Miscibility Scoping Study For
An Oil Reservoir In The Cooper Basin,
PThe Australian School of Petroleum
The University of Adelaide
This report details the methodology and results of a student honours project conducted on behalf of Santos Ltd. towards the requirements of the University of Adelaide undergraduate degree in Petroleum Engineering.
The aim of the project was to assess the potential of oil in the study reservoir -which I shall dub ‘Reservoir X' - to achieve miscibility with various injection gases. Of specific interest was the determination of the minimum pressure at which the ‘Reservoir X' oil and the injection gases would become miscible at reservoir temperature (i.e. the minimum miscibility pressure or MMP).
The report begins with a review of the common fluid sampling methods used in the Petroleum Industry. The vital importance of achieving a fluid sample which represents as close as possible the in-situ reservoir fluid is highlighted. Correct fluid sampling methodology is outlined.
A review of fluid analysis techniques is also included in the report. Pressure, Volume, Temperature (PVT) tests are examined and their relevance for different oil types considered. Data arising from these experiments is vital for all future Reservoir Engineering studies, so it is imperative that they are conducted appropriately.
In order to study the behaviour of ‘Reservoir X' oil under the conditions of a miscible gas flood it was decided to utilize a software package capable of simulating complex dynamic fluid behaviour. Available fluid data for an oil sample from the ‘Reservoir X' #13 well was used to calibrate an Equation-of-State (EOS) capable of predicting changes in oil properties under different conditions. A method of calibration was developed for ‘Reservoir X' oil and is detailed in the report. The EOS was successfully calibrated to match the relevant laboratory fluid data.
The EOS was used to estimate the MMP between the ‘Reservoir X' oil from #13 well and 26 separate injection gases of unique composition by a simulated Slim Tube displacement process. The gas which exhibited the lowest MMP with ‘Reservoir X' oil was a 30 mol% nC5/70 mol% CO2 gas mixture. The MMP of this injection gas with ‘Reservoir X' oil was 1540 psig at 285°F. The MMP between the ‘Reservoir X' oil and a pure CO2 injection gas was determined to be 2680 psig at 285 °F. The highest MMP seen in the study was between ‘Reservoir X' oil and a pure N2 injection gas. The MMP with pure N2 injection gas was determined to be 3800 psig at 285 °F.
Overall it was found that the presence of Nitrogen and Methane in the injection gas hindered the achievement of miscibility and raised the pressure required to achieve miscibility between the oil and injection gas. Presence in the injection gas of Ethane and Pentane tended to decrease the MMP and effectively aided the achievement of miscibility between the oil and injection gas.
The MMP values for different injection gases were also calculated utilizing existing correlations published in the literature. Correlations returned a wide range of MMP results, the majority of which returned a higher MMP than was calculated using Slim Tube simulation. It was clear from the results of this study that, whilst correlations are widely used for MMP determination, they can be inaccurate when applied outside of the context under which they were developed.
Whilst effects such as flow dispersion and reservoir heterogeneity may hinder the achievement of miscibility in a real miscible gas flood, the results indicate that a highly efficient displacement of the ‘Reservoir X' oil is possible via the process of miscible gas injection.