考虑杂质混入、水盾阻隔及孔隙尺寸效应的CO2近混相驱油典型特征研究

In view of the limitation of sedimentary environment, oil characteristics and gas sources in most oilfields of China, the implementation of CO2 near miscible flooding under the minimum miscible pressure can not only obtain the comparative oil recovery with less cost, but also greatly enhance the potential of CO2 flooding in China. However, miscibility, instead of miscible degree, is always the research focus and key factor. This proposal is to reveal the typical characteristics, dominant factors and mechanism of near miscible flooding based on the clear definition of near miscibility, identification method and principle of pressure interval with both physical, theoretical and simulation methods. Focusing on three key factors of the mixing of contaminants, water shielding and pore size effect, effect of factors such as the composition and content of the mixing gases, oil and water saturation, wettability, and the size and distribution of pores on miscibility degree will be studied quantitatively. Numerical simulation considering the pore size effect will be established accordingly. On this basis, the content limit of CO2 and the corresponding adjustment methods for the typical reservoirs of China can be obtained. Moreover, the mechanism and action mode of water shielding and water blocking will be studied from the angle of both bypassed flow, cross flow and Marangoni flow In sum, this proposal will provide theoretical basis and technical support for the promotion and implementation of CO2 near miscible flooding in China.

考虑到我国大部分油田在沉积环境、原油品质和气源等方面的限制，适当降低压力、实施CO2近混相驱不仅能获得与混相驱接近的驱替效果，有效降低开发成本，也能极大提高我国推广和实施CO2驱的潜力。然而，能否混相一直是该领域的研究重心和核心指标，对近混相的关注非常有限。本项目拟在明确近混相定义，建立近混相区间界定方法和划分原则的基础上，综合运用物理实验、理论分析和数值模拟相结合的方法，揭示近混相区别于混相/非混相的典型特征、主控因素和作用机理。从“杂质混入”、“水盾阻隔”及“孔隙尺寸效应”的角度出发，量化杂质气体（组成和含量）、油水饱和度、储层润湿性、孔隙大小和分布等因素对混相程度的影响规律，建立综合考虑孔隙尺寸效应的数学模型；获得我国典型油藏实施近混相驱的CO2含量下限及调控方法；从绕流、错流和马格朗尼流等多角度探索水盾阻隔的影响机理和作用机制，为我国推广和实施CO2近混相驱提供理论基础和技术支撑。

实施CO2近混相驱不仅能获得与混相驱相比拟的驱替效果，也能有效降低开发成本，实现温室气体埋存。本项目首次提出并建立了基于驱油效率和界面张力的非纯CO2-原油体系近混相压力区间综合界定方法；基于147组细管实验数据，筛选和评价了影响CO2-原油体系混相程度的9个主控因素，分别建立了纯/非纯CO2近混相驱压力区间经验公式和基于BP神经网络、支持向量机等机器学习方法的预测模型，相对误差小于10%；定量评价了油藏温度、原油组成和气体杂质混入等对CO2驱油混相程度的影响规律；首次从驱油效率贡献率的角度，考虑驱替机理的差异，对非纯CO2近混相驱油的采出过程进行了阶段划分，并量化了CO2纯度对分项相态机理的影响规律，揭示了近混相区别于混相和非混相的典型特征、主控因素和作用机理，探索了通过添加中间组分（C2-C6）调整非纯CO2近混相驱压力区间和CO2纯度下限的方法。在此基础上，评价了水盾阻隔效应影响下CO2近混相驱油的典型特征，为水驱油藏转CO2近混相驱油的转注时机及水气交替气窜防治提供了理论指导。一方面，水相的存在增加了CO2在多孔介质中的滞留时间，促进了原油-CO2体系的相互作用，推迟了气窜时间；另一方面，过厚的水膜阻碍了CO2-原油之间的多次接触，延缓和削弱了油气体系的混相程度。最后，明确了低渗、致密油藏CO2驱油过程中，不同驱替相带（原始油相带、注气前缘相带、混相/近混相带、注气后缘相带等）渗流能力的影响规律，发现混相程度和孔隙尺寸效应可显著影响油藏流体的渗流能力，采用流度可综合体现这两个主控因素的影响，流度越大，渗流能力越强。与混相驱相比，近混相驱油两相区缩小，但残余油下的气相相对渗透率显著降低，波及效率提高，最终采收率并未明显降低。本项目研究成果可为扩大和推广CO2近混相驱提供理论基础和技术支持。