NMR study on porous flow mechanisms in low permeability reservoirs with CO_2 flooding

For the reservoirs with ultra-low permeability and extra-low permeability,gas flooding is one of the important ways to improve oil recovery.Using sandsome core samples of Jinlin low permeability reservoirs and extra-low permeability reservoirs,core tests including miscible displacement,immiscible displacement,low pressure water displacement and high pressure water displacement were designed.Porous flow mechanisms based on CO2 miscible displacement,CO2 immiscible displacement and water displacement were analyzed by microcosmic angle of NMR.It shows that gas displacement results from most of movable fluid located in macro pores and small portion of crude oil enter small pores to become unmovable fluid;high recovery percents of movable fluid located in macro pores is a key reason for CO2 miscible displacement to enhance oil recovery.A effective way for enhancing oil recovery is based on CO2 miscible displacement to increase injection/production pressure difference and to improve recovery percentage of unmovable fluid located in small pores.Gas displacement is more effective than water displacement and miscible gas displacement is superior to immiscible gas displacement.