. Fig. 1 illustrates the synthetic route, with detailed steps as follows:

(1)

Pre-treatment: Ricinoleic acid (15 g) was purified by distillation under reduced pressure (0.1 MPa, 90°C, N₂ atmosphere) for 2 h to remove volatile impurities, yielding 14.2 g (94.7%) of a colorless viscous liquid. Benzene-1,2,4-tricarboxylic acid (10 g) was reacted with acetic anhydride (20 g) and p-toluenesulfonic acid (0.05 g) at 120°C for 5 h to form trimellitic anhydride, followed by purification. The crude product was dissolved in 50 mL of acetone at 60°C, filtered to remove catalysts, and cooled to 0°C for crystallization. White crystals were collected by vacuum filtration and dried at 80°C (yield: 8.9 g, 89.0%).

The performance evaluation employed a synthetic base drilling fluid formulation comprising: gas-to-liquid (GTL) synthetic oil, 25% CaCl₂ aqueous solution, 4% emulsifier, 2 wt% sedimentation stabilizer, 1.5 wt% organoclay, 18 wt% CaO, 4 wt% fluid loss additive, 1 wt% hydrophobic nano-silica, and 555 g barite (ρ = 1.8 g/cm³). Three comparative systems were prepared: Base fluid without stabilizer, Base fluid + sulfonated polyacrylamide, Base fluid + synthesized SS-1 stabilizer. The specific preparation steps are as follows: (1)

The synthetic base drilling fluid slurry is prepared by first measuring gas-to-liquid (GTL) synthetic oil using a graduated cylinder and weighing the corresponding 4% emulsifier, followed by transferring both components to a high-speed mixing cup where initial homogenization occurs at 11,000 rpm for 30 min. Subsequently, 25% calcium chloride (CaCl₂) aqueous solution is added with 20-min high-speed mixing (11,000 rpm), after which fluid loss additive, calcium oxide (CaO), and hydrophobic nano-silica are sequentially incorporated, each followed by identical 20-min mixing cycles. Upon complete dispersion of all individual additives, 555 g barite is incrementally introduced in small batches, culminating in final homogenization at 11,000 rpm for 90 min, yielding the base slurry ready for experimental use.

Molecular structures of the polymers were characterized using a Frontier FTIR spectrometer (PerkinElmer). Samples were prepared by grinding with potassium chloride (KBr) to form pellets. Spectra were acquired across the wavenumber range of 400–4000 cm⁻¹.

Thermal stability was assessed using a DZ-TGA-101 thermogravimetric analyzer. An Al₂O₃ crucible was zero-calibrated in the sample chamber. Precisely 0.4 g of sample was weighed into the crucible, which was then returned to the chamber. The TGA program heated samples at 10°C/min under N₂ atmosphere to a final temperature of 680°C. Thermograms of SS-1 were recorded and analyzed.

Particle size distributions of SS-1 suspensions were determined via dynamic laser scattering (DLS) using a Mastersizer 3000+ instrument (Malvern Panalytical, UK). To prevent particle aggregation during measurements, samples were subjected to ultrasonic agitation at a fixed frequency of 1500 Hz and repeated three times to get the average value.

The effect of temperature on the zeta potential value of drilling fluid was studied by BeNano 90 nm particle size and Zeta potential analyzer, and repeated three times to get the average value.

The impact of SBDF on the environment was evaluated through biological toxicity tests, and the 50% lethal concentration was determined using the fluorescent bacteria method [31].

Fig. 2 displays the FTIR spectrum of SS-1. Characteristic peaks were assigned as follows: 3400–3700 cm⁻¹ (O-H and residual N-H stretching vibrations), 1719 cm⁻¹ (symmetric C=O stretching), 1658 cm⁻¹ (amide C=O stretching), 1589 cm⁻¹ (aromatic C=C bending), 1423 cm⁻¹ (amide C-N stretching), 1113 cm⁻¹ (ester C-O-C stretching), and 1035 cm⁻¹ (S=O stretching of sulfonate groups from sulfonated asphalt). Additional peaks at 2918 and 2850 cm⁻¹ correspond to symmetric -CH₂- stretching from alkyl chains of ricinoleic acid and sulfonated asphalt, while the 799 cm⁻¹ band represents aromatic C-H out-of-plane bending in benzene-1,2,4-tricarboxylic moieties. The spectral features confirm perfect alignment with the designed molecular architecture of the sag stabilizer, indicating complete progression of the synthesis reaction.

