LABORATORY ANALYSIS OF RESERVOIR FLUIDS

Monday, September 21, 2009 by ShoXee

LABORATORY ANALYSIS OF RESERVOIR FLUIDS

Accurate laboratory studies of PVT and phase-equilibria behavior of
reservoir fluids are necessary for characterizing these fluids and evaluat-
ing their volumetric performance at various pressure levels. There are
many laboratory analyses that can be made on a reservoir fluid sample.
The amount of data desired determines the number of tests performed in
the laboratory. In general, there are three types of laboratory tests used to
measure hydrocarbon reservoir samples:
1. Primary tests

These are simple, routine field (on-site) tests involving the measure-
ments of the specific gravity and the gas-oil ratio of the produced
hydrocarbon fluids.

2. Routine laboratory tests

These are several laboratory tests that are routinely conducted to char-
acterize the reservoir hydrocarbon fluid. They include:

• Compositional analysis of the system
• Constant-composition expansion
• Differential liberation
• Separator tests
• Constant-volume depletion

3. Special laboratory PVT tests

These types of tests are performed for very specific applications. If a
reservoir is to be depleted under miscible gas injection or a gas cycling
scheme, the following tests may be performed:
• Slim-tube test
• Swelling test
The objective of this chapter is to review the PVT laboratory tests and
to illustrate the proper use of the information contained in PVT reports.

COMPOSITION OF THE RESERVOIR FLUID

It is desirable to obtain a fluid sample as early in the life of a field as
possible so that the sample will closely approximate the original reser-
voir fluid. Collection of a fluid sample early in the life of a field reduces
the chances of free gas existing in the oil zone of the reservoir.
Most of the parameters measured in a reservoir fluid study can be cal-
culated with some degree of accuracy from the composition. It is the
most complete description of reservoir fluid that can be made. In the
past, reservoir fluid compositions were usually measured to include sepa-
ration of the component methane through hexane, with the heptanes and
heavier components grouped as a single component reported with the
average molecular weight and density.
With the development of more sophisticated equations-of-state to calcu-
late fluid properties, it was learned that a more complete description of the
heavy components was necessary. It is recommended that compositional
analyses of the reservoir fluid should include a separation of components
through C10 as a minimum. The more sophisticated research laboratories
now use equations-of-state that require compositions through C30 or higher.
Table 3-1 shows a chromatographic “fingerprint” compositional analysis
of the Big Butte crude oil system. The table includes the mole fraction,
weight fraction, density, and molecular weight of the individual component.

CONSTANT-COMPOSITION EXPANSION TESTS

Constant-composition expansion experiments are performed on gas
condensates or crude oil to simulate the pressure-volume relations of
these hydrocarbon systems. The test is conducted for the purposes of
determining:
• Saturation pressure (bubble-point or dew-point pressure)
• Isothermal compressibility coefficients of the single-phase fluid in
excess of saturation pressure
• Compressibility factors of the gas phase
• Total hydrocarbon volume as a function of pressure

DIFFERENTIAL LIBERATION (VAPORIZATION) TEST

In the differential liberation process, the solution gas that is liberated
from an oil sample during a decline in pressure is continuously removed
from contact with the oil, and before establishing equilibrium with the
liquid phase. This type of liberation is characterized by a varying compo-
sition of the total hydrocarbon system.
The experimental data obtained from the test include:
• Amount of gas in solution as a function of pressure
• The shrinkage in the oil volume as a function of pressure
• Properties of the evolved gas including the composition of the liberated
gas, the gas compressibility factor, and the gas specific gravity
• Density of the remaining oil as a function of pressure

The differential liberation test is considered to better describe the sepa-
ration process taking place in the reservoir and is also considered to sim-
ulate the flowing behavior of hydrocarbon systems at conditions above
the critical gas saturation. As the saturation of the liberated gas reaches
the critical gas saturation, the liberated gas begins to flow, leaving behind
the oil that originally contained it. This is attributed to the fact that gases
have, in general, higher mobility than oils. Consequently, this behavior
follows the differential liberation sequence.
The test is carried out on reservoir oil samples and involves charging a
visual PVT cell with a liquid sample at the bubble-point pressure and at
reservoir temperature. As shown schematically in Figure 3-4, the pressure
is reduced in steps, usually 10 to 15 pressure levels, and all the liberated
gas is removed and its volume is measured at standard conditions. The vol-
ume of oil remaining VL is also measured at each pressure level. It should
be noted that the remaining oil is subjected to continual compositional
changes as it becomes progressively richer in the heavier components.
The above procedure is continued to atmospheric pressure where the
volume of the residual (remaining) oil is measured and converted to a
volume at 60°F, Vsc. The differential oil formation volume factors Bod
(commonly called the relative oil volume factors) at all the various pres-
sure levels are calculated by dividing the recorded oil volumes VL by the
volume of residual oil Vsc

SEPARATOR TESTS

Separator tests are conducted to determine the changes in the volumet-
ric behavior of the reservoir fluid as the fluid passes through the separa-
tor (or separators) and then into the stock tank. The resulting volumetric
behavior is influenced to a large extent by the operating conditions, i.e.,
pressures and temperatures, of the surface separation facilities. The pri-
mary objective of conducting separator tests, therefore, is to provide the
essential laboratory information necessary for determining the optimum
surface separation conditions, which in turn will maximize the stock-tank
oil production. In addition, the results of the test, when appropriately
combined with the differential liberation test data, provide a means of
obtaining the PVT parameters (Bo, Rs, and Bt
) required for petroleum
engineering calculations. These separator tests are performed only on the
original oil at the bubble point.