A Review of the Acoustic Determination of Liquid Levels in Gas Wells
- Kent L. Thomas (Phillips Petroleum Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- August 1968
- Document Type
- Journal Paper
- 784 - 785
- 1968. Society of Petroleum Engineers
- 4.6 Natural Gas
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- 139 since 2007
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In a recent article by Andsager and Knapp, a new method was presented foracoustically determining liquid levels in natural gas wells. The methodconsists of generating a sound wave at the wellhead, and measuring the timerequired for the wave to travel to the liquid level and back to the wellhead.The depth of the liquid level then can be obtained by integrating therelationship between velocity, distance and time.
The accurate determination of liquid levels by this technique is necessarilydependent upon one's ability to predict acoustic velocities of natural gas as afunction of both temperature and pressure.
Starting from the equation of the speed of sound in a compressible fluid,
Andsager and Knapp derived the following equation for a real gas.
When Eq. 3 was tested against 42 sets of experimental data, they found thatthe predicted values were too low. The ratio of the predicted to experimentalvelocity in each case was approximately equal to the square root of the gascompressibility factor.
In evaluating Eq. 3 during the integration of Eq. 1, Andsager and Knapp usedgas compressibility factors based on AGA data and specific heats from theEngineering Data Book. In using these specific heats to calculate K, theauthors were assuming K (T , p) to be independent of pressure since thespecific heats listed in this reference were evaluated at atmospheric pressure.It appears, therefore, that for the conditions of the experiments reported intheir paper
Data from Edmister for methane bears out the validity of Eq. 5 over thepressure range shown in Table 1.
Andsager and Knapp should be commended for providing an excellent workingequation for calculating acoustic velocities for the pressure range for whichEq. 5 is valid. Eq. 4, however, should be used with caution at high pressuresor in the neighborhood of the critical point of a gas since the approximationshown as Eq. 5 becomes invalid in these regions.
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