Theoretical and Experimental Investigation of Isothermal Compositional Grading
Authors: Ratulowski J. , Fuex A.N. , Westrich J.T. , Sieler J.J. 
Published: December 2007

A centrifuge system capable of producing a potential difference across a live oil column equivalent to 1000 ft of gravitational head was designed and tested. Initial tests on a simple ternary system yielded results similar to equation of state (EOS) models. A black oil sample from a Gulf of Mexico field, Bullwinkle J2-RB sand, exhibiting compositional gradients was segregated in the centrifuge. Experimental results from the centrifuge were similar to field values indicating that the large variation in composition observed for this field may be attributed to gravitational segregation alone.

From these results of these two sets of experiments and the subsequent analysis of the graded fractions, we can conclude that significant compositional variation of reservoir fluids not near their vapor-liquid critical points can be caused by gravity alone. The grading phenomenon is sensitive to the saturate/aromatic balance of the oil. Existing EOS models did not correctly predict the compositional variations in fluids like the Bullwinkle J2-RB because pseudocomponents generated from volatility distributions have a fixed saturate/aromatic character. It is subtle changes in the saturate/aromatic balance that drive the grading phenomena for these fluids.

Introduction

Compositional variation with depth has been observed in many reservoirs. These gradients result from a variety of causes and typically indicate nonequilibrium states. Gradients in composition can be observed in systems in equilibrium when chemical potential gradients are balanced by the gravitational potential gradient1–4. Temperature gradients can also contribute to concentration variation. However, in this situation a steady state not equilibrium state exists in the reservoir5–6. Equilibrium composition gradients have been observed and modeled by various authors in near critical reservoirs. Whitson3 provides a comprehensive summary of these efforts.

We have encountered a reservoir in the Gulf of Mexico, Bullwinkle J2-RB, with significant variation in GOR and saturation pressure with depth. The Bullwinkle fluid is not near its vapor liquid critical point at any location in the column. However, the saturation pressure varies by about 3 psia/ft and the GOR changes by nearly 700 SCF in 800 ft of oil column. The gradient in saturation pressure is even larger than equilibrium gradients typically predicted for near critical fluids. Existing EOS models could not be tuned to fit this behavior. Isotope analysis of the gas shows no significant variation in methane isotope ratios across the column. This indicates that at least the light ends are well mixed and suggests an equilibrium condition in the reservoir.

To develop tools to predicting composition variation away from well control, we need to understand the mechanism causing the gradients in fluid properties in the Bullwinkle J2-RB. An experimental apparatus to test the gravitational grading mechanism at Bullwinkle was designed. The results of these experiments and additional modeling work have provided us with a mechanism that can account for the grading in the Bullwinkle J2-RB.