致密油藏超前注CO ₂岩心驱替规律与孔喉动用特征

Displacement performance and pore throat characteristics of core by advanced CO ₂ injection in tight oil reservoirs

摘要

鄂尔多斯盆地致密储层天然能量不足,孔喉细小,注水开发难以实现有效驱替,而超前注CO₂是一种高效补能兼具环保效益的开发方式,基于核磁共振成像与T₂谱表征孔喉分布的原理,开展超前注CO₂岩心驱替试验,对比分析同步注CO₂与超前注CO₂、不同超前注气量和超前注气后闷井试验的驱油效果,明确不同超前注CO₂方式的驱替规律与孔喉动用特征,揭示其提高采收率的微观机制。结果表明:相较于同步注CO₂,超前注CO₂可提前动用注入端附近原油,延缓气窜时间,增加中孔与小孔的驱油效率;超前注气量需适中,过少时增产作用有限,过多易导致过早气窜,驱油效率下降;超前注气后闷井可充分发挥CO₂的扩散与溶解作用,推动油气前缘运移,进一步提高中小孔的驱油效率;超前注CO₂+闷井开发具备现场可行性。

Abstract

The tight reservoirs in Ordos Basin have insufficient natural energy and small pore throats, so it is difficult to achieve effective displacement through water flooding. Advanced or preceding CO ₂ injection is a development method with high efficiency and environmental benefits. In this study, core displacement experiments with advanced CO ₂ injection were conducted using nuclear magnetic resonance (NMR) imaging and T ₂ spectrum representation for investigating pore throat distribution. Comparative analyses were made on oil displacement effects between simultaneous and advanced CO ₂ injection modes, with different gas injection volumes and well shut-in time period after advanced CO ₂ injection. The displacement performance and pore throat mobilization characteristics of different CO ₂ injection methods were clarified, revealing the microscopic mechanisms for enhancing oil recovery. The results show that compared with simultaneous CO ₂ injection, advanced CO ₂ injection can mobilize residual crude oil at near the injection site earlier, delaying gas channeling and increasing displacement efficiency in medium and small pores. The advanced gas injection volume should be moderate, too little has limited production effects, while too much can lead to early gas breakthrough and decrease displacement efficiency. Well shut-in after advanced gas injection can fully leverage the effects of CO ₂ diffusion and dissolution, promoting oil and gas front migration and further enhancing displacement efficiency in medium and small pores. Therefore, advanced CO ₂ injection combined with well shut-in is feasible for field application.

关键词

超前注气 / 岩心驱替试验 / 核磁共振 / 驱油效率 / 孔喉动用特征

Key words

advanced gas injection / core displacement experiment / nuclear magnetic resonance / oil displacement efficiency / characteristics of pore throat utilization

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基金资助

中国石油重大战略项目(ZLZX2020-02-04-03)