Non-Isothermal Gravitational Composition Variation in a Large Deep Water Field
Authors: de Oliveira Padua Kleber Galvao 
Published: December 2007

Abstract
Significant fluid compositional variations at discovery conditions areobserved and discussed for some hydrocarbon reservoirs around the world. These variations may considerably affect the reservoir fluid properties and lead to different exploration and development strategies.
Mostly, they are associated with the thermodynamics (gravity, temperature), accumulation processes (genesis, migration) and reservoir characteristics(permeability, porosity, capillarity, geological structure).
In this work, a synergetic approach is used to examine the effects of the above factors on the oil composition distribution of a large deep water field which the temperature variation, because of the water depth (800 to 2000m), is in the opposite direction of the Earth's thermal gradient. The methodology includes the use of EOS fitting, thermodynamics, reservoir geochemistry and compositional simulation.
A single equation of state model, capable of accurately reproducing the wide range of experimental data, is adjusted to the PVT analyses. The successful matching is achieved by means of splitting the heaviest component and so reducing the molar concentration of the pseudo-component on the single regression procedure.
A new compositional equilibrium model which accounts for the effect of gravity and temperature gradient is developed, presented and applied to evaluate the steady state thermodynamic equilibrium. The overall results suggest the partial reservoir connection and the increase of oil segregation with temperature decrease toward deeper waters. Fluid composition and properties are extrapolated to different depths as function of their respective temperatures.
Reservoir geochemistry testing, such as computerized gas chromatography-mass spectrometry GC - MS, and analyses, such as the ratio of biological markers, are used for understanding the genesis and migration processes. The overall results indicate the same source rock (genesis), biodegradation, different migration pulses and a lateral migration path.
Field scale compositional simulations examine the dynamic effects of the accumulation process (secondary migration, entrapment) on the oil composition changes. The results suggest that geological structures, permeability, porosity and capillarity can also play an important role on the oil compositional distribution and leave spots of undrained water in the reservoir.

Introduction
Original fluid composition variations along the hydrocarbon column have been observed in many reservoirs around the world. They may have a significant impact on the reservoir fluid properties, leading to different exploration and development planning.
In light oils (API gravity &gt;35), an adequate knowledge of the compositional variation is particularly important to understand the oil formation volume factor variation and the development of miscibility. The former is meaningful in the calculation of the hydrocarbon volume in place. The later is vital when considering gas injection.
In heavier oils (API gravity &lt; 35),), the compositional changes are important to estimate the viscosity variation which affects the waterflooding strategy to be applied (highly viscous oil near the oil/water contact can be a serious handicap for down dip water injection).
The spatial distribution of the original reservoir fluid are believed to be caused by many factors, These can be categorized in an approximate manner as factors associated with the thermodynamics, reservoir characteristics, genesis and accumulation processes.
The thermodynamic associated factors are those related to the local temperature, pressure, composition, elevation of the fluid system(gravitational force), interfacial curvatures of the surrounding porous medium(capillary force), geothermal, geological temperature gradients (causing thermally driven convection and steady state thermal diffusion) and molecular diffusion.
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