CO2驱替方式对特低渗砂岩储层物理性质变化的影响

Impact of CO2 displacement methods on changes in physical properties of ultra-low permeability sandstone reservoirs

特低渗砂岩储层在注CO2过程中岩石物理性质会发生明显变化,且不同CO2驱替方式对储层物理性质的影响存在差异。在地层条件下(温度78℃、压力18 MPa)对物理性质相近高含水的储层岩心进行CO2-地层水交替驱替(CO2-WAG)和CO2驱替实验,对比驱替前后岩心物理性质变化的异同。结果表明:CO2驱替后的岩心渗透率降低幅度小于CO2-WAG驱替后的岩心,孔隙度都无明显变化;驱替后岩心的部分大孔隙中发现高岭石、碎屑颗粒及盐结晶堆积和附着;CO2-WAG驱替后岩心中大孔隙比例下降,中等孔隙比例增加,孔隙半径分布向中间集中,CO2驱替后岩心孔隙半径分布变化范围和幅度均小于前者;驱替后岩石物理性质变化存在差异的主要原因为岩石-CO2-地层水相互作用的程度不同、参与流动孔隙的半径范围不同和颗粒运移动力不同。

The physical properties of ultra-low permeability sandstone reservoirs change significantly during CO2 injection,and the effects of different CO2 displacement methods on reservoir physical properties are different as well.CO2-formation water alternate(CO2-WAG)flooding and CO2 flooding experiments were carried out on the fully saturated water cores of reservoirs with similar physical properties under formation conditions(78℃,18 MPa),and the similarities and differences of the changes on physical properties of the cores before and after flooding were compared.It is found that the decrease of core permeability after CO2 flooding is smaller than that after CO2-WAG flooding,but with almost unchanged porosity in both cores.After flooding experiments,some large pores in the core are blocked by debris particles,clay minerals and salt crystals.The proportion of large pores decreases while the proportion of medium pores increases.And the distribution of pore size concentrates in the middle.However,the changes of range and amplitude of the pore size distribution in the core after CO2 flooding are less than those after CO2-WAG flooding.The main reasons for the differences in the changes in the physical properties are:the different degree of rock-CO2-formation water interactions,the different range of pores radii in the fluid,and the different power of the particle migration.