Recommendations for Optimum Performance
The Echometer wellhead should be as near as possible to the casing annulus (or the tubing) preferably within 5
feet. Short (5-10 ft) lengths of pipe can mask the desired downhole signals. Longer (20-60 ft) lengths will generate
multiple reflections, which are hard to distinguish from collar reflections. Use a minimum of 90
0
ells and tees and
direct the blast straight into the well if possible. Two-inch connections are recommended, but one inch
connections are generally satisfactory if the length of 1 inch pipe is kept to a minimum.
Proper sensitivity setting is very important. Select the AUTO gain setting for the first shot. The background noise
level is indicated on the chart. Surface vibrations, leaking gas connections, gas “popping” out of the gas/liquid
interface and other unstable conditions, cause this noise. This background noise is not a part of the signals when
the pressure wave is generated. The instrument will automatically record the background noise at a low level and
larger collar and liquid level signals will be recorded at larger amplitudes, which simplifies the interpretation of the
chart. When operating in the AUTO mode, adjusting the sensitivity after the chart drive is tuned ON does not
change the automatic gain selection made by the electronics and software.
The pressure pulse travels down the well and is reflected by tubing collars and the liquid level. The signals from
upper collar reflections are strong, but the collar response becomes weaker as the pressure pulse travels long
distances to the bottom of the well so that the reflections from the lower collars may be weaker than the
background noise. The liquid level reflection varies from a strong signal in a shallow high pressure well to a very
weak signal in a deep low pressure well. Only signals stronger than the background noise are meaningful in the
recording. It is obvious that strong background noises must be reduced if the recording of lower collars and liquid
level is to be obtained.
Background noise can be classified as surface mechanical vibration noise or acoustic noise. The source of noise
can be determined easily by increasing the sensitivity until signal deflection is obtained. Closing the casing valve
between the microphone and the annulus will cause a reduction in the noise level if its source is acoustic noise. If
the signal level remains the same, then the noise is caused either by surface vibrations or by gas leakage from
extraneous lines connected on the same side of the closed casing valve as the microphone. The microphone is
shock mounted, but if the wellhead attachment vibrates excessively, unwanted signals are generated. Wellhead
vibration result from running gas engines, chattering check valves and other reciprocating surface equipment. It
may be necessary to eliminate wellhead vibrations to obtain better quality records in deep low-pressure wells. All
other lines leading to the casing annulus should be closed.
The main source of down-hole acoustic noise is gas "popping" out of a gaseous annular liquid column or liquid
falling into the wellbore. Downhole noise can also result from tubing and casing leaks. Generally, the down-hole
acoustic noise can be reduced in relation to the desired reflected signals by causing an increase in the casing
pressure. In order to do this, continue to pump the well with the casing vent valve closed. At low pressures, an
increase of 10-psi in the casing pressure almost always improves the record and it only depresses the liquid level by
30 feet.
If the signal from the liquid level is not detected due to excessive surface vibration noise or down-hole acoustic
noise, a larger signal from the liquid level can be obtained by generating a larger initial pressure pulse. Also,
increasing the sensitivity so that the background noise level exceeds 1/8 inch, generally will make interpretation
much more difficult and is not recommended.
