Page 113 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 52

sample chamber

pressure

p i
p b

p wf
r

Fig. 2.6 Subsurface collection of PVT sample

A special sampling bomb is run in the hole, on wireline, to the reservoir depth and the
sample collected from the subsurface well stream at the prevailing bottom hole
pressure. Either electrically or mechanically operated valves can be closed to trap a
volume of the borehole fluids in the sampling chamber. This method will obviously yield
a representative combined fluid sample providing that the oil is undersaturated with gas
to such a degree that the bottom hole flowing pressure p wf at which the sample is
collected, is above the bubble point pressure. In this case a single phase fluid, oil plus
its dissolved gas, is flowing in the wellbore and therefore, a sample of the fluid is bound
to have the oil and gas combined in the correct proportion. Many reservoirs, however,
are initially at bubble point pressure and under these circumstances, irrespective of
how low the producing rate is maintained during sampling, the bottom hole flowing
pressure p wf will be less than the bubble point pressure p b as depicted in fig. 2.6. In this
case, there will be saturated oil and a free gas phase flowing in the immediate vicinity
of the wellbore, and in the wellbore itself, and consequently, there is no guarantee that
the oil and gas will be collected in the correct volume proportion in the chamber.

In sampling a gas saturated reservoir, two situations can arise depending on the time at
which the sample is collected. If the sample is taken very early in the producing life it is
possible that the fluid flowing into the wellbore is deficient in gas. This is because the
initially liberated gas must build up a certain minimum gas saturation in the reservoir
pores before it will start flowing under an imposed pressure differential. This, so−called,
critical saturation is a phenomenon common to any fluid deposited in the reservoir, not
just gas. The effect on the producing gas oil ratio, immediately below bubble point
pressure, is shown in fig. 2.4 as the small dip in the value of R for a short period after
the pressure has dropped below bubble point. As a result of this mechanism there will
be a period during which the liberated gas remains in the reservoir and the gas oil ratio
measured from a subsurface sample will be too low. Conversely, once the liberated gas
saturation exceeds the critical value, then as shown in fig. 2.4 and discussed
previously, the producing well will effectively steal gas from more remote parts of the
reservoir and the sample is likely to have a disproportionately high gas oil ratio.