Visualization Experimental And Numerical Simulation Study On The Seepage And Oil-Displacement Characteristics Of Weak Gel In Porous Media

Although there is still a large amount of remaining oil in the reservoir, a large number of problems for the oil well have emerged, including high water cut, low production and poor economic benefits due to the facts that the reservoir is heterogeneous and the flow behavior of the injected water is different from the oil during the later period of the development of oil field in China. Finding an efficient way to enhance oil recovery（EOR） has become the critical task to maintain the sustainable production in oilfield. The weak gel displacement technology is a new EOR method, in which the weak gel with retarded crosslinking, low-strength and network crosslinking will be used to improve the sweep efficiency of the injected water by pluging the waterflood channeling-paths and diverting the injected water entering the smaller pore channels. At the same time, the weak gel can effectively reduce the water phase permeability, mobility ratio of water and oil, and enhance oil displacement efficiency. The conventional core displacement experiment method is currently employed to investigate the mechnisam of the weak gel profile control and flooding technology, by which the distribution and seepage flow characteristics of the weak gel in core, however, can not be directly observed. Therefore no detailed information about the plugging performance and mobility of the weak gel in porous media can be given. Due to the poor understanding for the seepage flow characteristics of the weak gel, there are still some deficiencies in dealing with the anisotropy, multiphase flow and non-Darcy flow features for the modeling of the weak gel profile control operation.In this paper, the seepage and oil-displacement characteristics of weak gel in core were systematically studied by nuclear magnetic resonance（NMR） visualization experiment and numerical simulation. Based on the study of fluid relaxation and imaging mechanism in porous media, the nuclear magnetic resonance core displacement system is firstly improved. Then the interaction mechanism of weak gel and water was investigated, and a new visual experiment method was proposed to study the seepage and oil-displacement characteristics of weak gel in the core. Finally, the one dimensional numerical simulations of the weak gel flooding of core scale were carried out based on the improved seepage model. The research works and achievements are as follows:（1）Based on the imaging principle of the NMR relaxation of fluid in porous media, the low field nuclear magnetic resonance core displacement system was improved to visually study the distribution of weak gel in porous media, and the key problems of nuclear magnetic resonance imaging of weak gel in porous media are resolved.（2）The method used to distinguish the nuclear magnetic resonance signal of weak gel, water and oil was investigated. By using the heavy water and manganese chloride inhibitors, water, oil and weak gel can be distinguished each other in T₂ spectrum and nuclear magnetic resonance image. Furthermore, the interaction mechanism of weak gel and water was also analyzed, which includes the interaction between weak gel and water, as well as the damage of the manganese chloride water on the weak gel.（3）Four important parameters, such as the migration velocity, the blocking height ratio, the dynamic diffusion and retention ratio, were firstly proposed to quantitatively describe the movement and distribution of weak gel in porous media. Based on this, a new method has been proposed to evaluate the weak gel seepage and oil-displacement characteristics by using pressure, flow rate, T₂ spectrum and nuclear magnetic resonance image.（4）The process of improving the sweep volume of water flooding by weak gel was directly observed in the nuclear magnetic resonance images, and the factors affecting gravity tonguing of weak gel were investigated. The effects of weak gel viscosity, core permeability and heterogeneity on the seepage characteristics of weak gel were further analyzed. The experimental results showed that the weak gel with the good aggregation and mobility, can effectively block the flow channeling-path, and improve the water flooding swept volume.（5）Based on the analysis of the mechanism of water flooding, polymer flooding and weak gel flooding, the distribution characteristics of remaining oil were analyzed by means of injection pressure, oil displacement efficiency, T₂ spectrum and nuclear magnetic resonance image. The experimental results showed that the weak gel has the dual characteristics of oil-displacement and profile control, which can significantly improve the efficiency of oil-displacement. The combination mode of water flooding, polymer flooding and weak gel flooding can be optimized according to the above experimental results.（6）Based on the experimental results of the seepage characteristics and oil-displacement characteristics of the weak gel in the rock, a mathematical model of three dimensional two-phase four component of weak gel flooding was improved. The finite difference method was used to solve the governing equations, and a one dimensional solution program which is suitable for core scale was compiled. The effects of weak gel strength, flooding slug length, and the parameters of accessible pore volume ratio on the oil displacement efficiency of weak gel flooding were analyzed.