低渗透油藏渗透率对CO2驱重力超覆的影响规律

Influence of permeability on gravity segregation during CO2 flooding in low-permeability reservoirs

为明确低渗透油藏渗透率对CO2驱重力超覆的影响规律及相关机理,采用气驱超覆物理模型开展不同渗透率岩心的CO2非混相和混相驱室内实验,以岩心气体突破时的上层采收率所占比重及岩心最终采收率等参数评价重力超覆程度,从而得出重力超覆规律,并分析其影响机理.实验结果表明,在较小渗透率变化范围内,渗透率增加对黏性力及重力的增加幅度影响较小,尤其在混相驱中,油气黏度和密度差异较小,渗透率增大后重力超覆的程度增加不明显;渗透率增加将增大非混相驱中气驱前缘移动速度,降低混相驱中油气混相程度,加之重力超覆程度加剧,最终采收率较低;韵律油层开发中可利用渗透率对重力超覆的影响规律,避免或利用重力超覆,尤其在CO2非混相驱中,正韵律油层的开发效果会更好.

In order to clarify the effect of permeability on gravity segregation and related mechanisms during CO2 flooding in low-permeability reservoirs,the CO2 immiscible and miscible flooding experiments on cores with different permeability are carried out according to gas gravity segregation model,and the recovery of top layer at gas breakthrough and final recov⁃ery are used to evaluate gravity segregation level and to reveal its mechanism.The results indicate that within a small per⁃meability variation range,the increase of permeability has little effect on the increase of the viscosity and gravity,especially in miscible flooding,the viscosity and density difference of oil and gas are small,the increase of gravity segregation is not obvious after the permeability increase;the increase of permeability also increases the moving velocity of gas front and de⁃creases the miscibility of oil and gas in immsicible flooding test,which produces a low final recovery with the influence of gravity segregation.The variation of gravity segregation with permeability can be applied to the production of rhythmic res⁃ervoirs to avoid or use the gravity segregation,especially in immiscible flooding,CO2 flooding performance in positive rhythm reservoirs could be better under the gravity segregation.