As a surfactant solution system, microemulsion has attracted much attention due to its ultra-low interfacial tension, high solubilization and thermodynamic stability in the process of enhanced oil recovery. Different from water phase system of polymer flooding and ASP flooding, the microemulsion system shows a special phase state, and its existence system may be water phase, oil phase or microemulsion phase. The microemulsion phase can be divided into upper phase, middle phase and lower phase microemulsion according to the composition of the system. Different phase microemulsions have different oil displacement efficiency, and the middle phase microemulsion reaches ultra-low interfacial tension with oil/water, and the oil displacement efficiency is the highest. In order to ensure the middle-phase microemulsion flooding as far as possible during the oil displacement process, it is necessary to study the phase change process of microemulsion and the formation conditions of microemulsion in detail, and clarify the influence of surfactant concentration, additive concentration, salt content, water–oil ratio and temperature on the microemulsion phase transformation and the formation mechanism of microemulsion. The research results have some guiding significance for the formulation selection and slug design of microemulsion flooding system.
Based on the features of microemulsion flooding in low-permeability reservoir, a three-dimension three-phase five-component mathematical model for microemulsion flooding is established in which the diffusion and adsorption characteristics of surfactant molecules are considered. The non-Darcy flow equation is used to describe the microemulsion flooding seepage law in which the changes of threshold pressure gradient can be taken into account, and the correlation coefficients in the non-Darcy flow equation are determined through the laboratory experiments. A new treatment for the changes of threshold pressure and the quantitative description of adsorption quantity of surfactant and relative permeability curves are presented, which enhance the coincidence between mathematical model and experiment results. The relative errors of main development indexes are within 4%. A software is programmed based on the model to execute a core-level small-scale numerical simulation in Chaoyanggou Oilfield. The fitting relative errors of the pressure, flow rate, and moisture content are 3.25%, 2.71%, and 2.54%, respectively. The results of laboratory experiments and numerical simulation showed that microemulsion system could reduce the threshold pressure gradient by 0.010 MPa/m and injection pressure by 0.6 MPa. The biggest decline in moisture content reaches 33%, and the oil recovery is enhanced by 10.8%.
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