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Given that the regulations governing pressure changes differ between injection wells and production wells, constant pressure (pressure gradient in deep formations) flooding was employed to reveal how the laws change for polymer injection.The primary causes of profile inversion were determined by analyzing the change law of the resistance factor and its corresponding derivative.Then, the main influencing factors were analyzed, and methods were implemented to restrain their influence.Based on the heterogeneity characteristic of a typical multi-layer reservoir of the Daqing oilfield, homogenous rectangular cores with a particular size of 30cm×4.5cm×4.5cm were set up with average permeabilities of 150×10-3μm2 , 600×10-3μm2 , and 1000 × 10-3μm2 , respectively.Using such models, three-core parallel experiments were carried out under the formation pressure gradient, and the rational injection mode was optimized.The results showed that profile inversion is caused primarily by the different change laws of the dynamic resistance factor of the polymer solution in high and low permeability layers, which intensifies the heterogeneity between layers.The polymers molecular weight and concentration greatly influence the injection profile,thus affecting the recovery.In addition, a rational slug combination could effectively restrain the profile inversion.