Fig. 3 shows the TGA analysis curve of SS-1. It can be observed that when heated to 260°C, the polymer exhibits good thermal stability with mass loss within 2%, likely attributed to the evaporation of intra- and intermolecular water and incompletely crosslinked low-molecular-weight sulfonates. As the temperature increased from 260°C to 300°C, the TGA curve declined rapidly with a mass loss rate reaching 24.58%, possibly due to thermal decomposition of minor molecular side groups. When the temperature rose from 300°C to 530°C, the thermogravimetric curve showed a slower declining trend with a mass loss rate of 18.94%, resulting from the degradation of the polymer backbone that disrupted the molecular structure. Beyond 530°C, the molecular structure of the polymer underwent complete degradation. The presence of sulfonic acid groups, benzene rings, and other rigid structural units enhanced the chain rigidity of the polymer, thereby improving the high-temperature stability of SS-1. On the contrary, Fig. 4 shows the TGA analysis curve of S-PAM. At 210°C, the thermogravimetric loss has decreased significantly to 30.59%. At 420°C, its molecular structure has completely degraded, and the thermogravimetric loss rate is 75.14%, which is 166% higher than that of SS-1. These results demonstrate that SS-1 has more outstanding thermal stability compared with S-PAM.

The rheological test data for three different systems of drilling fluids are presented in Tables 2–4, and the trend graphs are shown in Figs. 5–7. As shown in Figs. 5–7, the samples containing SS-1 exhibit higher viscosity and yield point compared to the blank control group and S-PAM. At a high temperature of 210°C, the AV, PV, and YP of SS-1 are, respectively: 45, 38 mPa·s, and 7.154 Pa which are superior to the blank group (31.5, 26 mPa·s, 5.621 Pa) and S-PAM (37, 30.5 mPa·s, 6.643 Pa). As the temperature gradually increases, the apparent viscosity, plastic viscosity, and shear stress of the drilling fluid first increase and then decrease. Furthermore, after aging at 210°C for 16 h, the viscosity of the blank group and the group with S-PAM decreased significantly, while the viscosity change range of the experimental group containing SS-1 was relatively smaller. This is attributed to the covalent bonds connecting the S-PAM molecules. Elevated temperatures break these chains, significantly reducing the solution’s viscosity. In the control group (without stabilizers), emulsification occurs during aging, leading to emulsifier deactivation. This disrupts the molecular structure and causes a sharp viscosity drop. However, adding SS-1—which leverages dynamic ionic bond recombination, a rigid framework, and electrostatic repulsion—results in nearly unchanged viscosity after aging. This indicates that increased temperature enhances the emulsification and dispersion capabilities of the drilling fluid, leading to an increase in its viscosity. However, when the temperature exceeds 180°C, high temperatures cause the emulsifiers in the drilling fluid to become ineffective, disrupting the spatial network structure of the drilling fluid, resulting in a decrease in both viscosity and yield point.

As shown in Fig. 8, the demulsification voltage of synthetic base drilling fluid initially increases before decreasing with rising temperature. This indicates enhanced electrical stability of the drilling fluid at elevated temperatures. However, beyond 180°C, the demulsification voltage shows a marginal reduction due to disruption of the oil-in-water structure, which compromises electrical stability. Notably, at 210°C, the system containing SS-1 exhibits a significantly higher demulsification voltage than both the blank-control group and the S-PAM formulation, with increases of 27.9% and 12.1%, respectively. This demonstrates that the base slurry incorporating SS-1 maintains superior stability, requiring higher voltage for demulsification.

As shown in Fig. 9, the settling coefficient increases progressively with rising temperature, indicating that elevated temperatures negatively impact drilling fluid sedimentation. At excessive temperatures, this sedimentation compromises overall fluid performance. Specifically at 180°C, the blank group exhibits a settling coefficient of 0.538, confirming performance degradation due to internal sedimentation, while formulations containing S-PAM and SS-1 demonstrate lower coefficients of 0.515 and 0.505, respectively. When the temperature exceeds 200°C, the settling coefficients further increase to 0.531 (S-PAM) and 0.509 (SS-1). These results demonstrate that the synthesized SS-1 polymer provides remarkable sedimentation stability to drilling fluids, outperforming the commercially available S-PAM additive, particularly under high-temperature conditions.

