Research On Reservoir Characterization And Numerical Simulation Of CO₂ Enhanced Oil Recovery For Low Permeability Oil Reservoir In A Oilfield In Northern Shaanxi

At a certain temperature and pressure, CO₂ will be miscible with oil and may recover 5%-25% of the original oil in place by several physical/chemical interactions occur between oil and CO₂, which include oil volume expansion, reducing oil viscosity, and decreasing the IFT between the fluids. Therefore, as an effective method for enhanced oil recovery, CO₂ flooding has become a leading and common technique all over the world and shown a great effect. While Ordos basin is a unique basin which has low permeability, strong heterogeneity and well-developed fractures. Sothe successful foreign CO₂ flooding technology can’t completely be suitable forOrdos basin.As a result, it is necessary to conduct CO₂ flooding evaluation before CO₂ injection in Ordos basin, so as to form a suitable CO₂ flooding technology series and provide a model for CO₂ enhanced oil recovery（CO₂-EOR） projects in other low permeability oil reservoir of Ordos basin and even China.Based on the theory and methods of reservoir characterization, geological modeling and numerical simulation, this paper chooses the oil field A as the target region, and first describe the features of reservoir for CO₂ flooding. Then select theoptimum member for CO₂ flooding based on the results of facies analysis, reservoir properties and properties distribution characteristics. Taking the optimum member as the main research object, establish the 3D geological model of the study area. Integrating the reservoir characterization and geological model results, perform CO₂ flooding numerical simulations, in which basic theories of reservoir characterization, modeling and numerical simulation on CO₂ floodingin low permeability reservoir is set up. The main results achieved and understanding are as follows:1. Based on the analysis of regional deposition backgrounds and well logging curves, combined with marker beds, lithology and formation thickness ratio, stratigraphic classification and correlation is conducted. The results show that Chang4+5 is divided into Chang4+51, Chang 4+52’and Chang4+522.2. The sedimentary microfacies types are subaqueous distributary channel and interdistributary bayof the delta front. The main microfacies isinterdistributary bay for Chang4+51,Chang 4+521, while subaqueous distributary channel for Chang4+522. The sand body developed in subaqueous distributary channel is the main oil reservoir, and also could be a target layer for CO₂ injection.3. Chang4+5 reservoir belongs to heterogeneous reservoir. However, compared with these three layers, Chang4+522is the weakest layer, which has high drilledsand ratio and well connected sand body.4. According to the results of reservoir characterization, select Chang4+522 as the most optimum target layer for CO₂ flooding. On the basis of micro-facies deterministic modeling method, establish the 3D geological model of the whole formation and the target layer of Chang4+522. It shows that porosity, permeability and NTG model have the same characteristics,which becomes weaker from northeeast to southwest. Chang4+522 has the best reservoir properties and is the main oil enrichment region, therefore, it is the best choice for CO₂ injection. It is also proved that the 3D model properties are in line with the reservoir characterization results, and have certain accuracy and can be used in later numerical simulation.5. Based on the results of reservoir characterization and geological model, select the most suitable area and sand body for CO₂ flooding, and simulate the CO₂ flooding process using Eclipse.By predicting and evaluating the flooding process, it reflects the effect of CO₂ flooding is better than the water flooding.