Characteristics of remaining oil viscosity in water-and polymer-flooding reservoirs in Daqing Oilfield

The experimental analysis of 21 crude oil samples shows a good correlation between high molecular-weight hydrocarbon components (C 40+) and viscosity.Forty-four remaining oil samples extracted from oil sands of oilfield development coring wells were analyzed by high-temperature gas chromatography (HTGC),for the relative abundance of C 21-,C 21-C 40 and C 40+ hydrocarbons.The relationship between viscosity of crude oil and C 40+ (%) hydrocarbons abundance is used to expect the viscosity of remaining oil.The mobility characteristics of remaining oil,the properties of remaining oil,and the next displacement methods in reservoirs either water-flooded or polymer-flooded are studied with rock permeability,oil saturation of coring wells,etc.The experimental results show that the hydrocarbons composition,viscosity,and mobility of remaining oil from both polymer-flooding and water-flooding reservoirs are heterogeneous,especially the former.Relative abundance of C 21- and C 21-C 40 hydrocarbons in polymer-flooding reservoirs is lower than that of water-flooding,but with more abundance of C 40+ hydrocarbons.It is then suggested that polymer flooding must have driven more C 40- hydrocarbons out of reservoir,which resulted in relatively enriched C 40+,more viscous oils,and poorer mobility.Remaining oil in water-flooding reservoirs is dominated by moderate viscosity oil with some low viscosity oil,while polymer-flooding mainly contained moderate viscosity oil with some high viscosity oil.In each oilfield and reservoir,displacement methods of remaining oil,viscosity,and concentration by polymer-solution can be adjusted by current viscosity of remaining oil and mobility ratio in a favorable range.A new basis and methods are suggested for the further development and enhanced oil recovery of remaining oil.