PVT analysis of the Volve Oil field consists for four reports covering three wells. These reports cover Constant Mass Expansion, Differential Liberation, Separator tests and Compositions analysis. However, it is seen that none of the reports are complete in themselves i.e. all standard tests were not performed on the same samples. The closest we get to a complete analysis is with the discovery well 15/9-19SR.
PVT (Pressure, Volume, Temperature) information is used to characterize the physical properties of reservoir fluids and determine changes during development and production. They primarily used to estimate the in-place oil & gas volumes, calculate flow properties of reservoir fluids, populate simulation models and prepare development plans.
Summary
- Sample Collection
- Well 15/9-19SR – PVT Analysis report
- Well 15-9-19A – PVT Analysis of Single Phase Sample Report & Recombination and PVT Analysis, Core Flooding Study
- Well 15/9-F-4 – PVT Analysis of MDT oil samples
- Bubble Point
- Solution Gas-Oil Ratio
- Formation Volume Factor
- Gravity
- Viscosity
- Compositional Analysis
PVT reports available for the Volve dataset:
| Sl no | Report Name | Date | Well |
|---|---|---|---|
| 1 | PVT Analysis | June 1993 | 15/9-19SR |
| 2 | PVT Analysis of Single Phase Sample | May 1998 | 15/9-19A |
| 3 | Recombination and PVT Analysis, Core Flooding Study | Sept 1998 | 15/9-19A |
| 4 | PVT Analysis of MDT oil samples | May 2008 | 15/9-F-4 |
Sample Collection
Well 15/9-19SR – PVT Analysis report
Discovery Well of Volve Oilfield. Test hydrocarbon potential in the Hugin and Heimdal formations.
Two bottom hole samples and one separator samples were collected from the well during the Drill Stem Test (DST) at 3885.16 m MD RKB and 3888.72 m MD RKB. The quality of the bottom hole samples was determined by measuring the bubble point pressure at reservoir temperature.
The quality of the separator sample was determined by measuring the bubble point pressure of the separator liquid at separator temperature, and the opening pressure of the gas bottle.
Well 15-9-19A – PVT Analysis of Single Phase Sample Report & Recombination and PVT Analysis, Core Flooding Study
Determine the minimum economic hydrocarbon volume in Hugin and map the extension of oil bearing formation.
The sample was collected using Xact Single phase sampler for the PVT Analysis of Single Phase Sample Report. The bottom hole sampling depth is 3748 m MD RKB.
Two sets of separator PVT samples from 15-9-19A were collected for the Recombination and PVT Analysis, Core Flooding Study. Opening pressure was measured for gas samples as a validity check. PVT analysis was carried out on one sample.
Well 15/9-F-4 – PVT Analysis of MDT oil samples
Well F-4 is the first injection well in the Volve development drilling program. The F-4 injection well will be located in a down flank position east of the F-12 production well, close to the east bounding fault of the structure.
Four MDT samples (two water and two oil samples) were collected from well 15/9-F-4. The sampling depth was 3245.5 m MD RKB
Quality check was performed on the oil samples by determination of the bubble point pressure at reservoir temperature, analysis of single stage separation properties and compositional analysis, and mud filtrate contamination study.
The mud filtrate contamination was determined by subtraction of the actual fractions in the oil after composition analysis of the mud filtrate. The sample with lower mud filtrate contamination was selected for further analysis.
Bubble Point
The pressure at which natural gas is first liberated from oil is known as the Bubble point pressure. It is measured experimentally using the Constant Mass Expansion technique.
The bubble point is in the range from 273.8 bar to to 213.1 bar. It is identified from the Rel Volume vs pressure relationship where, the trend becomes linear below the bubble point.
The initial reservoir pressure of 328 bar indicates that the reservoir was undersaturated. Later, due to water injection the reservoir pressure rose to around 335 bar indicating that the oil remained undersaturated.
The Y function is linear to pressure below the bubble point. This function is used to smoothen the total volume data at pressures below the bubble point.
Y=\frac{\frac{p_{b}}{p}-1}{\frac{V_{t}}{V_{b}}-1}
Solution Gas-Oil Ratio
The amount of surface gas that can be dissolved in a stock tank oil when brought to a specific pressure and temperature is called Solution gas oil ratio (Rs). The solution gas-oil ratio remains constant above the bubble point pressure.
The Rs for the Volve Oilfield remains constant throughout the field life at around 160 Sm3/Sm3. The Rs values are available for differential liberation test only for sample TS-29-06 (well: 15-9-19 SR). The sample had a Rs of 169 Sm3/Sm3.
The Solution Gas-Oil ratios of different samples indicate a possible range of Rs from 160-106 Sm3/Sm3. The samples were from a separator test where the fluid was flashed from reservoir to ambient conditions.
Formation Volume Factor
The formation volume factor (FVF) of oil is defined as the ratio of oil liquid volume at reservoir conditions divided by the oil liquid volume of the same sample at surface conditions.
The FVF from the Differential Liberation experiment for sample TS-29-06 (Well: 15-9-19SR) has a value of 1.56 vol/vol.
The Formation Volume Factors of different samples indicate a possible range of Bo from 1.51-1.38 Sm3/Sm3. The samples were from a separator test where the fluid was flashed from reservoir to ambient conditions.
Gravity
The oil gravity of the samples for stock tank condition is around 890 kg/Sm3 and the oil gravity at bubble point is around 720 kg/m3. The stock tank API gravity is 27.49, indicating black oil composition.
| Sample | Oil gravity @ Bubble Point (kg/m3) | Oil gravity @ Stock tank condition (kg/Sm3) |
|---|---|---|
| 20770 (15-9-19A) | 733.4 | 900.5 |
| 20605 (15-9-19A) | 857.5 | 894.7 |
| TS-29-06 (15-9-19SR) | 701 | 883 |
| TS-61-04 (15-9-19SR) | 702 | 883 |
| 6103-MA (15-9-F4) | 720.5 | 872.5 |
Viscosity
The Viscosity was determined at a reservoir temperature of around 107oC. A viscosity of below 1cP above bubble point pressure indicates that fluid is mobile.
Compositional Analysis
A significant portion of the PVT reports have compositional analysis for both the Oil and Gas phase. A later article will look into modelling the compositional analysis on an open source software.
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