Effects in the Determination of Oil Reserves Due to Gravitational Compositional Gradients in Near-Critical Reservoirs

摘要

Several factors, including gravity, temperature, reservoir and fluid properties, geological structure, and the accumulation process may have important effects on the spatial compositional variation of hydrocarbon fluids in reservoirs. Compositional gradient may be considerable in near-critical reservoirs and can have a profound impact on the estimation of initial in situ hydrocarbons, the prediction of oil/gas contact location (OGC), and, more importantly, on the reservoir development strategies. Some reservoirs have producing intervals with thicknesses that cover hundreds of feet from top to bottom, and sometimes even more than 7,000 ft. Over such a thickness, because of gravity segregation, the mole fraction of the lighter components decreases with depth, while the mole fraction of the heavier components increase from the top to the bottom of the reservoir. Modelling the thermodynamic behaviour of these fluids requires a properly tuned EOS capable of reproducing the available PVT data and the gravitational compositional gradation. We studied near-critical fluids (gas condensates and volatile oils) from the Cusiana Field in Colombia. PVT reports consisting of constant composition expansion (CCE) and constant volume depletion (CVD) tests were used to calibrate the fluid model and the EOS parameters. We used the Peng Robinson EOS with volume translation and the Whitson's methodology for defining and characterizing the pseudo-components. We evaluated the effect of an isothermal gravitational compositional gradient upon the determination of in-place hydrocarbon content, spatial fluid properties, and reservoir development planning scenarios.