Effects in the determination of oil reserves due to gravitational compositional gradients in near-critical reservoirs
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AbstractSeveral factors, such as gravity, temperature, type of reservoir fluid, geological structure, accumulation process, etc., may have important effects on the spatial compositional variation of hydrocarbon fluids in reservoirs.Compositional gradients, which may be considerable in near-critical reservoirs, can have a profound impact on the estimation of initial in-situ hydrocarbons, the prediction of oil/gas contact location (OGC), and more important on the reservoir development strategies.Some reservoirs have producing intervals with thickness that cover hundreds of meters from top to bottom, and sometimes even more than 2,000 meters. Over such a thickness, due to gravitational segregation, the mole fraction of the lighter components decreases with depth, while the mole fractions of the heavier components increase from the top to the bottom of the reservoir. Modeling the thermodynamic behavior of these fluids requires a properly tuned EOS capable of reproducing the available PVT data and the gravitational compositional gradation.We studied a volatile oil 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 mo del and the EOS parameters. We used the Peng Robinson EOS with volume translation and Whitson's methodology for defining and characterizing the pseudocomponents.The result of this study addresses the discrepancies in original hydrocarbon volumes that take place when the compositional gradient is not considered. Without considering a compositional gradient, the original oil and gas in place can be either over or underestimated depending upon the depth at which the sample is taken. We show that we can select a reference sample composition, corresponding to a certain depth, such that this can be used without compositional gradient and the estimates of the original hydrocarbons in place are equivalents to those obtained with compositional gradient.We provide guidelines and procedures to evaluate the effect of isothermal gravitational compositional gradient in the determination of in-place hydrocarbon content for volatile oil and gas condensate reservoirs.
