摘要
特高含水期后期,地层剩余油主要以油膜、盲端残余油等形态存在,水驱开发效果变差,采用黏弹性流体驱替是当前提高采收率的有效方法.为探索黏弹性流体驱替残余油的水动力学机理,基于N-S方程模拟黏弹性流体在微孔道内流动.采用Oldroyd-B本构方程描述流体的黏弹性,运用相场方法实时追踪驱替过程中两相界面.分别研究壁面油膜在黏弹性流体驱替下的运动和变形,黏弹性流体对盲端残余油的驱替效果,并分析残余油微观受力状况.采用有限元方法对模型进行求解,并验证模型与求解的正确性.研究结果表明:黏弹性流体可以有效驱动壁面油膜运动,相同注入流量下,黏弹性流体作用于壁面油膜上的水平应力差远大于水;作用于油膜上的法向应力分布不均匀导致油膜发生较大变形.同时黏弹性流体在盲端内的波及边界更深,有利于引发盲端残余油的运动,提高残余油采收率.黏弹性流体驱替对水湿油藏开发增产效果更明显.该研究初步揭示了黏弹性流体驱替残余油的水动力机理,为油田三次采油提供有效指导.
Microscopic residual oil in reservoirs after water flooding can be classified into four types： oil films, oil in the dead end, oil in pores throat, and oil clusters. The oil is trapped in pores throat when the viscous force is not sufficiently large to overcome the capillary forces with water flooding. The oil in the dead end is constrained by the rock configuration. The oil clusters mainly exist in the lower permeability portion of the porous media, which is constrained by the capillary force and rock configuration. In particular, the efficiency of water flooding is extremely low at the high water cut period. Recently, viscoelastic fluid flooding（polymer flooding） was widely applied to improve the recovery after water flooding in the petroleum industry, and the displacement efficiency was greatly improved. However, the poor understanding of viscoelastic fluid flooding restricts its further progress. The investigation of how the viscoelastic fluid mobilizes the residual oil is warranted. In this study, a direct numerical simulation method is employed to simulate immiscible two-phase flows in a micro channel. Viscoelastic effects are simulated using the Oldroyd-B rheological model. The position of the interface between two immiscible fluids is determined by using the phase field method. The model incorporates the wetting condition of the porous media. The dynamics of oil film and residual oil in the dead end are explored under the displacement of water and viscoelastic fluid. The forces exerting on the oil film in the micro channel are analyzed, which contribute to the difference of oil film movement and deformation between the water flooding process and viscoelastic fluid process. Moreover, the sweep efficiencies of water and viscoelastic fluid in the dead end are observed and compared. The concept of dimensionless velocity is introduced as the sweep boundary in the dead end to explain the discrepancy of recovery between water and viscoelastic fluid. The numerical solution used for the simulation is performed using a finite element method. Additionally, a benchmark of the flow of Oldroyd-B fluid past a cylinder between two parallel plates is given to validate the correction of the model and solution. Compared with water, viscoelastic fluid can more easily mobilize the residual oil which can greatly improve the displacement efficiency. The viscosity is not the only one mechanism of the viscoelastic fluid＇s mobilization of the oil film; ‘elasticity＇ of the fluid is another key driving mechanism which is reflected by the Weissenberg number. The horizontal stress difference of viscoelastic fluid acting on the residual oil film is considerably larger than that of water. The uneven distribution of normal stress caused by the viscoelastic fluid leads the residual oil film to deform significantly. Viscoelastic fluid can greatly improve the sweep efficiency in the dead end and this phenomenon is more obvious in the water-wet reservoir. The sweep boundary of viscoelastic fluid in the dead end is deeper than that of water which explain the reason why less residual oil trapped in the dead end with viscoelastic fluid flooding. The study provides effective guidance for tertiary oil recovery.
出处
《科学通报》
EI
CAS
CSCD
北大核心
2016年第36期3973-3981,共9页
Chinese Science Bulletin
基金
泰山学者建设工程专项经费
国家重大专项(2016ZX05011-001)
国家自然科学基金(41502131
51504276
51304232
51274226)
中国博士后科学基金(2015M580621)资助
