Page 104 - Fundamentals of Reservoir Engineering

CHAPTER 2

PVT ANALYSIS FOR OIL

2.1 INTRODUCTION

In Chapter 1, the importance of PVT analysis was stressed for relating observed
volumes of gas production at the surface to the corresponding underground withdrawal.
For gas this relationship could be obtained merely by determining the single or two
phase Z−factor, and using it in the equation of state. The basic PVT analysis required
to relate surface production to underground withdrawal for an oil reservoir is
necessarily more complex due to the presence, below the bubble point pressure, of
both a liquid oil and free gas phase in the reservoir.

This chapter concentrates on defining the three main parameters required to relate
surface to reservoir volumes, for an oil reservoir, and then proceeds to describe how
these parameters can be determined in the laboratory by controlled experiments
performed on samples of the crude oil.

The subject is approached from a mechanistic point of view in merely recognising that
PVT parameters can be determined as functions of pressure by routine laboratory
analysis. No attempt is made to describe the complex thermodynamic processes
implicit in the determination of these parameters. For a more exhaustive treatment of
1
the entire subject the reader is referred to the text of Amyx, Bass and Whiting .
Finally, a great deal of attention is paid to the conversion of PVT data, as presented by
the laboratory, to the form required in the field. The former being an absolute set of
measurements while the latter depend upon the manner of surface separation of the
gas and oil.

2.2 DEFINITION OF THE BASIC PVT PARAMETERS

The Pressure−Volume−Temperature relation for a real gas can be uniquely defined by
the simple equation of state

pV = ZnRT (1.15)

in which the Z−factor, which accounts for the departure from ideal gas behaviour, can
be determined as described in Chapter 1, sec. 5. Using this equation, it is a relatively
simple matter to determine the relationship between surface volumes of gas and
volumes in the reservoir as

p T 1 p
E = × sc × = 35.37 (scf /rcf) (1.25)
p T Z ZT
sc

Unfortunately, no such simple equation of state exists which will describe the PVT
properties of oil. Instead, several, so-called, PVT parameters must be measured by
laboratory analysis of crude oil samples. The parameters can then be used to express