Paper: Improved Analysis of Acoustic Liquid Level Depth Measurements Using A Dual Channel AnalogDigital Strip Chart Recorder

A new acoustic instrument has been developed for measuring the distance to the liquid level in the casing annulus of a well. The instrument features modem analog/digital technology to acquire and record acoustic reflections on a strip chart. The dual channel instrument accents the liquid level on a low frequency channel. Collars are accented and recorded on a second channel using automatic gain control to ease the counting of the number of tubing collars from the surface to the liquid level. The collar amplifier/filter response can be selected to accent sharp upper collar reflections or to accent lower collars in a deep, low-pressure well. Another feature of this instrument is the use of an automatic mode for selection of gain on both channels. The instrument automatically acquires and processes acoustic noise data before generation of the acoustic pulse and preselects the proper gain. In the automatic mode, the operator must turn on the amplifier power and the chart drive. These are the only instrument functions required for most tests. The sensitivity controls are not adjusted by the operator unless special recording is desired. Analysis forms are printed on the strip for entry of liquid level depth, casing pressure and casing pressure buildup rate. Software is supplied for use with a separate computer to determine bottomhole pressures even in wells which have gaseous liquid columns. The operator inputs the proper well parameters into the software and the program calculates the producing bottomhole pressure, the formation producing rate efficiency and the maximum production capability of the well. The results of the software analysis are manually entered in an additional form printed on the strip chart which serves as a permanent record and analysis of the acoustic test. A microprocessor, clock and timing circuit records a time and date stamp on each record to ease bookkeeping. The time between tests can be determined from these time stamps. If the casing pressure is measured during each test, the casing pressure buildup rate can be determined which aids in the calculation of casing annulus gas flow rate and producing bottomhole pressures. Additionally, one second time marks are placed on the strip chart which aid in chart analysis and the calculation of the gas specific gravity. This new system improves the ability of the operator to determine an accurate liquid level depth and analyze the liquid level depth with casing pressure data to obtain the bottomhole pressure and perform a better well performance analysis.