Hydrogen-bond-modulated flowable weak gel for EOR in ultra-high temperature and ultra-high salinity fracture-cavity ordinary heavy oil reservoirs

This paper presents a novel weak-gel oil flooding agent for ultra-high temperature and ultra-high salinity fracture-cavity ordinary heavy oil reservoirs and evaluates its performance. The agent has changed the characteristic that the traditional gel material can hardly flow. At 130 °C and 22 × 10⁴mg/L, the viscosity is 750mPa·s and almost not affected by shear. After aging at 130 ℃ for 50d, the viscosity retention rate reached 83.1 % and the dehydration rate was only 4 %. Rheological tests showed that the addition of guanidine Hydrochloride (GuHCl) greatly improved the flow performance of the system. Through cryo-scanning electron microscope (Cryo-SEM) and X-ray photoelectron spectroscopy (XPS) analysis, it was inferred that the -CONH₂ on the polymer backbone was protected by forming hydrogen bonds with GuHCl to prevent dehydration condensation with hydroxymethylated hydroquinone, which reduced the crosslinking density and enhanced the flow performance of the system. In the fracture model, when crude oil has a viscosity of 400mPa·s and the fracture scale is 8 mm, the weak-gel flooding agent's recovery rate reaches 68.54 %, 49.32 % higher than water flooding and 28.37 % higher than polymer flooding. In the fracture-cavity model, with a cave volume of 25 mL and crude oil viscosity at 400mPa·s, its recovery rate is 65.51 %, 54.51 % and 48.38 % higher than water and polymer flooding respectively. This shows the agent's good performance in such models, offering guidance for enhanced oil recovery in ultra-high temperature and ultra-high salinity fractured-cavity heavy oil reservoirs.