Microscopic Oil Displacement Characteristics of Low Permeability Glutenite Reservoir in Qaidam Basin

Abstract

Understanding of microscopic oil displacement characteristics is the key to improve recovery, which are obvious different between sandstone reservoirs and glutenite reservoirs. The sedimentary environment of glutenite reservoirs is more complex with lower porosity and permeability, smaller pore throat, and stronger reservoir heterogeneity. Water cut increases quickly with poor waterflooding performance. This paper presents its main influence factors and characteristics on microscopic oil displacement of low permeability glutenite reservoirs, in order to guide the successful development for glutenite reservoirs. Firstly, the pore structure characteristics is classified into 4 types which are studied by means of thin section, cast section, core observation and SEM. The effect of different pore structures on oil displacement efficiency is studied. Secondly, microscopic oil displacement characteristics and its key impact factors are studied by natural glutenite micromodels oil-water flooding experiment. Finally, the effects of injection pressure and injection multiples on the displacement efficiency of different pore structures are observed through microscope and image collection system, while the methods of improving oil displacement efficiency are simulated by experiments. The results show that the pore structure is classified into wide throat-big pore, wide throat-small pore, thin throat-big pore and thin throat-small pore. The oil displacement efficiency of wide throat-big pore is the highest, which is 67.7% with uniform displacement. While the oil displacement efficiency of thin throat-small pore is the lowest, which is 44.2% with non-uniform displacement. The none-water-production period of all types is very short which indicates that the main production of glutenite reservoir is produced after water breakthrough. The microscopic residual oil is mainly in the form of circumfluence. When the displacement pressure increases, the displacement efficiency will increase, too. But when the pressure exceeds a certain value, the displacement efficiency is basically unchanged. With the pressure increasing, displacement efficiency of the third pore structure type of core will increase by 19.7% at most, while the fourth will increase by 14%. When the injection multiple increases, the displacement efficiency will increase until the injection multiple of the third pore structure type of core exceeds 36PV and the fourth exceeds 69PV. Intermittent mining, intermittent water injection or surfactant injection are effective means to improve efficiency, the oil displacement efficiency can be increased by more than 10%. The results are successfully applied in Qinghai oilfield with optimized development strategy, which final oil recovery increases by 4%. Furthermore, this paper can also guide development strategy adjustment and optimization for the high water-cut glutenite reservoir.