Summary: Preformed particle gels (PPGs) have been broadly used in temporary plugging steering fracturing techniques. However, it is difficult to achieve both excellent mechanical and degradable properties in conventional PPGs. To address this issue, double crosslinked self-degradable preformed particulate gels (DCSPPGs) were prepared using starch, hydroxyethyl acrylate, and acrylamide, crosslinked with poly (ethylene glycol) diacrylate (PEGDA) and borax. The structure was characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), confirming the successful preparation of DCSPPGs. The mechanical properties of DCSPPGs indicate that the elastic modulus of DCSPPGs reached 22 918 Pa, which is much higher than that of most similar particle gels. Meanwhile, DCSPPGs can be degraded to a low-viscosity liquid within 36 hours without any visible solid residues. It shows that DCSPPGs have excellent mechanical and degradation properties. In addition, this study systematically evaluated the effects of particle size, temperature, and monovalent (Na+) vs. divalent (Mg2+ and Ca2+) ions on the swelling and degradation properties of the DCSPPGs. These findings reveal that DCSPPGs maintain exceptional degradation performance in high-temperature (150°C) and high-salinity (30% NaCl) environments. Core displacement experiments showed that the maximum breakthrough pressure of the core after DCSPPGs sealing could reach 27.69 MPa and that the permeability injury rate of the core after degradation was only 3.55%. It shows that DCSPPGs have excellent sealing performance and minimal formation damage post-degradation.

Horizontal wells’ multi-section and multi-cluster hydraulic fracturing plays an important role in the efficient development of shale gas. However, the influence of the perforating hole and natural fracture dip angle on the process of hydraulic fracture initiation and propagation has been ignored in the current researches. This paper presents the results related to a tri-axial large-scale hydraulic fracturing experiment under different natural fracture parameters. We discuss the experimental results relating to the near-wellbore tortuosity propagation of hydraulic fractures. Experimental results showed that the triaxial principal stress of the experimental sample was deflected by the natural fracture, which caused significant near-wellbore tortuosity propagation of the hydraulic fractures. The fractures in most rock samples were not perpendicular to the minimum horizontal principal stress after the experiment. As well, the deflection degree of triaxial principal stress direction and the probability of hydraulic fractures near-wellbore tortuosity propagation decreased with the increase of the natural fracture dip angle. After hydraulic fractures’ tortuous propagation, the hydraulic fractures will propagate in the direction controlled by the triaxial stress in the far-wellbore area. For reservoirs with natural fractures, proppant in hydraulic fracturing should be added after the fractures are fully expanded to prevent sand plugging in tortuous fractures. When the permeability of natural fractures is low, the volume of fracturing fluid entering natural fractures is small, and hydraulic fractures are easy to pass through the natural fractures.