Water-soluble polymers are widely used in oilfields. The rheological behaviors of these polymers in high-salinity solution are very important for stimulation of high-salinity reservoirs. In this work, a novel thermal-resistant and shear-stable amphoteric polyacrylamide (PASD), prepared from acrylamide (AM), sodium styrene sulfonate (SSS), and acryloxyethyl trimethylammonium chloride (DAC) monomers, was prepared by free-radical polymerization in high-salinity solution. The amphoteric polyacrylamide was characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy (¹H NMR), elemental analysis, thermogravimetric analysis (TG), and scanning electron microscopy (SEM). The amphoteric polyacrylamide exhibited excellent salinity tolerance. The slow increase in apparent viscosity of the polymer with increase in salinity was interesting. The amphoteric polyacrylamide showed perfect temperature resistance in high-salinity solution. The viscosity retention reached 38.9% at 120 °C and was restored to 87.8% of its initial viscosity when temperature was decreased to room temperature. The retention ratio of apparent viscosity reached 49.7% at 170 s⁻¹ and could still retain it at 25.8% at 1000 s⁻¹. All these results demonstrated that PASD had excellent thermal-resistance and shear-stability in high-salinity solution. We expect that this work could provide a new strategy to design polymers with excellent salinity-tolerance, thermal-resistance, and shear-stability performances.

The distribution of lower-order faults affects the development of oil and gas and the distribution of remaining oil, which is also the key to the development of fault block reservoirs in eastern China. The lower-order faults are characterized by short extension, small fault displacement, and difficult identification by traditional seismic interpretation methods. Therefore, it is necessary to predict the development law of faults in combination with the paleostress field during the fault formation period to improve the accuracy of fault seismic interpretation. In this paper, on the basis of reservoir structural analysis and rock mechanics experiments, the geological and mechanical model of the Sanduo period is established to predict the simulation of the paleostress field during the development of lower-order faults. The development of lower-order faults in rift basins is mainly controlled by the minimum principal stress and stress difference, and the shear stress in the profile controls the tendency of faults. Controlled by the paleostress field, the occurrence of lower-order faults in the Weizhuang area, Gaoyou Sag, and Subei Basin is very complex, and the strike of the fault is distributed as a tension shear broom on the plane. The prediction results can provide a reference for the excavation of remaining oil and the development of oil and gas.