特低渗油藏注CO2驱油微观机制

 利用核磁共振仪、原油气相色谱仪、细管实验和特低渗透岩心物理模拟实验,对细管实验的CO₂非混相驱和混相驱驱替原油过程中的产出物和残余油进行取样分析,并结合核磁共振实验分析结果得到特低渗透油藏注CO₂微观驱油机制.结果表明,CO₂非混相驱主要是萃取C₁₈以前的组分,而CO₂混相驱,不但可以萃取C₁₈以前的组分,也能萃取C₁₉—C₂₅的组分,初期主要萃取低碳分子的组分,随着驱替的进行,逐步萃取较高碳分子的组分;与原油组分和非混相驱的残余油组分相比,混相驱的残余油中轻质组分所占比例减少,中间组分和重质组分所占比例较大,导致残余油黏度变大;在非混相气驱和混相气驱时,由于残余油黏度发生变化和胶质沥青沉淀,使岩心孔隙中一部分流体变成不可动流体,核磁共振图谱发生左移现象.

Microscopic Mechanism of CO2 Flooding in Extra-low Permeability Reservoirs

By means of NMR, oil gas chromatography, slim tube experiments, and extra-low permeability core physical simulation experiments, the produced oil and residual oil acquiring from slim tube experiments both in CO₂ immiscible flooding and CO₂ miscible flooding are analyzed. Combined with the NMR results obtained from the physical simulation experiments for extra-low permeability core, the microscopic mechanism of CO₂ flooding in low permeability reservoirs is discovered. It includes three aspects: The components before C₁₈ are mainly extracted by CO₂ immiscible flooding, while the CO₂ miscible flooding not only could extract the components before C₁₈, but also could extract components C₁₉-C₂₅; low-carbon molecules are initially extracted, with the ongoing flooding, higher-carbon molecules are gradually extracted; Compared with crude oil and immiscible flooding residual oil, the components of miscible flooding residual oil consist of smaller proportion of light components and larger proportion of intermediate and heavy components, causing lager residual oil viscosity; The residual oil viscosity changes and the asphaltene precipitates caused by miscible and immiscible gas flooding make a part of pore fluid immobile and the graph of NMR shifted left.