The LaSaXing Oilfield has entered the ultra-high water cut stage, and the original evaluation indexes of water flooding development effect cannot reflect development status of the oilfield. In order to correctly evaluate the water flooding development effect of the oilfield at ultrahigh water cut stage, the indexes were extensively selected in terms of geological conditions, development technologies, production management, and economic benefits, and they were initially screened by observation principle, the Delphi method, and theoretical analysis. Then, the evaluation indexes were quantitatively selected by Pearson correlation analysis and combination of gray clusteringrough set to establish the index system evaluating water flooding development effect of oilfield at ultra-high water cut stage. The practice showed that the index system evaluating the development effect, which was scientific, objective, and strongly applicable, had great significance for comprehensive evaluation of the water flooding development effect of oilfield at ultra-high water cut stage, analysis of development potential, and formulation of development adjustment plan.
For the water-flooded reservoir at ultra-high water cut stage, the ratio of oil and water relative permeability and the water saturation deviate from the linear relationship and show the downwarping trend in the semilog coordinate, and the conventional water drive curves show the upwarping trend. Aiming at predicting the dynamic, recoverable reserve and recovery of oilfields at ultra-high water cut stage, a new formula characterizing the relation between the ratio of oil and water relative permeability and the water saturation was proposed, derivation of a new water drive curve was done by reservoir engineering methods, and the formula of recoverable reserve and recovery was derived. Comparison of the new water drive curve and the conventional one through numerical simulation and field application showed higher accuracy and better adaptability resulted from the new water drive curve, which solved the problems of upwarping and big prediction error in curves of ultra-high water cut stage. Keywords Ultra-high water cut stage Á Water drive curve Á Relative permeability curves Á Recovery Á Water cut
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