- Laboratory analysis for Black Oil.
The following pages give an outline of each test that is performed on the black oil sample(s) and also a specimen PVT report to show how the data may be presented to the client. All of the analyses are performed by industry recognized methods using standard PVT equipment.
- Validity Checks
This is the first test to be performed on receipt of samples in the PVT laboratory to check the samples prior to any analysis being performed on them.
- Compositional Analysis
The composition of the separator gas and liquid is determined on the sample using a suitably equipped Gas Chromatograph.The Chromatograph having been regularly checked using a standard samples
- Physical Recombination
This is performed using a calibrated container, which is filled to a known pressure with the separator gas. Based upon the corrected GLR the required amount of separator liquid is added. The saturation pressure at ambient temperature is then determined and compared with bottom hole samples and also with the measure bottom hole pressure.
- Pressure -Volume relations (Constant Composition Expansion)
An accurately measured volume of either the recombined fluid or the bottom hole sample is placed in a calibrated high pressure cell linked to a precision mercury pump. The cell is then heated to the reported reservoir pressure and allowed to stabilize for several hours. Once the sample has been stabilized at the required reference pressure and the reservoir temperature the CCE test can be commenced. From this test, the relative volume data above and below saturation pressure and the single phase compressibility can be calculated.
- Differential Liberation Test
This test is performed in a series of steps from the saturation pressure down to atmospheric pressure at reservoir temperature. From this test the following are calculated: - bo, bg, gas gravity, oil density and gas viscosity.
- Viscosity of liquid phase
The liquid viscosity can be determined by using a rolling ball viscometer. It can be used for liquids with viscosities down to about 0.1 centipoise. The equipment is calibrated with fluids of known viscosity prior to use in order to construct a full calibration for the equipment.
- Separation Tests
A portion of the reservoir fluid is subjected to flash separation to specified conditions of pressure and temperature, collecting the evolved gas and determining its composition and physical properties. The remaining liquid is then flashed either to a further separation stage or to stock tank conditions in a controlled manner. The gas and oil volumes are measured along with gas gravity and oil density.
- Laboratory analysis for a Volatile Oil
Most of the analyses carried out on this type of fluid are the same as those performed for a black oil. Validity checks, compositional analysis, pressure-volume relations, viscosity and separation tests are all performed in the manner already described. Instead of performing a DV test a Constant Volume Depletion test (CVD) is performed at reservoir temperature and in a series of six or more stages.
- Laboratory analysis for a Gas Condensate
The laboratory analysis of gas condensate fluids is performed by similar methods to those previously described. The validity checks, compositional analysis and recombination are carried out exactly the same as for the previous studies. The pressure-volume relations (CCE) is different in that a different PVT cell is used in order to visually examine the system as the pressure is decreased in order to view the dew-point of the sample at reservoir temperature. All other measurements are taken in the same way. Then a CVD test is performed as per a volatile oil analysis.
- Laboratory analysis for ‘Dry’ and ‘Wet’ Gases
Since in most cases there are very few if any changes in the system either in the reservoir or at the surface only a limited suite of analysis is performed for these types of fluid. Normally validity checks, compositional analysis, physical recombination and pressure-volume relations tests are performed with the addition of measurement of the ‘Z’ factor or compressibility of the gas.