Compositional tracking

The separator production data and fluid sampling data are used to predict daily values for:

• Wellstream Compositions
• Mole Rates
• Mass Rates
• Saturation Pressure and Type of Producing Wellstream

Wellstream Compositions

Compositions define the relative amounts of different components that make up a fluid. A wellstream composition represents the composition that a well produces at one point in time. EOS models enable convenient and flexible calculations for describing phase behavior of petroleum fluids. However, to use an EOS model, a composition is needed in addition to pressure and temperature. Temperature and pressure are almost always available, but measured wellstream compositions are typically not available every day. The purpose of this module is to make wellstream compositions readily available for every single well, every single day. The wellstream composition is calculated as described here.

Why do I need Wellstream Compositions over Time?

There are several reasons why engineers would like to know variations in wellstream compositions over time. Wellstream compositions can be used to, for example:

1. Indicate when the well is: (i) producing at BHPs below the in-situ saturation pressure, (ii) producing from several layers with different in-situ fluid compositions, or (iii) experiencing gas coning in the perforated interval (most relevant for conventional reservoirs),
2. Assist in history matching and production performance forecasting,
3. Compare well-to-well production behavior throughout a field in a consistent, “surface-process insensitive” manner – indicating differences in well performance and/or in-situ fluid spatial variations,
4. Allocate oil and gas rates and/or components to individual wells,
5. Assist in condensate tracking – study relative contributions from a condensate gas cap,
6. Assist in fluid initialization and well classification exercises,
7. Understand compositionally sensitive processes such as gas enhanced oil recovery,
8. Study the sensitivity of surface processes on rates, liquid yields and GORs,
9. Perform separator train optimization,
10. Normalize for changing separator conditions. Separator conditions might vary considerably over time, hence, part of the GOR variation seen over time is due to changing separator conditions. By knowing wellstream compositions over time, one can express GOR in terms of a fixed surface process and remove “noise” in production data (“GOR normalization”),
11. Assist in facility design.

Mole Rates

Molar rates, i.e. the total number of moles (\(n\)) produced every day, are calculated as:

$$\begin{equation} \label{eq:mole-rates} n = n_o + n_g = \frac{q_{om}}{v_o} + \frac{q_{gm}}{v_g} \end{equation}$$

where \(q_{om}\) is the measured oil rate, \(v_{o}\) is the oil molar volume, \(q_{gm}\) is the measured gas rate, and \(v_{g}\) is the gas molar volume.

Gas rates, even when measured at separator conditions, are reported at standard conditions; hence we can use the ideal gas law for the molar volume of the separator gas. The molar volume of the separator oil must be calculated using an EOS calculation.

Mass Rates

Mass rates, i.e. total mass (\(m\)) produced every day, are calculated as:

$$\begin{equation} m = MW \cdot n \end{equation}$$

where \(MW\) is the average molecular weight of the wellstream and \(n\) is the mole rate.

Saturation Pressure & Type

If the wellstream composition changes over time, the saturation pressure of the producing wellstream will also change. The saturation pressure type (bubblepoint or dewpoint) might also change depending on the fluid system. For a specific wellstream composition and reservoir temperature, the producing fluid composition is calculated every day.

Want to know more? Here is a presentation on compositional tracking.