Abstract: In order to reflect the effect of wettability and pore structure on seepage characteristics of surfactant flooding in low permeable reservoir,an extended capillary number (ECN) theoretical model is put forward,which contains wettability and pore structure on the basis of traditional capillary number (TCN) theory and combined physical simulation and mathematical treatment methods.ECN and residual oil saturation (ROS) values are collected in the different low permeability cores through experiments,and cross correlation chart between ECN and ROS is drawn.A nonlinear processing method of the relative permeability curve (RPC) is established with ECN,and reservoir engineering application is conducted.The results show that ECN theory can embody the wettability and pore structure in low permeability reservoirs,and the chemical flooding EOR needs combined action of interfacial tension reduction and reservoir wettability change in low permeability reservoirs.There is a nonlinear,negative correlation between ECN and ROS,and ROS will decrease drastically only when the increasing ECN exceeds a critical value.The nonlinear characterization method has a good agreement with the experimental results.Finally,the new method could reflect the effect of wettability and displacement characteristics of surfactant flooding,and thus provides a new tool for reservoir engineering and numerical simulation of chemical flooding in low permeability reservoirs.

Key words: pore structure, wettability, residual oil saturation, extended capillary number, relative permeability curve, surfactant, low permeability