Optimization Method for Unstable Water Injection Parameters Considering Time-Varying Physical Properties

Unstable water injection in reservoirs leads to continuous changes in reservoir physical properties, making it difficult to determine reasonable water injection parameters. Conventional core experimental analysis, numerical simulation, and field experimental methods have limitations such as high cost, insufficient experience, and lack of data in optimizing unstable water injection parameters. The multiphase flow numerical well testing model is established through numerical methods for well testing analysis, with comprehensive considerations and high reliability of interpretation results. However, the model is difficult to establish and requires a large amount of calculation, making it difficult to apply in practice. Moreover, the existing parameter characterization functions for time-varying reservoir properties have problems such as inconsistent functional forms, excessive fitting parameters, and limited applicability, making their practical application difficult. Therefore, in this study, a universal time-varying characteristic function of physical parameters is constructed, and a well testing analysis model for oil-water two-phase injection wells is established. The pressure solutions are obtained by numerical method for well testing analysis. The characteristics of typical well testing curves and the changes in well testing curves under different time-varying parameter conditions are studied and analyzed, and the interpreted permeability time-varying parameters are compared over time to guide the optimization direction of water injection parameters. Through calculation and analysis of field examples, the research results indicate that by considering the time-varying law of reservoir physical properties and based on cumulative inflow flux, a universal permeability time-varying function is constructed, which reduces fitting parameters and reduces the difficulty of function application. By comparing this parameter over time, it is found that in reservoirs with long-term water injection and high permeability channels (microfractures), the parameter increases, reflecting the further expansion of high permeability channels between oil and water wells due to excessive injection volume or short injection shutdown time. Therefore, the injection volume should be reduced or the injection shutdown time should be extended. The proposed method can provide a practical well testing analysis method for optimizing water injection parameters in long-term unstable water injection development oilfields.