This paper is based on a study performed on an offshore Middle East field. The field is a giant complex mostly carbonate oil field, which is characterized by a thin oil column, a low permeability associated with fractures, a large transition zone and a lateral variation in fluid properties. Even after an extensive and efficient water-flood development, there are substantial amounts of oil remaining in the reservoir due to the highly oil-wet nature of the rock. Various Enhanced Oil Recovery (EOR) techniques have been envisaged to enhance oil production. The most mature one is the immiscible hydrocarbon Water Alternating Gas (WAG) injection. This High Pressure (HP)-WAG project started in September 2012 after the encouraging results of the continuous Low Pressure (LP) gas injection trial performed in 2008.

This paper presents the latest analysis of the performances of this HP-WAG project. The HP-WAG project performances is evaluated through (i) the oil gain (versus a water-flood baseline), (ii) the water injectivity evolution over the WAG cycles, (iii) the gas management and (iv) the well and surface integrity. The paper also aims to share the methodology for analyzing the contribution of the main mechanisms occurring over the WAG cycles: the oil-gas interaction mechanisms and the desaturation mechanisms. The oil-gas interactions that occur in immiscible gas injection cases lead to significant long-lasting WAG effects thanks to both the swelling effects that continue even once the oil is saturated and a permanent mobility ratio improvement. The contribution of both macroscopic and microscopic oil desaturation is also described and quantified in this paper.

The work presented in this paper has evidenced the HP-WAG technique benefits and has improved the understanding of the impacts of the main mechanism occurring in the reservoir. This knowledge paved the way towards more extensive WAG deployment on the field. It also emphasized the need of laboratory experiments to calibrate the three-phase models and the absolute need of compositional models to capture the entire WAG benefits even in immiscible gas injection cases.

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