致密油储层低摩阻纳米磁流体压裂液构筑及渗吸驱油机制

The effective displacement system is difficult to build because of poor percolation capacity in tight oil formation. The production of an oil well declines rapidly and the oil recovery is also low by the dissolved gas drive or depleted mining after hydraulic fracturing. The oil displacement fracturing by the fracturing fluid imbibition combine rock cracking, oil replacement by water imbibition and energy supplement, and it is expected to increase the oil recovery in tight formation. Therefore, a strong magnetic response, high hydrophobic nanoparticles are prepared with surface modification and a strong magnetic response, low friction nano-magnetic fluid fracturing fluid is constructed by non- covalent interactions between nanoparticles and micelles. The rheological characteristics and drag reduction mechanism of fracturing fluid are also investigated by advanced tests and analysis methods. The imbibition behavior of fracturing fluid in the process of fracturing and soaking is modeled to investigate the interaction between nano-magnetic fluid and tight rock, resulting in transform of fluid occurrence form. The oil displacement mechanism and stimulation of an interaction between interface control of surfactant and directional migration of magnetic nanoparticles under magnetic field is studied by CT scan, micro etching model and core flow experiment. It is expected to improve oil/water replacement efficiency between the fractured network and the matrix by the multi-field coupling of magnetic field, chemical field and flow field, providing a theoretical basis for the engineering application feasibility of the nano-magnetic fluid fracturing fluid.

致密油储层渗流能力差，难以构建有效驱替系统，仅靠压裂后储层溶解气驱或衰竭式开采，单井产量递减快、采收率低。渗吸驱油压裂将岩石造缝、渗吸置换及能量补充有机结合，有望较大幅度提高致密储层原油采收率。因此，本项目拟应用表面修饰方法制备具有强磁响应高疏水纳米颗粒，借助纳米颗粒与胶束的非共价键作用构筑一种强磁响应低摩阻纳米磁流体驱油压裂液，应用先进测试与分析方法研究纳米磁流体压裂液的流变特征及减阻机理；模拟压裂及焖井过程中纳米磁流体压裂液渗吸行为，考察纳米磁流体压裂液与岩石相互作用对孔隙中致密油赋存形式的影响规律；结合CT扫描、微观蚀刻模型与岩心流动实验等手段揭示磁场作用下微纳孔隙中表面活性剂界面调控与磁性纳米颗粒定向运移协同驱油的微观机制与效果，期望通过磁场、化学场与渗流场的多场耦合作用改善压裂缝网与基质的油水置换效率，为纳米磁流体压裂液渗吸置换采油方法的工程应用可行性提供理论依据。

致密油储层渗流能力差，难以构建有效驱替系统，仅靠压裂后储层溶解气驱或衰竭式开采，单井产量递减快、采收率低。渗吸驱油压裂将岩石造缝、渗吸置换及能量补充有机结合，有望较大幅度提高致密储层原油采收率，而压裂液特殊性能将直接影响渗吸压裂增产效果。因此，基于纳米磁流体智能流体，构筑了一种低摩阻纳米磁流体渗吸驱油压裂液。首先，以三氯化铁、硫酸亚铁、油酸钠、氢氧化钠等为原料，通过化学沉淀法和表面改性，制备了强亲油性、低密度、高磁化强度的油酸钠改性四氧化三铁磁性纳米颗粒。颗粒形貌较为规则，粒径为20nm左右，密度为1.107 g/cm3，饱和磁化强度为58 emu/g。其次，构筑了具有稳定颗粒-胶束网状结构的纳米磁流体压裂液（粘弹性表活剂3.5 wt% + KCl 2.0 wt% + 改性纳米磁性颗粒0.3 wt%+杀菌剂0.2 wt%+消泡剂0.5 wt%）；利用冷冻电镜分析了纳米磁性颗粒-胶束网络结构的相互作用机制。然后，通过流变实验和动态携砂实验测试表明，磁场作用会提高纳米磁流体压裂液的粘弹性，影响纳米磁流体压裂液携砂性能的主要影响因素依次为体系弹性、粘度、排量、砂比。同时通过减阻测试表明，磁性纳米颗粒与表面活性剂胶束具有协同减阻作用，减阻率达75%以上。最后，通过自发渗吸、岩心驱替及微观渗流刻蚀模型等实验表明，纳米磁流体压裂液可以将岩石表面油性润湿反转为中性-弱亲水性润湿，同时具有在磁场作用下磁性纳米颗粒“吸油”、“携油”定向运移和纳米颗粒的“楔形”驱油作用，能有效改善驱油效果，提高原油采收率。相对于零场，2000 Gs磁场可以将纳米磁流体压裂液自发渗吸驱油采收率提高57%，渗吸采收率可达32.81%；将纳米磁流体压裂液驱替原油采收率提高39%，最终岩心驱替原油采收率高达57.19%。该纳米磁流体压裂液实现了降低摩阻和渗吸驱油的目的。研究成果不仅为提高致密油储层压裂改造提高原油采收率提供一种环境友好的新方法，而且为纳米磁流体工程应用提供理论依据，具有良好的工程应用前景。