Comprehensive Fluid Sampling Program Maps Compositional Gradients in Near-Critical Fluid Systems in a Giant Offshore Field

Abstract This paper describes a fluid characterization study and equation of state (EOS) modelling for an offshore carbonate field composed of five reservoirs. The fluid column grades from gas condensate to black oil, passing through a critical point. Crest miscible gas injection was implemented soon after field start-up and peripheral water injection recently started for full pressure maintenance. The outcome of the sampling program has had a profound effect on the field development strategy. The understanding of the compositional gradient of the four main reservoirs of the field has been considered critical for the success of the project. More than a hundred downhole samples were taken at different depths during appraisal and development phases to delineate the gradient and model the fluid behavior. The detailed fluid database includes conventional PVT analysis as well as slim tube, swelling tests and equilibrium constant experiments. We calibrated the equations of state and regressed the observed gradient to implement it in the compositional reservoir simulator. All the reservoirs showed complex, variable slope, compositional gradients. The observed data was used to fit a function with depth which accounts for the gravity segregation of the components and the steady heat flow effect. The saturation pressure changes with variable gradient across the thick fluid column. This had a major influence on the field development strategy; oil producers were drilled in the peripheral, deeper areas of the field to be able to reach higher pressure drawdown. In contrast, gas is injected at the crest, where highly volatile oil and gas condensate is located. Slim tube experiments at selected depths proved that the injected gas is miscible below the initial reservoir pressure. The current study describes the challenge of modelling and characterizing the reservoir fluids when a complex compositional gradient exists. It also shows the importance of understanding the distribution of the fluid properties for the success of the field development. We believe that the extent of the fluid sampling program to delineate the compositional gradients is unprecedented in the industry.