Polymer flooding has drawn more and more attention in the world for its high incremental oil recovery factor and relative low costs compared with water flooding and other chemically enhanced oil recovery techniques. However, for many oilfields, such as Daqing Oilfield, China, that have already been flooded with polymers, how to further improve recovery remains a big problem. Traditional intralayer, interlayer and plane heterogeneity studies cannot accurately characterize the remaining oil distribution after polymer flooding. To solve this problem, we established a method to quantitatively describe the reservoir’s architecture. Then, the architecture elements were dissected hierarchically and the interface of each architecture level in Daqing Oilfield was identified. The distribution pattern and development potential of the remaining oil after polymer flooding under the influence of reservoir architecture was analyzed. The results show that, regarding the sedimentary process from north to south in Daqing Oilfield, the channel becomes narrower, the thickness decreases, the point bar’s width increases and the thickness of the meandering river decreases. The braided bar scale becomes larger and the thickness becomes smaller in the braided river. According to the reservoir’s architecture, the remaining oil was divided into four categories of plane remaining oil (abandoned channel occlusion type, interfluvial sand body occlusion type, inter-well retention type and well pattern uncontrollable type) and three types of vertical remaining oil (in-layer interlayer occlusion type, rhythm type and gravity type). About 40% of the original oil in place (OOIP) of Daqing Oilfield has not yet been produced, which indicates that there is great potential for development. This study is important for improving oil recovery in polymer-flooded reservoirs.

CO2 flooding technology for EOR not only meet the needs of oilfield development, but also solve the problem of carbon emission, which has become a global research hotspot. CO2 flooding includes miscible and immiscible flooding. The advantage of immiscible CO2 flooding is the ability to achieve high recovery in different reservoirs or fluid conditions compared with miscible flooding. But there are no reports about quantitative analysis of immiscible CO2 flooding at the micro level due to the expense and complexity of the experiments. In this paper, the process of immiscible CO2 flooding was simulated based on the Navier-Stokes equation in porous media by COMSOL Multiphysics. An ideal homogeneous rock structure model was established to study the influence of interfacial tension, injection velocity, injection viscosity and gravity on immiscible CO2 flooding. The porosity of the model is 34.7% and the permeability is 36.9mD. The simulation of pressure is 10 MPa and the temperature is 80 ℃. It was found that with the injection of CO2, the contact interface of two phases gradually changes from near-piston flow to non-piston flow under immiscible condition. Decreasing the interfacial tension and increasing the injection velocity significantly change the flow paths of CO2 and increase the sweep area of CO2. The difference between CO2 and oil viscosity is one of the factors influencing the occurrence of fingering. Increasing the viscosity of CO2 injection effectively suppress viscous fingering and improve the sweep effect. Gravity is one of the factors affecting the effect of immiscible CO2 flooding. Phase field simulation was used to study immiscible CO2 flooding for the first time. It was found that increasing the viscosity of CO2 injection could significantly enhance recovery. In order to increase the viscosity of CO2, a thickener can be added to the supercritical CO2. This study provides micro-level theoretical support for the development of process parameters in oilfield, and further provides new ideas for CO2 EOR.