Water Alternating Gas (WAG) injection is normally implemented after water injection to increase microscopic 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 between gas and oil. The macroscopic WAG-effect is mostly associated with recovery of attic oil or other pockets of oil not yet swept by water.

This study presents the results from simulation of a WAG process for an offshore turbidite reservoir with undersaturated oil. The aim of the work is to qualify the increased drainage potential from WAG injection by understanding the reservoir response, which includes cycling of saturations in two- and three-phase flow as 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 analysis of 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-side requirements, is the basis for estimating an extra WAG potential of about 3% of additional OIIP recovered compared to water injection. WAG injection is further modelled on the field scale without any topside constraints to get the free reservoir response of WAG, and to investigate the impact of relative permeability and 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 and 1D models.

The results from the workflow qualify the enhanced oil recovery from WAG compared to a water injection scheme. 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.

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