Page 115 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 54

gas meter

p sep
gas
T sep
sample

p st
T st
separator

oil stock tank oil
well
sample

Fig. 2.7 Collection of a PVT sample by surface recombination

The well is produced at a steady rate for a period of several hours and the gas oil ratio
is measured in scf of separator gas per stock tank barrel of oil. If this ratio is steady
during the period of measurement then one can feel confident that recombining the oil
and gas in the same ratio will yield a representative composite sample of the reservoir
fluid. In fact, a slight adjustment must be made to determine the actual ratio in which
the samples should be recombined. This is because, as shown in fig. 2.7, the oil
sample is collected at separator pressure and temperature whereas the gas oil ratio is
measured relative to the stock tank barrel, thus the required recombination ratio is

REQUIRED MEASURED SHRINKAGE

scf scf stb
R sep = R × S
sep.bbl stb sep.bbl

Dimensionally, the measured gas oil ratio must be multiplied by the shrinkage factor
from separator to stock tank conditions. This factor is usually determined in the
laboratory as the first stage of a PVT analysis of a surface recombination sample by
placing a small volume of the oil sample in a cell at the appropriate separator
conditions and discharging it (flash expansion) to a second cell maintained at the field
stock tank conditions. During this process some gas will be liberated from the separator
sample, due to the reduction in pressure and temperature, and the diminished stock
tank oil volume is measured, thus allowing the direct calculation of S. In order to be
able to perform such an experiment it is important that the engineer should accurately
measure the pressure and temperature prevailing at both separator and stock tank
during sampling and provide the laboratory with these data.

One of the attractive features of surface recombination sampling is that statistically it
gives a reliable value of the producing gas oil ratio measured over a period of hours;
furthermore, it enables the collection of large fluid samples. Of course, just as for
subsurface sampling, the surface recombination method will only provide the correct