Page 144 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 131

against the borehole wall through which reservoir fluids flow into a series of
chambers which are then sealed to maintain reservoir pressure conditions (see
Section 6.3.6). A variety of downhole analysis techniques are used to identify the
fluids and estimate flowing potential. This type of sampling allows reservoir fluids to
be characterized without resorting to more expensive well-testing.

6.2.6.2. Surface samples
Surface sampling involves taking samples of the two phases (gas and liquid) flowing
through the surface separators, and recombining the two fluids in an appropriate
ratio such that the recombined sample is representative of the reservoir fluid.
The oil and gas samples are taken from the appropriate flowlines of the same
separator, whose pressure, temperature and flowrate must be carefully recorded to
allow the recombination ratios to be calculated. In addition, the pressure and
temperature of the stock tank must be recorded to be able to later calculate the
shrinkage of oil between the point at which it is sampled and the stock tank. The oil
and gas samples are sent separately to the laboratory where they are recombined
before PVTanalysis is performed. A quality check on the sampling technique is that
the bubble point of the liquid sample at the temperature of the separator from
which the samples were taken should be equal to the separator pressure.
The advantages of surface sampling and recombination are that large samples may
be taken, that stabilised conditions can be established over a number of hours prior to
sampling and that costly wireline entry into the well is avoided. The subsurface
sampling requirements also apply to surface sampling; if P wf is below P b ,thenit is
probable that an unrepresentative volume of gas will enter the wellbore, and even
good surface sampling practice will not obtain a true reservoir fluid sample.

6.2.6.3. PVT analysis
Typical analysis in the laboratory consists of sample validation, a compositional
analysis of the individual and recombined samples, measurement of oil and gas
density and viscosity over a range of temperatures and determination of the basic
PVT parameters B o , R s and B g .
For the details of PVT analysis refer to Fundamentals of Reservoir Engineering by
L.P. Dake.
It is of particular interest to note the different data requirements of the disciplines
when the laboratory tests are performed. During the compositional analysis,
petroleum engineers are satisfied with a compositional analysis of the hydrocarbons
which extend up to around the C 6 components, with C 7+ components being
lumped together and characterised by a pseudo-component. Process engineers
require a more detailed compositional analysis, typically extending up to C 30 .This is
because the heavy ends play a more important role in the phase behaviour at the
lower temperatures and pressures experienced during surface processing. For
example, the long-chain hydrocarbons will form solids (such as wax) at surface
conditions, but will remain in solution at reservoir conditions.
Part of the PVT analysis will include passing the reservoir fluid sample through a
series of expansions to simulate the separator conditions. At the design stage, process