Effects in the Determination of Oil Reserves Due to Gravitational Compositional Gradients in Near-Critical Reservoirs
Authors: Barrufet M.A. , Jaramillo J.M. 
Published: March 2010

Abstract
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 PVTdata 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 odel and the EOS parameters. We used the Peng Robinson EOS with volume translation and the Whitson's methodologyfor 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.

Introduction
The effects of gravity upon composition in a static hydrocarbon fluid column have been studied since the late 1800s when Gibbs(1) provided the theoretical model for evaluating a gravitational compositional gradient at constant temperature. Early in the 1900s, Muskat(2) concluded that the effect of gravity on compositional variations in a reservoir was negligible. However, recent investigations(3, 4) indicate that the effects of a gravity field become significant as the critical point of the fluid is approached.
In 1980, Schulte(5) investigated the effects that different hydrocarbon characterization schemes have on the predicted gradient and matched his predictions with experimental data from the Brent Field. Schulte's results suggest that the aromatic content of oil can strongly affect the vertical distribution. In 1983, Holt et al.(6) investigated the effect of temperature on compositional variations, however, the formulation includes gravity and temperatureindependently. Jacqmin(7) discussed the effect of natural convection and diffusion segregation due to gravity and showed that the combined effects may result in a significant variation of compositions in both horizontal and vertical directions. Hamoodi et al.(8) discussed a field case which exhibited both areal and vertical compositional variations. Bedrikovetsky(9) used irreversible thermodynamics to present a comprehensive discussion and the formal mathematical treatment of compositional gradient, including gravitational and thermal effects.
Gravity-induced compositional gradients attracted the attention of many researchers whose theoretical studies have paved the way for further research on this topic. For practical reservoir engineering applications, if a significant compositional gradient occurs, what role will it play? To what extent will compositional gradient affect reservoir production strategies? No early papers c