CO_(2)驱后水气交替注入驱替特征及剩余油启动机制

Displacement characteristics and remaining oil startup mechanism of WAG after CO_(2) flooding

水气交替注入(WAG)是特低渗透油藏提高采收率的有效手段之一,但在CO_(2)连续气驱后实施WAG驱,仍然存在驱替特征模糊、剩余油启动机制不明确等问题。以海拉尔油田贝14区块为研究对象,借助Micro⁃CT研究WAG驱启动剩余油的微观作用机制,同时通过长岩心驱替实验研究CO_(2)驱后水气交替注入的驱替特征。Micro⁃CT实验结果表明:目标区块大孔隙的体积比例超过85%,在被CO_(2)全部动用后成为了气窜通道,采收率仅47.95%;CO_(2)驱后WAG驱不仅启动次级大孔隙中的剩余油,对中小孔隙的剩余油也有不同程度的动用。长岩心实验结果表明:在CO_(2)驱后开展WAG驱,水和气段塞需要交替注入一定量(0.40 PV左右)后采收率才能大幅度增加,气水比和段塞尺寸存在最优值,分别为1∶1和0.10 PV,该条件下WAG驱的采收率增幅主要由第3、4交替轮次所贡献,10轮次的水气交替注入可在CO_(2)驱的基础上提高采收率18.68百分点。研究成果可为特低渗透油藏CO_(2)驱后进一步提高采收率提供理论支撑。

Water alternating gas injection(WAG)is one of effective techniques to enhance oil recovery of ultra-low permeability reservoirs,but WAG flooding after continuous CO_(2) flooding still has problems such as ambiguous dis⁃placement characteristics and unclear remaining oil startup mechanism.Taking Bei 14 block of Hailar Oilfield as re⁃search object,the microscopic mechanism of WAG flooding to start up the remaining oil is studied with Micro-CT,and the displacement characteristics of WAG after CO_(2) flooding are studied by long-core displacement experiments.The results of Micro-CT experiment show that the volume of large pores in target area accounts for more than 85%,becoming channeling paths after produced by CO_(2),with oil recovery of only 47.95%.The subsequent WAG flooding can not only start up the remaining oil in secondary-level large pores,but also produce the remaining oil in small and medium pores in various degrees.The results of long-core experiment show that WAG flooding after CO_(2) flood⁃ing requires specific amount(about 0.40 PV)of water and gas plugs injected alternately to increase recovery signifi⁃cantly.There are optimal values for gas-water ratio and slug size,being 1∶1 and 0.10 PV,respectively.In this con⁃dition,the incremental recovery of WAG flooding is mainly contributed by the 3rd and 4th rounds.10 rounds of WAG can increase oil recovery by 18.68 percentage points after CO_(2) flooding.The research results provide theoreti⁃cal support for further improving oil recovery after CO_(2) flooding in ultra-low permeability reservoirs.