Simulation Study for Field Implementation of WAG as Initial DrainageStrategy

摘要

Water Alternating Gas(WAG)injection is normally implemented after water injection to increasemicroscopic and macroscopic sweep efficiency.The microscopic sweep is enhanced by trapping of gas,three-phase flow effects including lower residual oil saturation,and possible component transfer betweengas and oil.The macroscopic WAG-effect is mostly associated with recovery of attic oil or other pocketsof oil not yet swept by water.This study presents the results from simulation of a WAG process for an offshore turbidite reservoir withundersaturated oil.The aim of the work is to qualify the increased drainage potential from WAG injectionby understanding the reservoir response,which includes cycling of saturations in two-and three-phase flowas well as changes in fluid properties due to interaction between gas and oil.A hierarchical workflow has been followed in order to conduct a systematic and comprehensive analysisof the WAG process.This includes the use of simulation models from full-field scale models(FFM),via box-models,to simpler 1D models.The full-field model,with appropriate well constraints reflecting top-siderequirements,is the basis for estimating an extra WAG potential of about 3% of additional OIIP recoveredcompared to water injection.WAG injection is further modelled on the field scale without any topsideconstraints to get the free reservoir response of WAG,and to investigate the impact of relative permeabilityand fluid properties on production curves as well as comparison to water injection.Furthermore,grid effects,two-and three-phase flow properties and the effect of oil swelling is studied in smaller box models and1D models.The results from the workflow qualify the enhanced oil recovery from WAG compared to a water injectionscheme.Approximately 1/2 of the increase in recovery factor is attributed to gas-oil relative permeability,about 1/5 is attributed to a change in fluid properties from oil swelling,and 1/3 is due to increased well-lift.