Drilling fluids, under the action of sedimentation stabilizers, achieve amphiphilic bridging at the solid-liquid interface through directional anchoring of hydrophilic groups onto barite particle surfaces via electrostatic adsorption, while hydrophobic groups embed into the continuous oil phase. The polymer backbone incorporates thermal-resistant groups—including rigid aromatic rings (e.g., trimellitate residues) and sulfonic groups—imparting thermal stability above 200°C to the system. The three-dimensional network formed by intermolecular crosslinking generates synergistic effects: (1) Steric Hindrance: Isolated networks at crosslinking nodes restrict barite particle migration, enhancing static sedimentation stability; (2) Rheological Regulation: Shear-thinning behavior maintains controllable rheology under high-temperature/high-pressure conditions. The mechanistic schematic is illustrated in Fig. 10.

Zeta potential serves as a critical indicator of clay particle stability in drilling fluids. As shown in Fig. 11, the initial base slurry exhibits zeta potentials of −34.11 mV (base slurry), −34.42 mV (S-PAM), and −34.85 mV (SS-1), with absolute values >30 mV confirming excellent colloidal stability per DLVO theory [32]. Upon heating from 25°C to 210°C, these values shift to −25.12 mV (base slurry), −30.02 mV (S-PAM), and −40.96 mV (SS-1), revealing a unique positive correlation between |zeta potential| and temperature exclusively for SS-1-modified systems. This anomalous behavior arises from three mechanisms: (1) SS-1’s PEI-derived -NH₃⁺ groups electrostatically occupy clay surface sites, counteracting the EDL compression and chain collapse observed in blank/S-PAM systems where high temperatures accelerate counterion migration and suppress carboxyl/sulfonate ionization; (2) Hydrophobic moieties in SS-1 reduce diffuse EDL thickness, mitigating electrolyte-induced compression; (3) Extended PEI chains form hydration layers that increase interparticle distances, weakening van der Waals forces while enhancing electrostatic repulsion. Furthermore, SS-1’s rising |zeta potential| with temperature stems from thermally promoted ionization of -SO₃⁻/-COO⁻ groups, chain extension exposing buried charges, restructuring of aqueous hydrogen-bond networks facilitating faster electrophoretic mobility, and counterion desorption reducing charge shielding–collectively re-expanding the EDL.

Fig. 12 reveals a negative correlation between clay particle size and temperature exclusively in SS-1-modified samples, contrasting with the opposite trend observed in control groups. The median particle size (D50) of SS-1-treated clay decreases from 9.96 µm at 25°C to 5.06 µm at 220°C, indicating improved dispersion and reduced agglomeration at elevated temperatures. This phenomenon arises from three synergistic mechanisms: (1) Enhanced ionization of sulfonic groups at high temperatures strengthens electrostatic repulsion, promoting polymer chain extension; (2) Increased steric hindrance from expanded molecular chains forms thicker adsorption layers that prevent particle approach; (3) Elevated adsorption capacity exposes more anchoring sites, ensuring uniform particle surface coverage. Conversely, systems without SS-1 or with S-PAM exhibit significantly increased average particle sizes under thermal stress, indicating severe agglomeration due to S-PAM molecular degradation and chain scission above 200°C. These findings align with zeta potential measurements (Fig. 10), collectively demonstrating SS-1’s superior high-temperature stabilization capability.

The EC₅₀ of SBDF was tested by the luminescent bacteria method according to the relative industry standard (Q/SY 111–2007, Garding and determination of the biotoxicity of chemicals and drilling fluids—Luminescent bacteria test) [33], and the results are listed in Table 5. The EC₅₀ results of the three groups of samples are as follows: 15200 mg/L (S-PAM), 49250 mg/L (Blank control group),48900 mg/L(SS-1). According to the biological toxicity grade classification standard, when the EC₅₀ was larger than 25000 mg/L, WBDF was nontoxic. If the EC₅₀ exceeded 30000 mg/L, the emission standard was achieved [34]. Therefore, the experimental samples with the addition of SS-1 and the blank group were considered completely non-toxic. On the contrary, the samples with the addition of S-PAM were toxic and could not meet the environmental protection emission requirements, having certain limitations.