Yunhai Zhao

Xanthan gum (XG), valued for its environmental friendliness and multifunctionality, has been extensively utilized in drilling fluids. However, its inadequate thermal stability at elevated bottomhole temperatures encountered in deep drilling operations has constrained its broader application. In this study, we successfully synthesized a graft copolymer (XG-g-Nano SiO2-AMPS) through grafting XG with KH570-modified nano-SiO2 and thermally stable monomer AMPS. The modified product exhibited significantly reduced molecular weights, with number-average (Mn) and weight-average (Mw) molecular weights of 3.17 × 105 g/mol and 3.88 × 105 g/mol, respectively. Thermal analysis revealed a 31% mass loss at high temperature, representing an 14% reduction compared to native XG, while biotoxicity assessment showed an EC50 value exceeding 106 mg/L, indicating low toxicity. Performance evaluation demonstrated excellent viscosity-enhancing properties and superior filtration control, with API filtrate volumes (FLAPI) of 3.0 mL (freshwater) and 3.4 mL (saturated brine), along with high-temperature/high-pressure filtrate volumes (FLHTHP) of 12 and 15 mL, respectively, after 16 h of thermal aging at 150 °C. This breakthrough substantially improves XG’s thermal resilience, extending its applicability in deep high-temperature wells.

To address wellbore instability and the technical challenges associated with high-density water-based drilling fluid loss control in deep shale gas formations of the Sichuan-Chongqing region in China, a novel nano-micro sealant designated CLG-Seal was synthesized via molecular structural optimization. The molecular structure of newly developed CLG-Seal exhibits distinct core–shell structural characteristics. The inorganic nano-silica constitutes the rigid core of CLG-Seal, which guarantees its plugging performance. The hydrophobically associating polymer which is coated on the surface of nano-silica constructs the flexible shell of CLG-Seal, endowing the CLG-Seal with excellent gel-forming capacity, adhesion film-forming capacity, deformability and perfect dispersibility. Transmission electron microscopy and scanning electron microscopy were employed to characterize the morphology of the CLG-Seal nanomicron-scale plugging agent. The sealing performance and underlying mechanisms of CLG-Seal were subsequently evaluated via particle plugging apparatus tests, displacement experiments, and etched glass micromodel simulations. Field trials conducted in the third section of Well WY3-2-3HF validated the application effectiveness of this agent in drilling fluid systems. The results indicate that the nano-micro sealant CLG-Seal exhibits a median particle size of D₅₀ is 146 nm, which can be modulated by adjusting the synthesis conditions. The nano-micro sealant CLG-Seal significantly mitigates fluid loss in low-permeability microfractures and fissures. Notably, a concentration of merely 3% is sufficient to achieve optimal nano-micro plugging performance. The results of the mechanism study indicate that while the CLG-Seal particles are close to each other, the polymer chains with flexible long chain structure which are coated on the surface of nano-silica constructs tend to be intertwined, forming a cross-linked network structure of gel film, thereby increasing the interaction between nano-micron particles and forming an impermeable plugging film. In addition, due to the nanoscale effect, the CLG-Seal has a strong tendency to adsorb onto the surface of shale rock through hydrogen bonding with the shale matrix. The hydrophobically associating polymer with high elastic modulus and excellent mechanical properties can enhance the pressure-bearing capacity of the filter cake through elastic deformation. Therefore, these nano-micron particles can form a strong sealing film on the filter cake and at the micropores of shale rock, thereby creating a dense mud cake on the outside of the shale formation. Field trial results demonstrate that the incorporation of the nano-micro sealant CLG-Seal into the drilling fluid for the third section of Well WY3-2-3HF reduced the PPA fluid loss to 4.6 mL. This value represents a substantial reduction compared to adjacent wells and signifies a remarkable improvement over the drilling fluids previously employed in the Longmaxi Formation of this block. Furthermore, the treated drilling fluid exhibited a superior filtration control pressure capacity of 10.5 MPa. The operation was completed successfully without any lost circulation or wellbore instability, and achieved a drilling footage of 42 h with an average penetration rate of 7.81 m/h. The mud weight was reduced by approximately 0.08–0.10 g/cm³ compared to offset wells. These results confirm the excellent application efficiency of the newly developed CLG-Seal in field operations.

The flow behavior of shale gas horizontal wells is relatively complex, and this should be regarded as the main reason for which conventional pipe flow models are not suitable to describe the related dynamics. In this study, numerical simulations have been conducted to determine the gas-liquid distribution in these wells. In particular, using the measured flow pressure data related to 97 groups of shale gas wells as a basis, 9 distinct pipe flow models have been assessed, and the models displaying a high calculation accuracy for different water-gas ratio (WGR) ranges have been identified. The results show that: (1) The variation law of WGR in gas well satisfies a power function relation. (2) The well structure is the main factor affecting the gas-liquid distribution in the wellbore. (3) The Beggs & Brill, Hagedorn & Brown and Gray models exhibit a high calculation accuracy.