Page 11 - Acoustic Fluid Level Measurements

Acoustic Fluid Level Equipment and Procedures 4-1

Petroleum Extension-The University of Texas at Austin

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Acoustic Fluid Level Equipment and Procedures

In this chapter:
• Requirements for state-of-the-art equipment used to acquire and analyze acoustic records
• Background history of equipment and patents
• State-of-the-art equipment used in the field
• Recommended practices for using acoustic fluid level equipment

This chapter presents the specialized equipment and Low-frequency waves propagate with less at-
procedures necessary for acquiring acoustic records. The tenuation than high-frequency waves. Thus,
objective of this chapter is to outline the history, practi- the pulse should have a slow rise time and long
cal application, and complexities of generating a viable wavelength to obtain distinct echoes from deep
acoustic pulse, as well as building microphones that detect wells. The pulse should have a spectrum shifted
the pressure pulse and signal processing equipment that toward low frequencies (1 to 10 Hz).
records and displays acoustic signals. Also presented are • Clearly defined echoes from discontinuities of
best practices and recommended operating procedures cross-sectional area require a pulse of minimum
for installing the sound source, preparing the well, and duration in time with a short wavelength and fast
acquiring and recording an optimum, high-quality acoustic rise time. Thus, the pulse should have a spectrum
signal with minimal interference. with high-frequency content (20 to 80 Hz).
Thus, the designer is faced with the problem of
ACOUSTIC PULSE GENERATION AND creating a pulse generation system that satisfies both
SIGNAL ACQUISITION objectives, which is very difficult in practice. As a
The characteristics of the acoustic pulse used in echo- consequence, some systems emphasize low frequencies
metric surveys of oil and gas wells are described in to provide high-amplitude echoes from deep reflectors,
chapter 3. Acoustic pulses need sufficient amplitude and while other systems stress high frequencies to achieve
appropriate frequency content in order to generate clear better definition of echoes from shallow- and medium-
and distinct echoes from the fluid level and all other depth wellbore discontinuities.
cross-sectional area discontinuities in the wellbore over This problem is also addressed through signal
distances from a few hundred to several thousand feet. processing techniques (filtering and variable gain) ap-
Designing a pulse generation and recording system that plied to the received signal, either in real time or by
satisfies these requirements has to take into account the post-processing, to enhance the quality of the displayed
following two opposing characteristics of sound propa- record and thus facilitate the analysis.
gation and reflection that were discussed in chapter 3: The pressure of the gas in the well has a major impact
• Acoustic pulse attenuation increases as the on received signal quality since it affects the attenuation
square of the frequency content of the pulse. of the pulse, causing less attenuation in high-pressure

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4-1