Page 12 - Acoustic Fluid Level Measurements

Methods of Determining Distance to the Liquid Level 5-1

methods to obtain the distance using acoustic velocity University of Texas at Austin

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Methods for Determining Distance to the Liquid Level

In this chapter:
• Best methods to obtain accurate results of fluid level depth for various well configurations
• Converting time record to distance
• Identifying collar echoes and echoes from wellbore discontinuities
• Determining acoustic velocity from gas properties
• Correlations and equations of state
• Calculating velocity from acoustic records in similar wells or past surveys

This chapter presents some recommendations for obtain- v = average acoustic velocity of gas between
ing the most accurate estimates of the distance to the the source and the reflector (ft/s)
liquid level from acoustic surveys. A number of different Δ t = round trip travel time (seconds)
Petroleum Extension-The
As discussed in chapter 3, the acoustic velocity is a
are explained, from using collar echoes, tubing joints, or function of pressure, temperature, and composition of the
collar count to past acoustic surveys in the same region. gas. Consequently, it differs from well to well and also at
various points in a given well because of the increase of
CONVERTING ACOUSTIC PULSE TRAVEL pressure and temperature as a function of depth and the
TIME TO DISTANCE possible stratification of the gas column due to the differ-
ence in density of the various hydrocarbon components,
Echo signals are registered as the time required for the especially when gas is not being produced from the cas-
sound to travel from the pulse generator (gas gun) to the ing annulus.
wellbore cross-sectional area change (anomaly) and back For fluid level surveys in real wells, the following
to the microphone housed in the gas gun. This time is four methods are used to determine the average acoustic
known as the round trip travel time (RTTT) and, generally velocity:
speaking, is measured with an accuracy of ±1 millisecond. • Determination using identification and counting
The conversion of travel time to the actual distance of echoes from tubing or casing collars
from the microphone to the anomaly can be made using • Determination using the distance to a known
equation 5.1, if the average acoustic velocity for the gas anomaly in the wellbore
present in the wellbore between the gun and the anomaly • Calculation from gas gravity or composition
can be determined: • Estimation based on experience or previous
vΔt measurements
D = —– Eq. 5.1
2 All of these four methods involve varying degrees
where: of uncertainty, but generally, it is considered that the
D = distance between the sound source and the first two (the collar count and anomaly methods) yield
reflector (feet) the best estimates of the distance to the liquid level.

For distribution by Petroleum Extension-The University of Texas at Austin
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