Blocking mechanism of multicomponent thermal fluid huff-and-puff and unblocking with surfactant oil in offshore heavy oil cold production wells

In Nanpu Oilfield of the Bohai Sea, heavy oil cold production wells are produced using multicomponent thermal fluid huff-and-puff with temperatures below 150 °C. However, severe reservoir plugging occurs after huff-and-puff, leading to decreased or halted oil production. Through laboratory experiments on the causes of reservoir plugging, it was clarified that the viscosity of heavy oil increases after emulsification with injected water, enhancing the liquid resistance effect of droplets(aggregates) passing through pore throats, thereby increasing the “startup pressure”. When the displacement pressure in the emulsification zone is lower than the startup pressure, reservoir plugging occurs. Experiments were conducted on dilution and viscosity reduction using an surfactant oil unblocking system composed of fatty alcohol polyoxyethylene ether sodium hydroxypropyl sulfonate, dodecyl trimethylammonium salt and sodium succinesulfonate in a molar ratio of 3∶1∶1. After adding 2% of this system, the diluted oil and the heavy oil containing 50% water were mixed according to the dilution-heavy ratio of 2∶8. Compared with the heavy oil containing 50% water (7 698 mPa·s), the viscosity was reduced by 98.5%, and the interfacial tension was reduced by 74.5% compared with that of the heavy oil and water (21.86 mN/m). In addition, the starting pressure of the emulsification zone was reduced to 1.86 MPa. Through numerical simulation, the injection volume of diluted oil and the surfactant concentration were optimized, predicting that injecting 2 000 m³ of diluted oil and adding 2% of the surfactant to the first 1 000 m³ of diluted oil could achieve unblocking in the emulsification zone, resulting in a net oil increase of 4 016.5 m³. Field tests show that after unblocking a well, the daily oil production reached 5.9 m³/d, with a cumulative oil increase of 980.7 m³, indicating a significant unblocking and production increase effect. This technology provides a guidance for plugging removal and increasing oil production in offshore heavy oil cold production wells.