PVT tests

The normally used PVT experiments include:

• Composition Analysis
• Constant composition expansion
• Constant volume depletion
• Differential expansion
• Swelling test
• Separator test

NOTE:Except for separator tests, all experiments are conducted at constant temperatures. The actual experiments performed will vary depending on the type of fluid produced from the reservoir.

Uncertainties in Laboratory Data include:

• Reservoir fluid sampling
• Sample consistency
• Laboratory measurement accuracy

Composition Analysis

Composition analysis is normally performed on all fluid samples to provide the composition of the reservoir fluid.

The data obtained usually includes:

• Components of the fluid samples
• Mole fraction of each component
• Weight fraction of each component (optional)
• Heavy end analysis of heptanes-plus (Mole fraction, Specific gravity, Molecular weight)

Constant Composition Expansion

Constant Composition Expansion is also referred to as:

• Flash vaporization
• Flash liberation
• Pressure-volume relations
• Flash expansion

Constant Composition Expansion is performed on:

• Black oils
• Volatile oils
• Retrograde gases

Constant Composition Expansion measures:

• Undersaturated fluid compressibility and density
• Bubble point or dew point pressure
• Total volume
• Liquid volume

Relative Volume

Above the saturation pressure, the cell volume is measured relative to the volume at saturation pressure.

Vrel = V/Vsat

where:
V_rel - relative volume
V - volume of the hydrocarbon system
V_sat - volume at the saturation pressure

Isothermal Compressibility Coefficient

Also above the saturation pressure, the isothermal compressibility coefficient of the single phase fluid is usually determined from the expression:

where:
C - isothermal compressibility coefficient, psi^-1

Relative Total Volume

Below the saturation pressure, the two-phase volume, V_t, is measured relative to the volume at saturation pressure and expressed as:

RelativeTotalVolume = Vt/Vsat

where:
V_t - total hydrocarbon volume


It should be noted that no hydrocarbon material is removed from the cell, thus, the composition of the total hydrocarbon mixture in the cell remains fixed at the original composition.

Constant Volume Depletion

Constant volume depletion is performed on volatile oils and retrograde gases. At pressures equal to or below dewpoint, it measures:

• Dew point pressure
• Composition changes of the gas phase with pressure depletion
• Compressibility factor at reservoir pressure and temperature
• Recovery of original in-place hydrocarbons at any pressure
• Retrograde condensate accumulation, i.e., liquid saturation

Differential Expansion

Differential expansion is also referred to differential vaporization and differential liberation.It is a test performed on black oils. For pressures below the bubble point, it measures:

• Relative oil volume or differential oil formation volume factor
• Solution gas-oil ratio
• Gas Z-factor
• Oil density

The differential oil formation volume factors, B_od (commonly called the relative oil volume factors), at all the various pressure levels are calculated by dividing the recorded oil volumes, V_L, by the volume of residual oil, V_sc, or:

Bod = VL/Vsc

The differential solution gas-oil ratio, R_sd, is also calculated by dividing the volume of gas in solution by the residual oil volume. The relative oil volume data can be converted to a shrinkage curve by dividing each relative oil volume factor, B_od, by the relative oil volume factor at the bubble point, B_odb, or:

Sod = Bod/Bodb

where:
B_od = differential relative oil volume factor at pressure, bbl/STB
B_odb = differential relative oil volume factor at the bubble point pressure, bbl/STB
S_od = differential oil shrinkage factor, bbl/bbl of bubble point oil

The shrinkage curve has a value of one at the bubble point and a value less than one at subsequent pressures below bubble point pressure. In this suggested form, the shrinkage curve describes the change in the volume of the bubble point oil as reservoir pressure declines.

Swelling Test

Swelling tests assess compositional effects from gas injection. They measure:

• Volume change of the liquid upon gas addition
• Changes in saturation pressure and type (dew point or bubble point)
• Composition of the equilibrated liquid and vapor phases(optional)

The original saturation volume is used as a reference value and the results are represented as relative total volumes.

Vrel = Vsat /(Vsat )orig.

where:
V_rel = relative total value
(V_sat)_orig. = original saturation volume

Separator Test

Separator tests are used to obtain an optimum pressure for separator operation. That optimum occurs when there is simultaneously:

• A maximum in stock tank API gravity
• A minimum in oil formation volume factor (minimize oil shrinkage)
• A minimum in gas-oil ratio

Separator tests measure:

• Composition of the produced gases
• Gravity of the stock-tank oil
• Producing gas-oil ratio
• Formation volume factor of the oil

Separator tests are performed on:

• Black oil
• Volatile oil

The separator volume factor and separator gas-oil ratio are calculated by:

Bof = Vo/(Vo)st

Rsf = (Vg)sc/(Vo)st

where:
V_o - volume of oil at pressure and temperature, bbl
(V_o)_st - volume of stock tank oil at 60oF, bbl
(V_g)_sc - volume of gas removed from separator, scf

The oil formation volume factor and the total solution gas-oil ratio at the bubble point pressure are then calculated by:

Bofb = (Vo)pb/(Vo)st 

Rsfb = (Vg)sc/(Vo)st 

where:
B_ofb - bubble point oil formation volume factor, as measured by flash liberation, bbl of the bubble point oil/STB
(V_o)_pb - volume of oil at bubble point, bbl
R_sfb - bubble point solution gas-oil ratio as measured by flash liberation, scf/STB
(_Vg)_sc - total volume of gas removed from separators, scf

See also

Curtis Whitson Petroleum Engineering Videos#PVT Lab Tests