致密储层不同驱替方式下超临界CO2蓄能返排效果实验研究

Experimental study of supercritical CO2 storage and flowback under different displacement methods in tight reservoirs

目前超临界CO2辅助压裂技术已成为致密油开发的有效方式之一,在压裂过程中,压裂液的蓄能返排效果成为近年来关注的热点之一。为了对超临界CO2驱替过程中储层蓄能效果和流体的返排效果进行定量评价,自主设计实验装置对不同驱替方式下超临界CO2蓄能返排效果进行研究。在油藏温度和压力下,对比单独注气、单独注水及水气交替注入方式下岩心系统的平均压力变化以及控压返排效果,并对水气交替注入方式的注气段塞进行优化。实验结果表明:以超临界CO2作为前置液的压裂可以大幅度提高后续注入滑溜水的蓄能效果;注入的超临界CO2使原油及束缚水体积发生膨胀,其膨胀程度受超临界CO2的溶解量影响较大,而受岩心系统压力影响较小;先注超临界CO2再注滑溜水的驱替方式蓄能效果最佳,且能实现超临界CO2封存,滑溜水的返排效率及出油效率最高;从蓄能返排效果来看,0.5 PV的超临界CO2段塞注入量最优。

At present,the supercritical CO2 assisted fracturing technology has become one of the effective methods for the development of tight oil reservoirs.In the process of fracturing,the energy storage and flowback effect of fracturing fluid has become one of the focuses in recent years.In order to quantitatively evaluate the energy storage in the formation and fluid flowback efficiency during the supercritical CO2 displacement,this paper independently designed an experimental apparatus to investigate the efficiency of energy storage and flowback under different displacement modes of the supercritical CO2.The changes of mean pressure and flowback efficiencies of the core system under continuous gas injection,continuous water injection and alternate water and gas injection were compared by the laboratory experiment under reservoir temperature and pressure,and the gas injection slug with alternating water and gas injection was also optimized.The results show that the supercritical CO2 as a preflush can increase the energy storage efficiency of subsequent injection of slippery water.The injected supercritical CO2 expands the volume of crude oil and irreducible water,and the degree of expansion is greatly affected by the dissolved amount of supercritical CO2,but less affected by the core system pressure.The displacement method of injecting supercritical CO2 followed by injecting slippage water shows the best energy increasing efficiency,which can achieve supercritical CO2 storage,and the highest flowback efficiency of slippage water and oil recovery.From the perspective of the energy storage and fluid flowback efficiencies,the best volume of the supercritical CO2 slug is 0.5 PV.