为解决致密油开采初期产量递减快、采收率低的问题,结合扶余致密油藏岩石和流体性质,采用数值模拟方法,研究了注入时机、注入压力对CO2吞吐采油效果的影响,优化工程参数,分析了致密油CO2吞吐过程中的油藏压力分布特征、界面张力分布特征以及致密油CO2吞吐开发的可行性。研究表明:油层压力系数降至0.65左右时开展CO2吞吐,CO2可与原油充分接触,有利于发挥CO2原油溶解作用,提高开采效果;注入压力接近最小混相压力时,CO2以游离相进入基质,更易进入基质深处,与致密油充分接触,并在原油中溶解混相,改善开发效果。溶解平衡后,CO2与致密油界面张力随时间快速增加,以此确定闷井时间。研究确定了CO2吞吐采油最佳工程参数,即注入量为8 000 t,注入速度为120 t/d,注入压力为27 MPa,闷井时间为30 d,计算该方案下增油量为3 084 t,换油率为0.39。水平井现场试验也证明CO2吞吐技术应用于扶余致密油藏,采收率提高了1.38～3.33个百分点。研究结果对拓展致密油藏CO2吞吐开采技术的应用具有重要意义。
To address the issues of high production decline rate and low recovery rate in the early stage of tight oil development,considering the rock and fluid properties of Fuyu tight oil reservoir, numerical simulation methods were used to study the effects of injection timing and injection pressure on the effect of CO2 huff-n-puff.The engineering parameters were optimized, the characteristics of reservoir pressure distribution and interfacial tension distribution are analyzed,the operability of CO2 huff-n-puff development of tight oil is studied.The results indicate that CO2 huff-n-puff should be implemented when the oil reservoir pressure coefficient drops to around 0.65.At this time,CO2 can fully contact with crude oil,which is beneficial for the dissolution,thereby improving the recovery efficiency.As the injection pressure approaches the minimum miscible pressure,CO2 in a separate phase enters the matrix,making full contact with the tight oil deep within.The dissolved miscible phase improves the development effect.After reaching dissolution equilibrium,the interfacial tension between CO2 and tight oil increases rapidly over time,the time of well shut-in can be determined at this moment.The optimal engineering parameters for CO2 huff-n-puff recovery in this study are:the injection volume is 8000 t,the injection rate is 120 t/d, the injection pressure is 27 MPa,and the time of well shut-in is 30 d. According to this scheme, the increase of crude oil production is calculated to be 3 084 t,and the oil replacement ratio is calculated to be 0.39. The in-situ test of horizontal well also confirms that the application of CO2 huff-n-puff technology in Fuyu tight oil reservoir increased the recovery rate by 1.38 to 3.33 percentage points.The research results are of great significance for further expanding the application of CO2 huff-n-puff technology in tight oil reservoirs.
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