Fluid SamplingPrepared by Saif Ur Rehman

Sampling To determine the physical properties of the reservoir fluids, we do

sampling. We determine:

1. Bubble-point pressure

2. Formation volume factor

3. Solution gas-oil ratio

4. Total formation volume factor

5. Coefficient of isothermal compressibility of oil

6. Oil viscosity

7. Z-factor

8. Formation volume factor of gas

9. Gas viscosity

A sample can obtained by two ways:

1)Bottom-hole OR subsurface sampling: The well is shut in and liquid at the bottom of the wellbore is

sampled.

2)Separator or Surface Sampling: Production rate is carefully controlled, and separator gas and

separator liquid are sampled.

Reservoir must be sampled before the pressure drops below the bubble-point pressure. No sampling method will give the original reservoir sample after bubble-point pressure drop.

Bottom-hole sample will contain less gas because some gas has evolved after bubble-point pressure.

Separator sample be recombined at the wrong ratio because free gas is produced from the reservoir with the reservoir liquid.

Reservoir Fluid Study We will discuss the five major test performed during a

reservoir fluid study which are as under:

1. Compositions measurements2. Flash Vaporization3. Differential Vaporization4. Separator Test5. Viscosity measurements

Compositions measurements

Determining the composition of every one of the hundreds of different chemical species present in a reservoir is impossible. Even determining the composition of a major fraction of the crude is difficult. Therefore only the composition of the light components usually C1 to C6 are determined, and all of the heavier components are grouped together in a plus component like heptane plus (C7+). The plus component consists of hundreds of different chemical species.

Flash Vaporization

A sample is placed in a laboratory cell which has a reservoir pressure and temperature. Pressure is reduced by increasing the volume in increments. Gas is produced when pressure is below the bubble-point pressure. No gas or liquid is removed from the cell.

At each step, pressure and volume are measured. Vt is the total volume in which gas and oil both are include.

This procedure is called flash vaporization, flash liberation, pressure-volume relations, constant composition expansion, or flash expansion.

We find bubble-point pressure by this method

Pressure is plotted against total volume, the pressure at which slope changes is the bubble-point pressure of the mixture. The volume at this point is the volume of the bubble-point liquid denoted ad Vsat.

Volume of the bubble-point liquid per mass of the fluid is term as specific volume at the bubble point.

Total volume Vt divided by volume at bubble-point is term as relative volume. We will use the symbol (Vt/Vb)F

Differential Vaporization The sample in the

laboratory cell is brought to bubble-point pressure and temperature is set at reservoir temperature.

Pressure is reduced by increasing cell volume and all the gas is expelled from the cell while pressure in the cell is held constant by reducing cell volume.

The gas is collected and its quantity and specific gravity are measured. The volume of liquid remaining in the cell, Vo is measured.

This process is repeated in steps until atmospheric pressure is reached. Then temperature is reduced to 60’F and volume of remaining liquid is measured. This is called residual oil from differential vaporization or residual oil.

This process is called differential vaporization, differential liberation, or differential expansion.

Volume of liquid in a cell divided by the volume of the residual oil is term as relative volume and denoted by BoD

The volume of gas removed during each step is measured both at cell condition and at standard conditions. The z-factor is calculated as

Where R refers to conditions in the cell. Formation volume factor of the gas removed are

calculated with these z-factors as

Separator Test A sample is placed in the laboratory

cell which has reservoir temperature and bubble-point pressure. Then liquid expelled from the cell through two stages of separation. The vessel representing the stock tank is a stage of separation if it has lower pressure than the separator. Pressure in the cell is held constant at the bubble-point by reducing cell volume as the liquid is expelled. The temperature of the separator and stock tank usually are set to represent the condition in the field. The stock tank is always at atmospheric pressure.

The formation volume factor of oil is calculated as

The solution gas-oil ratio is calculated as

Where S indicates the result of separator test and b indicates bubble-point condition in the reservoir.

Specific gravity of the separator gas and stock-tank gas are measured.

The composition of the separator gas is determined.

A separator volume factor is calculated by volume of separator liquid measured at separator conditions divided by the volume of stock-tank oil at standard condition, SP bbl/STB

Viscosity measurements

Oil Viscosity

Oil viscosity is measured in a rolling-ball viscometer or a capillary viscometer.

Gas Viscosity

Measurement of gas viscosity is very tedious. Obtaining accurate measurements on a routine base is difficult. Thus, gas viscosity is estimated from correlations using the values of gas specific gravities measured in differential liberation.