Page 114 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 53

The problems associated with sampling an initially saturated oil reservoir, or an
undersaturated reservoir in which the bottom hole flowing pressure has been allowed to
fall below bubble point pressure, can be largely overcome by correct well conditioning
prior to sampling. If the well has already been flowing, it should be produced at a low
stabilized rate for several hours to increase the bottom hole flowing pressure and
thereby re-dissolve some, if not all, of the free gas saturation in the vicinity of the well.
Following this the well is closed in for a reasonable period of time during which the oil
flowing into the wellbore, under an ever increasing average pressure, will hopefully re-
dissolve any of the remaining free gas. If the reservoir was initially at bubble point
pressure, or suspected of being so, the subsurface sample should then be collected
with the well still closed in. If the reservoir is known to be initially undersaturated the
sample can be collected with the well flowing at a very low rate so that the bottom hole
flowing pressure is still above the bubble point. With proper well conditioning a
representative combined sample can usually be obtained.
One of the main drawbacks in the method is that only a small sample of the wellbore
fluids is obtained, the typical sampler having a volume of only a few litres. Therefore,
one of the only ways of checking whether the gas oil ratio is correct is to take several
downhole samples and compare their saturation pressures at ambient temperature on
the well site. This can be done using a mercury injection pump and accurate pressure
gauge connected to the sampler. The chamber normally contains both oil and a free
gas phase, due to the reduction in temperature between wellbore and surface. Injecting
mercury increases the pressure within the chamber until at a saturation pressure
corresponding to the ambient surface temperature all the gas will dissolve. This
saturation pressure can be quite easily detected since there is a distinct change in
compressibility between the two phase and single phase fluids. If it is experimentally
determined, on the well site, that successive samples have markedly different
saturation pressures, then either the tool has been malfunctioning or the well has not
been conditioned properly.

In addition, it is necessary to determine the static reservoir pressure and temperature
by well testing, prior to collecting the samples. Further details on bottom hole sampling
techniques are given in references 2 and 3 listed at the end of this chapter.

b) Surface recombination sampling

In collecting fluid samples at the surface, separate volumes of oil and gas are taken at
separator conditions and recombined to give a composite fluid sample. The surface
equipment is shown schematically in fig. 2.7.