Page 77 - Introduction to Petroleum Engineering

SOURCES OF FLUID DATA 61
Viscosity of water increases by about 50% from 14.7 psi (1 atm) to 8000 psi. The
effect of pressure on water viscosity can be estimated using the following correlation
with pressure in units of psi:

µ
−
×
9
+
P
w = . 10 0 .00004 + . 31 10 P 2 (3.25)
µ
watm
,
1
3.7 SOURCES OF FLUID DATA
The best information about oil, gas, and water in a formation is obtained from fluid
samples that are representative of the original in situ fluids. A well should be
conditioned before sampling by producing sufficient fluid to flush any contaminating
drilling or completion fluids from the well.
Surface sampling at the separator is easier and less expensive than subsurface
sampling. For surface samples, the original in situ fluid is obtained by combining sep-
arator gas and separator oil samples at the appropriate GOR. The recombination step
assumes accurate measurements of flow data at the surface. 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.
Once a sample has been acquired, it is necessary to verify the quality of the
sample. This can be done by compositional analysis and measurement of such
physical properties as density and molecular weight. Gas chromatography is the
most typical instrument for compositional analysis.
After sample integrity is verified, several procedures may be performed to measure
fluid properties that are suitable for reservoir engineering studies. The most common
procedures include the following tests: constant composition expansion (CCE),
differential liberation (DL), and separator tests.

3.7.1 Constant Composition Expansion

A CCE test provides information about pressure–volume behavior of a fluid without
changes in fluid composition. The CCE test begins with a sample of reservoir fluid
in a high‐pressure cell at reservoir temperature and at a pressure in excess of the res-
ervoir pressure. Traditionally, the cell contained oil and mercury. Pressure was altered
by changing the volume of mercury in the cell. Modern systems are designed to be
mercury‐free by replacing mercury with a piston. The piston is used to alter pressure
in the cell, as illustrated in Figure 3.8. The cell pressure is lowered in small incre-
ments, and the change in volume at each pressure is recorded. The procedure is
repeated until the cell pressure is reduced to a pressure that is considerably lower
than the saturation pressure. The original composition of the fluid in the cell does not
change at any time during the test because no material is removed from the cell. The
fluid may be either oil or a gas with condensate. If the fluid is oil, the saturation
pressure is the BP pressure. If the fluid is a gas with condensate, the saturation
pressure is the DP pressure.