J.N. McCoy & D. Becker, Echometer Co. & A.L. Podio, University of Texas at Austin
A computerized system has been developed for acquisition, processing and analysis of acoustic data used to determine the distance to the liquid level in the casing annulus of a well. The system utilizes a gun-microphone assembly such as has been used in the past. However, the analog signals from the microphone are digitized by a high speed analog to digital converter and stored in a lap-top computer, allowing much greater versatility in signal processing and analysis. The computerized system processes the acoustic digital data to automatically identify the liquid level reflection pulse and counts the number of collar reflections to the liquid level signal. After acquiring the acoustic data, this system first determines the collar reflection signal rate or frequency. Then, the digital signal data between the initial pulse and the liquid level pulse are filtered with a narrowband filter centered at the collar reflection frequency, in order to improve the signal to noise ratio. Using this narrow-band filtering, the collar reflections can be distinguished much further down the wellbore. The program uses a cross-correlation technique to automatically count the number of collar reflections to the liquid level depth is calculated utilizing the number of collar reflections and the average tubing joint length. Digital filtering and signal processing allow the automatic interpretation of the level position in the majority of the field cases, even those where conventional analog recording and broad-band filtering were inadequate for accurate measurements. The system also provides additional processing techniques, under operator control, to obtain accurate results in those wells where conditions are very unfavorable: shallow liquid levels, gaseous liquid columns, noisy well bores, annular constrictions, etc. A number of field cases are presented to illustrate the primary application to optimization of pumping wells. The system also acquires the casing pressure, determines the casing pressure buildup rate, converts it to an equivalent annular gas flow rate and calculates the effective gradient of the gaseous liquid column in the annulus. The annular liquid level depth and casing pressure distribution determine the producing bottom- hole pressure which is combined with a well data base that contains the production rate, static reservoir pressure and other parameters used in the generation of a well performance analysis for the operator.