Reservoir fluid composition and PVT properties play an important role in liquid rich shale development. Accurate estimation of reservoir fluid richness has a significant impact on drilling targets and well completions. Traditional PVT properties are widely used for reservoir management, production optimization, and economic evaluation.
PVT data from shale reservoirs are rarely available in public literature due to difficulty to obtain a representative in-situ fluid sample in extremely low permeability reservoirs. This makes understanding shale PVT a very challenging task. Statoil/Talisman spent two years to collect a large number of PVT samples and measured PVT properties for Eagle Ford field. A wide range of reservoir fluids from dry gas, gas condensate, and volatile oil to black oil were collected and studied thoroughly.
Based on the large PVT database, a new methodology was developed to estimate in-situ reservoir fluid composition and corresponding PVT properties based on readily available field data. Any of the following field data, API gravity of surface condensate/oil, gas-oil ratio (GOR1), surface gas composition, or mud logging gas data is sufficient to derive the majority of PVT properties: including reservoir fluid composition, saturation pressure, formation volume factors, and Natural Gas Liquids (NGLs).
The new method has been applied to many business areas with significant improvements. The first application example is production optimization for new wells without PVT data. All PVT inputs can be derived reasonably from only surface gas composition. The second example is application to shale play evaluation in the early phases. This new method allows for fairly accurate estimation of fluid richness based on very limited field data. The last example is using mud logging gas data to provide real time reservoir fluid type and GOR interpretation. The interpretation provides useful guidance for drilling target zones and well completion design.
To summarize, the new method proposed in this paper is a technical breakthrough for Shale PVT with many potential applications. Thanks to reservoir fluid self-consistency in shale plays, it is very practical to estimate reasonable PVT properties based on existing field data without large sampling and laboratory programs.