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128 Principles of Applied Reservoir Simulation

estimating bubble point pressure, formation volume factor, and isothermal oil
compressibility have been proposed by Levitan and Murtha [1999].

13.3 Fluid Sampling

All laboratory measurements of fluid properties and subsequent analyses
are useless if the fluid samples do not adequately represent in situ fluids. The
goal of fluid sampling is to obtain a sample that is representative of the original
fluid in the reservoir. It is often necessary to condition the well before the sample
is taken. A well is conditioned by producing any nonrepresentative fluid, such
as drilling mud, from within and around the wellbore until it is replaced by
original reservoir fluid flowing into the wellbore. Fluid samples may then be
taken from either the surface or subsurface.
Subsurface sampling requires lowering a pressurized container to the
production interval and subsequently trapping a fluid sample. This is routinely
accomplished by drill stem testing, especially when access to surface facilities
is limited. It is generally cheaper and easier to take surface samples from
separator gas and oil.
If a surface sample is taken, the original in situ fluid, that is, the fluid at
reservoir conditions, must then be reconstituted by combining separator gas and
separator oil samples. The recombination step assumes accurate measurements
of flow data at the surface, especially gas-oil ratio. Subsurface sampling from
a properly conditioned well avoids the recombination step, but is more difficult
and costly than surface sampling, and usually provides a smaller volume of
sample fluid. The validity of fluid property data depends on the quality of the
fluid sampling procedure.

Exercises

Exercise 13.1 Data set EXAM9.DAT models depletion of a gas reservoir with
aquifer support. Initial reservoir pressure is approximately 1947 psia. (A) Run
the model at a temperature of 226°F and record time, pressure, gas rate, and
water rate at the end of the run. Report the gas viscosity in the gas PVT table