Improved Characterization of Heptanes-Plus Fractions Of Light Crudes

Abstract

Heptanes plus fractions have strong effects on the

physical properties and phase behaviour of petroleum

fluids. It is therefore very important to properly

characterize plus fractions. A step to achieving

improved characterization is to obtain more realistic

molecular weights. Most of the current methods of

heptanes plus characterization assume their molecular

weights are accurate. However, what is commonly

measured in the laboratory is the molecular weight of

the complete fluid; the molecular weight of the heptaneplus

fraction is then estimated using Kay's mixing rule.

Unfortunately, physical properties like molecular

weight obtained using 'equivalent fluid' principles by

mixing pure components, do not give the same values

with actual measurements. Therefore, while a very

accurate estimate of the molecular weight ofa reservoir

oil could be available, that of the heavy fractions,

which is 'inferred' could be unreliable, because of the

mixing rule.

A simple technique has been formulated to achieve

improved characterization ofpetroleum fluids and the

heavy fractions. We suggest fine tuning' Kay's mixing

rule in order to achieve a match between actual

measured molecular weight and the 'equivalent fluid'.

Experimental data from over 4()O PVT reports from over 100 fields in the Niger Delta were collected and

studied A correlation was established between oil

gravity and molecular weight and compared with other

commonly used correlations. Statistical error analysis

was undertaken. Heptanes plus molecular weights

which were generally estimated using Kay's mixing

rule were found to be generally high and hence fine

tuned using a simple technique.