基于油水渗流差异的剩余油形成机理研究——以大庆长垣杏树岗油田为例

Remaining oil formation mechanism based on oil-water permeability difference:A case study of Xingshugang oil field in Daqing placanticline

为了从油水渗流差异角度解释剩余油的形成机理,通过开展岩心铸体薄片观察和扫描电镜测试、压汞测试、相对渗透率测试以及水驱油测试等一系列实验,将砂岩的孔喉系统、油水相对渗透率和剩余油类型进行综合分析,认为储层孔喉系统发育特征及其内部流体渗流特征是造成剩余油形成的主要因素,并且不同类型剩余油的形成机理存在差异.研究结果表明:低渗、中渗、高渗和特高渗砂岩的孔喉半径分布峰值分别为9.1%,25.3%,28.4%,40.7%,其渗流临界喉道半径分别为0.25,0.63,2.50,2.50μm,无效渗流孔喉半径的百分比分别为52.2%,42.3%,37.7%,17.5%.该数据反映了砂岩渗透率与其内部孔喉系统复杂程度的负相关关系,并且砂岩的孔喉系统越复杂,无效渗流孔喉体积的百分比越高.在注入水无法进入的无效渗流孔喉位置,存在因渗流条件差而形成的水驱后不可动剩余油.以油水黏度比为13.33的测试组为例,渗透率逐渐增加的3组样品从开始出现油水两相流到处于油水等渗状态的含水饱和度跨度分别为17.1%,29.8%,36.2%,反映了在渗透率越低的砂岩中水相的出现对油相的抑制作用越明显.在发生渗流的孔喉系统中,存在受不规则喉道结构和界面张力的影响而形成的不规则状剩余油.以与对照组渗透率比值依次为1.6和2.6的2组高渗样品为例,其驱替效率比值依次为1.2和1.4,反映了非均质储层中不同部位驱替效率的差异.在注入水优势渗流通道的分割包围区内,存在受不同位置渗流能力差异影响而形成的水驱后可动剩余油.

The aim of this study is to explain the formation mechanism of remaining oil from the perspective of oil-water seepage differences. A series of experiments were carried out, such as core casting thin section observation, scanning electron microscope test, mercury injection tests, relative permeability tests, and water displacement test. The comprehensive analysis of the pore throat system, the relative permeability of oil and water, and the types of remaining oil in the sand stone showed that the development characteristics of reservoir pore-throat system and its internal fluid flow characteristics are the main contributory factors in the formation of remaining oil, while the formation mechanisms of different types of remaining oil vary greatly. The results showed that the peak distributions of pore throat radius of low permeability, medium permeability, high permeability, and extra high permeability sand stone are 9.1%, 25.3%, 28.4% and 40.7%, respectively. The critical seepage throat radius of these four permeability types of sand stone are 0.25, 0.63, 2.50 and 2.50 μm, respectively. And these distribution probabilities of invalid seepage pore throat radius are 52.2%, 42.3%, 37.7% and 17.5%, respectively. It shows the negative correlation between the permeability and complexity of pore-throat system. The more complex the pore throat system of sand stone is, the higher the percentage of ineffective seepage pore throat volume is. Unmoveable remaining oil is caused by the influence of flow resistance in the area whose radius is below the radius of the critical flow throat where the injected water cannot enter. Taking the test group with oil-water viscosity ratio of 13.33 as an example, the water saturation spans of the three samples with gradually increasing permeability from the beginning of oil-water two-phase flow to the same permeability state were 17.1%, 29.8% and 36.2%, respectively, revealing that the water phase in the sand stone with lower permeability has more obvious inhibitory effect on the oil phase. Irregular remaining oil is caused by the influence of irregular pore throat structure and interfacial tension in the throat where the injected water has entered. In addition, taking two high-permeability(permeability ratio to the control group was 1.6 and 2.6, respectively) samples into consideration, the displacement efficiency ratios are 1.2 and 1.4, respectively, which reveals the differences of displacement efficiency between different parts in heterogeneous reservoir. Moveable remaining oil is caused by the influence of seepage abilities at different locations in the area surrounded by the dominant seepage channel of injection water.