PROPERTIES OF FLUID RESERVOIRS – PVT TESTS

FLASH VAPORIZATION. A sample of the reservoir liquid is placed in a laboratory cell. Pressure is adjusted to a value equal to or greater than initial reservoir pressure. Temperature is set at reservoir temperature. Pressure is reduced by increasing the volume in increments.

This procedure is called FLASH VAPORIZATION, there is a graphic that represents the process of this porcedure where the plot reporduces part of an isotherm of a pressure-volume diagram. The shape is similar to that shown in below.

PRESSURE VS VOLUME

The pressure at which the slope changes is the bubble point pressure of the mixture. The volume at this point is the volume of the bubble -point liquid. Often it is given the symbol Vsat. The volume of the bubble point

FLASH VAPORIZATION

liquid can be divided by the mass of reservoir fluid in the cell to obtain a value of specific volume at the bubble point. Specific volume at the bubble point also is meaasured during other tests and is used as a check on the quality of the data.

All values of total volume, Vt, are divided by volume at the bubble point, and the data or information is reported as relative volume. Sometimes the symbol  V/Vsat is used; however, we will use the sysmbol (Vt/Vb)f. The sysmbol (Vt/Vb)f means total volume divided by volume at the bubble point for a flash vaporization.

DIFFERENTIAL VAPORIZATION:

The sample of reservoir liquid in the laboratory cell is brought to bubble-point pressure, and the temperature is set at reservoir temperature. Pressure is reduced by increasing cell volume, and the cell is agitated to ensure equilibrium between the gas and liquid. Then, all the gas is expelled from the cell while pressure in the cell is held constant by reducing cell volume.

DIFFERENTIAL VAPORIZATION PROCEDURE

The gas is collected, and its quantity and specific gravity are measured. The volume of liquid remaining in the cell, V0, is measured. The process is repeated in steps until atmospheric pressure is reached. Then  temperature is reduced to 60·F, and the volume of remaining liquid is measured. This is called residual oil from differential vaporization or residual oil.

Each of the values of volume of cell liquid, Vo, is divided by the volume of the residual oil. The result is called relative oil volume and is given the symbol BoD. In addition, the volume of gas removed during each step is measured both at cell conditions and at standard conditions.

The total volume of gas removed during the entire process is the amount of gas in solution at the bubble point. This total volume is divided by the volume of residual oil, and the untis are converted to standard cubic feet per barrrel of residual oil. The symbol RsDb represents atandard cubic feet of gas removed per barrel of residual oil. The gas remaining in solution at any lower pressure is calculated by subtracting the sum of the gas removed down to and including the pressure of interest from the total volume of gas removed. The result is divided by the volume of residual oil, converted to SCF/residual bbl, and reported as RsD.

SEPARATOR TESTS:

A sample of reservoir liquid is placed in the laboratory cell and brougt to reservoir temperature and bubble point pressure. Then the liquid is expelled from the cell through two stages of separation.The vessel representing the stock tank is a stage of separation

SEPARATOR TESTS

if it has lower pressure than the separator. Pressure in the cells held constant at the bubble point by reducing cell volume as the liquid is expelled.The temperatures of the laboratory separator and stock tank usually are set to represent average in the field. The stock tank is always at atmospheric pressure. The pressure in the separator is selected by the operator. The specific gravities of the separator gas and stock tank gas are measured. Often the composition of the separator gas is determined. Finally, a separator volume factor is calculated. It is the volume of separator liquid measured at separator conditions divided by the volume of stock-tank oil at standrad conditions, SP BBl/ STB.

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