致密油藏多级压裂水平井CO_2吞吐机理

Mechanism of CO_2 huff & puff of multi-stage fractured horizontal well in tight oil reservoir

致密油藏衰竭开采产量递减快、采收率低,急需实施能量补充措施。CO_2以良好的注入性、易溶于油等特性,且具有降低原油黏度、使原油体积膨胀等增油机理,因而受到普遍关注。文中以鄂南红河油田为例,采用物理模拟和数值模拟相结合的方法,研究了致密油藏多级压裂水平井CO_2吞吐机理和影响因素。研究结果表明:CO_2注入压力越高,焖井时间越长,周期注入量和注入速度越大,裂缝间距越小,CO_2吞吐效果越好;CO_2吞吐能够有效增加储层弹性能,但随着吞吐轮次增加,CO_2越来越难以进入基质深部接触更多的原油,增加的有效弹性能逐渐减少;CO_2吞吐多轮次间混相程度是交互变化的,对比不同轮次注入和焖井阶段,第1轮次CO_2与原油间能够实现混相,但随着吞吐的进行,油藏亏空越来越严重,气相逐渐出现,且波及范围增大,油气界面张力增加,逐渐过渡到非混相状态;延长焖井时间能够发挥CO_2溶解扩散作用,但随着焖井时间延长,CO_2波及体积增幅减小。

Due to the extremely tight formation, the production declines tast and the primary oil recovery factor is very low. Thus, it is urgent to implement the energy supplementary. Because of the high injection capacity, high solubility in oil, the capacity to reduce oil viscosity, and the capability to expand the crude oil volume, CO2 is widely concerned by reservoir engineers. In this paper, the meehanism and influencing taetors of CO2 huff ＆ puff, which was applied to multi-stage fractured horizontal well in the target tight oil reservoir of Honghe Oilfield, were studied using the physical simulation and numerical simulation methods. The simulation result shows that increasing the injection pressure, the soaked time, the injection volume, injection rate, or reducing the space of fraeturing could improve the CO2 huff ＆ puff efleet. CO2 huff ＆ puff can efleetively increase the reservoir energy by increasing cycles. However, it is difficult tor CO2 to go deeper into the matrix and contact with fresh oil by increasing cycles. The miscibility degree of CO2 huff ＆ puff alternately changes by increasing cycles. At soaking stage of the first cycle, CO2 and crude oil are miscible. As the cycle increases, the reservoir energy deficit is more obvious, gas phase gradually appears, the intertaeial tension increases and the miscible state transits to immiscible state. Due to the CO2 diflusion and dissolution, the oil recovery increases as the soaking time increases. However, the swept volume of CO2 decreases as the soaking time increases.