Determination of Fluid Level in Oil Wells bv the Pressure-wave Echo Method

Determining the distance to remote objects by observing the time requiredfor sound to traverse the intervening space is an old practice. Attempts havebeen made to use this method for determining the fluid level in oil wells butas far as the writer is aware it was not found to be practicable, becausevibrations in gas in the audible frequency range do not travel as well and asfar as low-frequency pressure waves, which are capable of transmitting agreater amount of energy than can be transmitted by audible waves. Wood, in hisbook "Sound," states that in a given medium the range of soundtransmission will vary inversely as the square of the frequency. The fact thatthe sound from a whistle having a low tone has a much lower attenuation ratethan that from a whistle of high pitch is well known. Also, the pressure wavescaused by cannon fire often rattle windows at points so distant that no soundof the cannon is heard. In 1932 P. E. Lehr and H. T. Wyatt, of the Shell Oil Co., experimented with theprinciple of the concussion, or pressure-wave echo, method of determining fluidlevels in oil wells. They released compressed gas into the top of the casing, thereby creating a pressure wave, which traveled down the well and back again.Their experiments proved conclusively that, with the properapparatus forrecording the echoes and determining the velocities at which the waves travel, the method could be used successfully for determining fluid levels in oilwells. Wells are often equipped with from one to three tubing catchers andthese together with the top of the liner are frequently above the fluid. Eachof these obstructions reflects a part of the pressure wave. In such wells theremay be, therefore, as many as five pressure-wave reflections. Thus it isobvious that in order to identify the various reflections and to determinethere from the fluid level in a well it is necessary to obtain a graphic recordof these reflections.