气水交替改善CO2驱油效果的适应界限*

针对低渗特低渗透油藏CO2驱油效果差、气窜现象严重等特点,开展了CO2驱气水交替注入(WAG)方式改善CO2驱油效果研究,评价了岩心渗透率及其非均质性对气水交替驱油效果的影响;选取天然露头和人造非均质岩心,对气水交替的注入速率、注入参数及注入量进行优选,进行了WAG驱的适应性评价。研究表明,对于0.5×10^-3、1×10^-3和5×10^-3μm^2的低渗特低渗均质岩心,气水交替驱能够实现良好的流度控制作用,延长CO2的窜逸时间,且渗透率越低,气窜时间越晚;渗透率级差为5、10和50的非均质性岩心,渗透率级差越小,气水比越高,提高采收率效果越好。渗透率级差大于10时,气窜时间明显提前,特别是当级差大于50时,气水段塞无法有效启动低渗基质中的剩余油,快速气窜而无经济效益。利用气水交替在适应界限范围内可显著降低CO2流度,延长CO2窜逸时间,启动基质中的剩余油,提高剩余油采收率。图16表2参20。

Experimental Study on the Adaptation Limit of WAG to Improve CO2 Flooding Effect

In view of the characteristics of poor CO2 flooding effect and serious gas channeling phenomenon in low permeability and ultra-low permeability reservoirs,water alternate gas(WAG)improving CO2 flooding effect was investigated.Taking reservoir physical simulation as the main research method,the impacts of low permeability and ultra-low permeability core and heterogeneity on the WAG flooding effect was studied.Selecting natural outcrops and artificial heterogeneous cores,and the injection method,injection speed,injection parameters and injection timing were researched and optimized for WAG flooding.The results showed that WAG flooding could achieve good mobility control and prolong CO2 channeling time for low permeability and ultra-low permeability homogeneous cores of 0.5×10-3μm2,1×10-3μm2 and 5×10-3μm2,and the lower the permeability,the later the gas channeling time.As for the heterogeneous cores with the permeability differential of 5(1×10-3/5×10-3μm2),10(1×10-3/10×10-3μm2)and 50(1×10-3/50×10-3μm2),the smaller the permeability differential,the higher the gas-water ratio,the higher the enhanced oil recovery.When The permeability differential was 10,the gas channeling time was obviously advanced,especially when the permeability differential was greater than 50,the WAG slug could not start the remaining oil in the low-permeability matrix,and only a small part of the crude oil could be displaced in gas injection stage.WAG in use within limit adaptation could significantly reduce the CO2 mobility and prolong CO2 channeling time,start the residual oil in matrix and enhanced the residual oil recovery.