thermodynamics flow measurement pipes

Increased Uncertainty of Flow Measurement due to Changes of Thermodynamic Properties of Gas Condensate Reservoirs

Case Study

Case Study

SECTOR CHALLENGE

Wet-gas flow meters play a critical role in the allocation of wet-gas output and the optimisation of reservoir performance in the oil and gas industry. Over time, the dynamic conditions within a well may lead to an increase in the liquid content observed by surface wet gas meters, thereby affecting the reliability of their readings. Modern flow meters are equipped to compute hydrocarbon density and viscosity based on preconfigured compositions, as well as adapt to variations in liquid content.

Nevertheless, it is acknowledged that the gas composition can evolve with depth and temperature changes in the well. Consequently, it is essential to present the results of an investigation focused on elucidating the impact of compositional changes on the major thermodynamic properties in various gas condensate reservoirs and production scenarios across a spectrum of pressures and temperatures. To effectively forecast reservoir performance using numerical simulators and ensure accurate flow meter readings, it is necessary to understand how compositional variations affect the phase behaviour properties of these hydrocarbons.

TÜV SÜD's APPRoach

We approached the issue of wet-gas flow metering from a reservoir engineering standpoint, with a specific emphasis on the need to re-evaluate the scope and purpose of a permanently installed wet gas meter. Our objective was to examine the uncertainties associated with wet-gas flow measurement using a conventional Venturi meter installed at the surface, considering the alterations in gas composition throughout the lifetime of the gas well.

We systematically quantified the sources of uncertainty inherent in a typical Venturi meter, originating from variations in its geometry and temperature, pressure and differential pressure measurements. Additionally, we explored the impact of fluctuations in the composition of gas condensate reservoirs on the thermodynamic properties of the fluid. These compositional changes could lead to an increased uncertainty of the wet-gas flow meter, particularly over extended periods.

As an initial step in our thermodynamic modelling, we fine-tuned the equation of state (EoS) using data on gas-condensate-fluid compositions, including the parameters for the heavier C7+ fraction. The adjustments to the EoS primarily focused on modifying the density and molecular weight of the heavy fraction, allowing for deviations of up to 10% from experimental values, to better align with saturation-pressure data. Then, after optimisation of the EoS using the available experimental datasets, we have employed the optimised EoS to determine the phase behaviour and generate the phase envelopes of variety of gas condensate compositions.

The phase envelopes for the three distinct gas condensate scenarios are illustrated in the accompanying figure, together with select experimental saturation points. The saturation datasets were instrumental in refining the accuracy of the phase envelope model.

diagram showing gas condensate phase envelope scenario SECTOR IMPACT

The assessment of gas condensate reservoir composition variations has been conducted to analyse their impact on the thermodynamic properties of reservoir fluids. It has been concluded that changes in compositions of gas condensates would result in change in the phase envelope of the associated fluids. Such variations could potentially introduce elevated levels of uncertainty in the wet-gas flow meter situated on the surface over extended periods.

Gas composition and density measurements, as well as assessments of water-cut, present notable measurement challenges. However, an effective wet-gas meter should be capable of accommodating long-term variations in density and composition. Failure to do so would increase the uncertainty of the wet-gas meter during time potentially leading to gross measurement errors.

Furthermore, it is worth noting that mass transfer within the hydrocarbon phase can significantly influence the scaling and corrosion tendencies of brine when water is present in the fluid stream. This interaction of factors underscores the complexity and importance of monitoring and analysing wet-gas flow in the energy industry.

Dr Shima Ghanaatian, CCUS Consultant

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