Modeling of a Large Gas-Capped Reservoir With Areal and Vertical Variation in Composition

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Abstract Compositional modeling of one of the giant middle east reservoirs overlain by a primary gas cap is presented. The reservoir is 43 Km long and 23 Km wide and is noted for its large variation in composition in the areal extent. The H2S content varies from above 10 mole % in the North to less than 1% in the South. Current technology in compositional reservoir simulators accounts for vertical variation in composition; however the problem presented for this reservoir includes significant areal variation in composition. This paper presents a successful attempt in addressing this challenge. Reservoir Fluid sampling included 11 bottom hole and 5 recombined surface oil samples. 10 samples are considered representative of the virgin reservoir oil. Experimental data available include extended true boiling point analysis up to C20+ of two samples, constant composition expansion, differential liberation and oil viscosity measurements. One gas cap sample was recently collected. The sample experimental data include analysis up to C20+, constant composition expansion and constant volume depletion measurements. Additional experimental data available include a swelling test and two slim tube displacement tests. An equation of state model, capable of accurately predicting the wide range of experimental data available, spanning the full compositional spectrum of the reservoir fluid, was developed. The methodology adopted in the development and validation of the equation of state characterization is presented. An isobar map of the saturation pressure and areal compositional distribution maps based on reservoir fluid analysis were constructed and assumed representative of the mid-reservoir depth of the most productive layer. By applying the saturation pressure versus depth gradient, base saturation pressure maps for all reservoir layers were derived. The compositional variation was correlated against mid-reservoir depth and saturation pressure. The total reservoir compositional distribution was then determined based on the average gradient of composition versus saturation pressure, and base saturation pressure and compositional distribution maps. The model was successfully initialized in non-equilibrium mode by defining the composition for each grid block. Stability of the model was verified, and the calculated saturation pressure distribution per layer was validated against the established distribution based on collected samples and the saturation pressure versus depth correlation. Original in-place fluid volumes were compared to black-oil model initialization data, including the original oil in place, solution gas in place and free gas volumes. The model size is 39,780 grid blocks, 7 components and includes 400 wells. The memory requirement is 380 Megabytes.