磁激励下纳米磁流体-残余油界面赋存状态及残余油的启动机制

For the nanoparticle magnetic fluid has the advantage of both magnetic and nanofluid, it should have the good prospects for application in residual oil development. The application of nanoparticle magnetic fluid in residual oil development is a new subject, so many issues are remained to be solved. With the influence of magnetic field, the adsorption of magnetic nanoparticle on the interface between nanoparticle magnetic fluid and residual oil, and its influence on the phase interface properties, the relationship between the azimuth and strength of magnetic field and the residual oil retention position and many other factors determine the nanoparticle magnetic fluid displacement ability, however their mechanism are not yet clear. Thus making sure the environment feedback on nanoparticle magnetic fluid in magnetic field excitation, understanding the residual oil activating mechanism has important value for accelerating the nanoparticle magnetic fluid application. As the interface occurrence state between nanoparticle magnetic fluid and residual oil, and the activating mechanism to residual oil under magnetic field excitation are ambiguous, theoretical analysis and laboratory experiments are employed to systematacially study the problem. Firstly, the magnetic nanoparticle adsorption characteristic on the interface under magnetic field excitation, and its influence on interface property and morphology are studied by microscopic displacement experiment and theoretical arithmetic. Secondly, based on the microscopic displacement experiment, and the relationship of capillary number, Weber number and the porous medium assumption, the generation condition of the magnetic nanoparticle stabilized emulsion is built up, and its migration characteristic is also revealed. Then, according to the throat model displacement experiment and the multi-phase Lattice Boltzmann theory, the migration evolution law of oil segment, oil droplet, oil column, dead end oil, and cluster oil are studied, and the residual oil activating mechanism under magnetic field excitation is revealed. Finally, the principal component analysis and laboratory findings are employed to determine the main factors that affect the residual oil recovery. In accordance with the internal logic of parameters, fluid mechanics and seepage theory, the dimensional analysis method is introduced to make sure the relationship between the main factors and the residual oil recovery. Through the subject study, the foundation of nanoparticle magnetic fluid used in residual oil development under magnetic field excitation is laid and the key technical support is also built up.

纳米磁流体在启动残余油方面具有良好的应用前景，但由于纳米磁流体在残余油开发中的应用属于新兴课题，对于磁激励下磁性纳米颗粒在纳米磁流体-残余油相界面上的吸附及吸附对相界面性质的影响、磁场方位和强度与残余油滞留位置的关系等诸多影响纳米磁流体携油能力因素的作用机理并不清楚，针对以上问题采用理论分析和实验相结合的手段对其进行系统研究。首先，通过微观驱替实验结合理论计算确定磁激励下磁性纳米颗粒在油水界面上的吸附特性及其对界面性质和形态的影响；其次，基于驱替实验结合毛管数、韦伯数和孔隙假设间的关系，揭示磁性纳米颗粒稳定的乳状液的生成条件并明确其在纳米磁流体中的运移特征；然后，根据孔喉模型驱替实验研究油膜、油滴、油柱、盲端油和簇状油的运移演化规律，揭示磁激励下纳米磁流体对残余油的启动机制；最后，引入主成分分析结合实验结果确定影响残余油采出程度的主要因素，根据物理参数间的内在联系结合流体力学与渗流理论，采用量纲分析的方法建立主要因素与残余油采出程度的优化匹配关系。本课题的研究为磁激励下纳米磁流体驱替残余油的研究奠定了基础并形成了关键技术支撑。

鉴于目前油气资源的紧张局势及开发难度的日益增大，纳米流体由于其具有粒径小、比表面积大等独特的优点而成为油气开发领域研究的热点问题。纳米磁流体除具有纳米流体的优点外，还具有磁响应的优势，因此纳米磁流体在油气开发中必定具有相当的应用潜力。但目前纳米磁流体在油气开发中的应用报道较少，针对纳米磁流体与残余油界面赋存状态及残余油启动机制尚不明确的问题，从纳米磁性颗粒的筛选、纳米磁流体表/界张力评价、粘度评价等方面对纳米磁流体的性质进行系统的评价；进行纳米磁性颗粒影响情况下的油/水乳状液评价实验，并对纳米磁性颗粒影响下的乳状液性能进行评价，确定纳米磁性颗粒对乳状液性能的影响。设计纳米磁流体驱替残余油的实验，并监测纳米磁流体的影响动态，确定纳米磁流体驱替残油油的效果。结果表明：纳米磁流体的表面张力能稳定在65mN/m左右，且随着纳米磁流体中纳米磁性颗粒浓度的降低，纳米磁流体的表面张力会增加，接近于水的表面张力值；纳米磁性颗粒颗粒大部分明显附着于乳状液的油水界面上；乳状液粒径随纳米磁性颗粒浓度增加呈下降趋势；随纳米磁性颗粒浓度增加，乳状液粘度降低。纳米磁流体稳定的乳状液初期表现出牛顿流体的曲线特征，随着时间变长，乳状液逐渐表现出拟塑性流体的流变性曲线特征；1%的纳米磁流体与煤油的界面张力稳定在16mN/m附近。不同浓度下纳米磁流体的黏度一般低于水的黏度，但是在高剪切作用下，会出现大于水的黏度的现象，存在黏度的极值。与水驱相比，纳米磁流体驱在岩心实验中能够将采收率提高平均提高16%左右，最高可达17.78%；同时能将残余油饱和度降低22%，最高可降低27.83%，同时使岩心的残余油饱和度稳定在30%以下。在1%浓度的纳米磁流体驱替下，与水驱相比，见水时间相对提前了7%左右，但是无水采油量却提升5%左右。纳米磁流体驱替过程中驱替压力基本上随时间变化呈稳定上升趋势，区别于水驱的逐渐趋于稳定的趋势。课题的研究能够为纳米磁流体在油气开发中的应用提供借鉴作用。