Calculating Gas Oil Ratio with Logging Data and Quantitatively Identify Fluid Properties of Low Porosity and Permeability Reservoirs

Low porosity and permeability reservoirs are characterized by small reserving space and low oil and gas saturation, which leads to small difference between oil and gas logging response and difficulty in reservoir fluid properties identification. The oil and gas reservoirs are deeply buried and developed below 3 800 m in X sag, which are characterized by poor physical properties, strong heterogeneity, complex pore throat structure and irregular distribution of oil, gas and water. Gas logging is capable of directly measuring the content of each component of combustible gas in drilling fluid, which can effectively reflect the gas-oil ratio of oil and gas reservoirs. Response equations of gas-oil ratio and gas components are established based on the contents of gas components (T_G, C₁) measured by mud logging and the gas-oil ratio measured by MDT pump. Then overdetermined linear equations are established with various conventional logging response equations. Consequently, the gas-oil ratio of the reservoir is calculated by using the optimization algorithm, so as to quantitatively identify the fluid property of the reservoir combining with the identification standard of oil and gas reservoir type. In the application of the actual wells, the gas-oil ratio calculated by logging data is compared with the MDT pumping analysis result, and the relative error is less than 7.80%. This shows that the optimized calculation of gas-oil ratio is reasonable and reliable. This method is effective and worth promoting.