We targeted high-temperature and highly saline old oil fields, whose environmental conditions could be attributed to the significantly high heterogeneity cause by long-term water flooding. The Huabei Oilfield was chosen as the research object. We developed a hydrophobic functional monomer–polymer with temperature and salt resistance by introducing the temperature-resistant and salt-resistant monomer NVP and a hydrophobic functional monomer into the main chain for copolymerization. We used a crosslinking agent with phenolic resin to prepare a weak gel system that showed temperature and salt resistance and investigated its temperature and salt resistance, infective property, plugging performance, liquid flow ability, micropore throat migration, and plugging characteristics. The results obtained using the infrared spectroscopy technique revealed the successful preparation of the phenolic resin crosslinker. The weak gel exhibited good temperature and salt resistance when the polymer concentration was 2000 mg/L, the cohesion ratio was 1:1.5, the additive concentration was 2000 mg/L, the reservoir temperature was 120 °C, and the injected water salinity was 40,300.86 mg/L. The average viscosity retention rate of the 90-day weak gel reached more than 80% and its microstructure was examined. The coreflow experiment results revealed that the weak gel system was characterized by good infectivity. After plugging the weak gel, the effect on the direction of the liquid flow was evident and the flow rate of the low permeability layer increased to a maximum of 48.63% under conditions of varying permeability levels. A significant improvement in the water absorption profile was achieved. The plugging was carried out through a sand-filling pipe under varying permeability conditions and the pressure measuring points in the sand-filling pipe were sucessfully pressurized. The migration ability of the weak gel was good and the blocking rate was >85%.
The class III reservoir in the Daqing Oilfield has poor sand body development, poor reservoir physical properties, and poor effects of measures. Its water drive recovery degree is low and the remaining reserves are large. It is the key target oil layer of the Daqing Oilfield. Due to the sedimentary characteristics and reservoir physical properties of class III reservoirs, conventional EOR technology (chemical flooding) and conventional stimulation and injection measures (fracturing) have poor potential tapping effects on class III reservoirs. According to the special reservoir conditions and development characteristics of the class III reservoir in the Daqing Oilfield, fracture-flooding technology is innovatively proposed, which greatly improves the recovery of remaining oil in class III reservoirs. The rapid injection of hydraulic surface activators into the formation and displacement of the remaining oil in class III reservoirs through rock core flooding experiments were simulated in this paper. The nuclear magnetic resonance (NMR), confocal scanning laser, and computed tomography (CT)-scanning technologies were applied to study the remaining oil distribution after fracture flooding. The results show that: (1) After fracture flooding, the peak value of the T2 spectrum curve of NMR shifts to the left and the degree of middle and small pore space production increases obviously. (2) Confocal scanning laser study shows that the remaining oil in thin membranous and clustered forms on pore surfaces is highly utilized. (3) CT scan study shows that the remaining oil in membranous and clustered forms is effectively utilized after fracture flooding. In summary, fracture-flooding technology can improve the washing efficiency and sweep volume of class III reservoirs, thus enhancing the recovery efficiency of class III reservoirs.
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