Abstract:

In order to investigate characteristics of volatile contaminant release and migration during CO₂ geological sequestration, a contaminant migration model with multiphase displacement process in a CO₂-aqueous phase-residual oil phase system was proposed. Numerical simulations were performed to figure out the multiphase displacement and the contaminant migration, and further to explore the effects of interphase mass transfer, initial profiles of the residual oil and injection rates on the volatile contaminant migration. The results indicate that the volatile contaminant enters into the supercritical CO₂ phase due to the multiphase displacement process, and migrates with the mobile phase in the subsurface formation. An interphase mass transfer region (IMTR) gradually forms. The evolution of IMTR directly reflects the release characteristics of contaminant and migration behaviors of CO₂ coerced with contaminant. A larger interphase mass transfer coefficient results in a narrower IMTR and a faster decay rate of the oil phase. When the initial saturation of the oil phase is larger, the IMTR becomes much narrower and the oil phase decays much slower. With increasing CO₂ injection rate, IMTR increases and the oil phase decays rapidly. The proposed model and the results intuitively describe the migration characteristics of the volatile contaminants in a variety of geological reservoirs during CO₂ geological sequestration. The proposed model can also be used for the risk analysis and evaluations of CO₂ storage.

Key words: CO₂, sequestration, multiphase flow, displacement process, interphase mass transfer, contaminant migration

摘要：

为了研究封存CO₂注入过程可挥发性污染物的释放和迁移特性,本文建立了CO₂-水-残余油多相流驱替过程中的污染物迁移模型。采用数学模拟方法分析了多相驱替过程和污染物迁移过程,并研究了相间传质特性、初始油相分布和CO₂注入速率等对污染物迁移过程的影响。研究表明:CO₂驱替过程将促使可挥发污染物进入CO₂相并随之在地层中迁移,逐渐形成相间传质区域。相间传质区域的演化反映了污染物释放和CO₂裹挟污染物迁移的特性。可挥发污染物的传质系数越大,相间传质区域越窄,油相饱和度衰减越快;油相初始饱和度较大时,其饱和度衰减相对缓慢,对应的相间传质区域也较窄。当CO₂注入速率增大时,相间传质区域增大,油相饱和度衰减变快。本文模型可用于不同地质储层环境下封存CO₂时可挥发性污染物的迁移特性分析,并用于封存CO₂的风险分析与评价。

关键词: CO₂, 封存, 多相流, 驱替过程, 相间传质, 污染物迁移