Improving Polymerflood Performance Via Streamline-Based RateOptimization:Mangala Field,India

摘要

This paper describes a novel streamline-based rate allocation approach to maximize oil recovery frompolymerfloods and its application to an ongoing polymerflood in the Mangala field,India which is one ofthe largest polymerfloods in the world.With over 130 injectors and producers,the field-scale optimizationis challenging because of the long simulation times,operational constraints at the well,group and field leveland changing field conditions.The FM-1 unit which is the main producing zone of the Mangala field comprises of multi-layered singlestoried fluvial channel sands of thickness ranging 3-10m with excellent flow characteristics.The grossthickness of FM-1 is about 90m.The sand is under a five-spot pattern polymerflood with a well spacingof aproximately 200m.A finite difference simulator is used for modeling the polymerflood performancewith all relevant physics(mobility control,adsorption,residual resistance factor,polymer rheology/shearthinning,inaccessible pore volume and time-dependent degradation).Streamlines are generated from theflux field extracted from the finite difference simulation and are used to calculate pair-wise efficiency foreach injector-producer streamline bundle.The pair-wise efficiency quantifies how much oil is recoveredgiven a barrel of water injection.The streamline-based optimizer iteratively reallocates the fluids to injectorsand producers by diverting the injected fluid to high efficiency pairs located in unswept oil regions.Afteroptimization,injection efficiency plots as well as streamline/time of flight plots are used to examine andvisualize the flow pattern changes leading to the improved flood performance.The reservoir model is first calibrated with 10 years of waterflood history.The polymer injectionconcentration and tapering schemes are determined based on extensive laboratory experiments.Theoptimization focuses on reallocating the well rates while maintaining the well,group and field levelconstraints.Two optimization scenarios were studied to examine the potential benefits of rate allocation:(1)optimization of rate allocation for both injectors and producers,(2)optimization of rate allocation forinjectors only.Both the scenarios yield improved oil recovery(~5 MMSTB),accelerated production andreduced polymer usage(utility factor).The streamline and injection efficiency plots clearly reveal regionsof improved sweep efficiencies,providing an intuitive explanation of the enhanced performance.