Properties of Black Oils

Reservoir Fluid Study

• To provide data for reservoir calculations

• To provide physical property data for well flow calculations

For surface facility design

• The reservoir calculations are the main driving force for the

various tests.

• Over recent years reservoir simulation capability has

generated the need to extend compositional description from

C7+ to in some cases C29+.

• PVT report provides source of all reservoir engineering

properties for behaviour over exploration, development and

production

Sampling

Clearly the sample has to representative of

the reservoir contents or the drainage area.

Desirable to take samples early in the life of the reservoir.

Either sub-surface or surface sampling.

Sub-Surface Sampling

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

reservoir 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.

Surface Sampling

Production rate is carefully

controlled and separator gas and oil

are sampled.

The gas and liquid are recombined

at the producing ratio to obtain a

sample representing the reservoir

liquid.

It is important to emphasize that an

oil reservoir must be sampled

before reservoir pressure drops

below bubble point pressure (Pb).

Reservoir Fluid Study has five main procedures (or tests):

1. Composition

2. Flash Vaporization ( Constant Composition Expansion)

3. Differential Vaporization

4. Separator Tests

5. Oil and Gas Viscosity

A Black oil Reservoir Fluid Study provides the

following physical properties as a function of reservoir

pressure:

1. Bubble point pressure (Pb)

2. Oil formation volume factor (Bo)

3. Solution gas-oil ratio (Rs)

4. Total formation volume factor (Bt)

5. Oil viscosity (μo)

All of the above properties will be determined as a

function of reservoir pressure during its reduction from

initial value to bubble point to very low pressure.

In addition, the Reservoir Fluid Study provides the

followings factor at various separator pressures: -

1- Gas deviation factor (Z-factor)

2- Gas formation volume factor (Bg)

3- Gas viscosity ( μg)

Composition: Measurements of every one of the

hundreds of different chemical species is impossible.

Even determining the composition of the major fraction

of the crude oil is difficult.

Generally, in every case, composition of all light

components are determined from methane to hexane and

all of heavier components are grouped as a plus

component such as heptane plus C7 +.

1- Composition

1. A sample of reservoir fluid is placed into a laboratory cell.

2. Pressure of the cell is adjusted to P > initial reservoir Pressure.

3.Temperature of the cell is adjusted to temperature equals to

reservoir temperature.

4. Pressure is reduced and temperature is kept constant during all

phases pf the test.

5.No liquid or gas is removed from the cell.

6.At each step, pressure and volume of the reservoir fluids are

measured.

2. Flash Vaporization (Constant

Composition Expansion)

• Determination of the correlation between pressure and

volume at reservoir temperature.

• The system never changes during the test.

• The gas remains in equilibrium with the oil throughout the test.

• The behaviour below the bubble point does not reflect

reservoir behaviour, where gas has greater mobility than the oil.

• This test determines the Bubble Point pressure

corresponding to the reservoir temperature.

Note

Pressure is plotted versus volume. This figure shows two

straight lines of different slopes. The intersection of these lines

determines the bubble point pressure of the sample.

EXAMPLE :

The data from a flash vaporization on a black oil at 220oF are given

below. Determine the bubble-point pressure.

Solution:

1. Plot pressure versus volume.

2. Draw two straight lines, each one passing suitable

data points.

3. Determine the intersection point (Pb, V) of the two

lines.

3. Differential Vaporization

1.Temperature is set at the reservoir temperature.

2.The sample is set in the laboratory cell and brought to

the bubble point pressure.

3. Pressure is reduced by increasing the cell volume and

the cell is agitated to ensure equilibrium between gas and

liquid.

4.Then all gas is expelled from the cell while pressure is

held constant by reducing cell volume.

5.The gas is collected and its quantity and specific gravity

is measured. The volume of liquid remaining in the cell

(Vo) is measured.

3. Differential Vaporization

6.Calculate the relative oil volume as follows:

Relative Oil Volume (BoD) = Volume of cell liquid/

Volume of residual oil

7.Calculate Z-factor for the removed gas

Z-factor= VR PR TSC / VSC PSC TR

Where

R = cell condition, SC = standard conditions.

Gas formation volume factor is calculated :

BgD = 0.0282 Z T/P cu ft/scf

Calculate Relative total volume at any pressure:

BtD = BoD + BgD (RsDb - RsD)

Below bubble point in reservoir gas liquid

separation in the reservoir is a constant changing system.

A test has been design to attempt to simulate this process.

In the differential vaporization test liberated

gas is removed from the cell step wise.

At each step below bubble point, volumes

densities , gas expansion and compressibility determined.

Bubble point starting point.

Note

Differential Vaporization vs. Flash Vaporization

Flash liberation consider

ed to be

representative of the

process in the reservoir

above bubble point

pressure.

Differential liberation

considered to be

representative of the

process in the reservoir

below bubble point

pressure.

The primary 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.

4- Separator Test

The test involves placing a hydrocarbon sample at its saturation

pressure and reservoir temperature in a PVT cell. The volume of

the sample is measured as Vsat.

The hydrocarbon sample is then displaced and flashed through a

laboratory multistage separator system—commonly one to three

stages.

The pressure and temperature of these stages are set to represent

the desired or actual surface separation facilities.

The gas liberated from each stage is removed and its specific

gravity and volume at standard conditions are measured.

The volume of the remaining oil in the last stage (representing the

stock-tank condition) is measured and recorded as Vost.

These experimental measured data can then be used to determine

the oil formation volume factor and gas solubility at the bubble-

point pressure as follows:

Oil viscosity is measured using Rolling Ball

Viscometer or Capillary viscometer.

Gas viscosity is usually obtained from

correlation using gas specific gravity.

5. Oil and Gas Viscosity