The economic due diligence that determines the price to pay for a gas property can be sensitive to the time value of money. This calculation is therefore intimately linked to predicting flow rates. With the invention of highly sensitive technology, gas-phase viscosities can now be measured accurately. This enables the investigation of previously ignored effects such as the influence of equilibrium water vapor on gas-well deliverability.

This paper discusses the technology of gas-phase viscosity measurement and then compares existing literature data. The paper then presents, by means of a case-history, the influence of water of condensation on viscosity and concomitantly on production rates. Moreover, material-balance calculations are provided that show the role of water vapor in gas properties evaluation.

It is concluded that calculated gas flow rates can be significantly impacted by incorporating the influence of equilibrium water vapor whereas the influence of water vapor on gas in place is minimal.


Gas-phase properties measurement has been the subject of much research for many years. Lee and Eakin, Eakin and Ellington, Giddings and Kobayashi and Saeedi and Rowe1–4 report early gas-phase measurements of viscosity and density along with correlations for each. Each of these references was at conditions less than 10,000 psi and without water content. Lee5 assembled a large body of gas-phase data that have been very useful for practitioners in the oil industry for many years. The Gas Processors data book6 has also been widely used but the limits of application have needed to be updated in light of much higher pressure reservoirs. Moreover, the measurements of rheological properties in the presence of water have not been made.

Due to the paucity of density and viscosity data available at high pressures and the influence of equilibrium water on these parameters, testing was initiated at Hycal Energy Research Laboratories Ltd. to investigate these parameters.


Cambridge Applied Systems developed a viscometer based on measuring the resistance to movement of a piston when in the presence of a magnetic field. Figure 1 shows a schematic of the apparatus. The drive level of the magnet is adjusted to where the magnetic field causes movement of the piston through the fluid in the viscometer and the time of travel is recorded. The greater the difference between the travel time for the highest expected viscosities and the lower viscosities, the more sensitive the viscosity measurements will be. Thus, the recommended calibration protocol is to minimize the drive level and then measure the travel time and calibrate travel time to viscosity based upon some known viscositystandards.

The equations that govern the calibration are shown as Equations 1, 2 and 3. Equation 1 is a simple linear expression relating travel time to viscosity, whereas Equations 2 and 3 are quadratic expressions defining the dependence of α?and ®?on travel time.

(Equation (1) is available in full of paper) (Equation (2) is available in full of paper) (Equation (3) is available in full of paper) (Equation (4) is available in full of paper)

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