Viscoelastic Surfactants with High Salt Tolerance, Fast-Dissolving Property, and Ultralow Interfacial Tension for Chemical Flooding in Offshore Oilfields

摘要

Injected chemical flooding systems with high salinity tolerance and fast-dissolving performance are specially required for enhancing oil recovery in offshore oilfields. In this work, a new type of viscoelastic-surfactant (VES) solution, which meets these criteria, was prepared by simply mixing the zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (HDPS) or N-octyldecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (ODPS) with anionic surfactants such as sodium dodecyl sulfate (SDS). Various properties of the surfactant system, including viscoelasticity, dissolution properties, reduction of oil/water interfacial tension (IFT), and oil-displacement efficiency of the mixed surfactant system, have been studied systematically. A rheology study proves that at high salinity, 0.73wt.% HDPS/SDS- and 0.39wt.% ODPS/SDS-mixed surfactant systems formed worm-like micelles with viscosity reaching 42.3 and 23.8 mPas at a shear rate of 6 s(-1), respectively. Additionally, the HDPS/SDS and ODPS/SDS surfactant mixtures also exhibit a fast-dissolving property (dissolution time 25min) in brine. More importantly, those surfactant mixtures can significantly reduce the IFT of oil-water interfaces. As an example, the minimum of dynamic-IFT (IFTmin) could reach 1.17 x 10(-2) mNm(-1) between the Bohai Oilfield crude oil and 0.39wt.% ODPS/SDS solution. Another interesting finding is that polyelectrolytes such as sodium of polyepoxysuccinic acid can be used as a regulator for adjusting IFTmin to an ultralow level (10(-2) mNm(-1)). Taking advantage of the mobility control and reducing the oil/water IFT of those surfactant mixtures, the VES flooding demonstrates excellent oil-displacement efficiency, which is close to that of polymer/surfactant flooding or polymer/surfactant/alkali flooding. Our work provides a new type of VES flooding system with excellent performances for chemical flooding in offshore oilfields.