PRODUCTION PROBLEMS 



1129 



higher frequency harmonics are recorded. UnHke other low frequency 



recorders, this instrument has an exceptionally high and rapid response 



to a steep front wave like that from the fluid reflection. A view of the 

 complete equipment is shown in Figure 700. 



Operation of Wave Reflection Equipment. — The Reflecto- 



grain recorder is installed in a field car and when measurements are 

 to be made, the car is positioned near the well and connected to the 

 microphone by a shielded cable. The casing-head unit is connected 

 to the well by a two-inch union and a standard gate or block valve. 

 The distance between the casing-head and the casing unit should be 

 as short as possible to minimize loss of reflection energy. The sound 

 wave utilized to make these measurements is created by a specially 

 loaded Reflectogram cartridge, which produces a low-frequency, 

 high-energy sound pulse. When the shell is fired, the pulse enters 

 the casing-head and travels down the annular space between the 

 tubing and casing, being gradually dissipated and partially reflected 

 by the various obstructions in the annulus. Small reflections are 

 produced by tubing collars ; stronger reflections are received from 

 such larger obstructions as tubing catchers and liner tops, while the 

 fluid surface reflects practically all of the wave energy impinging 

 upon it, and gives a correspondingly greater reflection on the record. 

 Production of oil and gas need not be halted during the tests. The 

 unit is designed to withstand high pressures. 



Calculating the Depth to Fluid 



In most wells, each tubing collar between the casing head and 

 the fluid will be shown as a small reflection on the tape record. The 

 fluid level will be shown by a larger reflection. The depth to the 

 fluid may therefore be determined by simply counting the number 

 (to the nearest 0.5) of tubing collar reflections between the shot and 

 the fluid-level reflection, and then multiplying this count by the 

 average length of tubing. This method is shown in Figure 701, 

 where 91.7 tubing-collar reflections exist between the shot and the 

 fluid. Since the average length of tubing in this well is 30.1 feet, 

 the depth to fluid is 30.1 • 91.7 = 2760 feet. 



Occasionally, wells will be encountered with small diameter 

 casing or other unfavorable conditions which impede the travel of 

 the sound wave down the annulus. In such wells, the tubing-collar 

 reflections may be weak and may therefore not be recorded entirely 

 to the fluid level. The depths may then be determined by simple 

 proportion. The procedure is to count the number of tubing-collar 

 reflections to a given point as far down the record as possible, then 

 calculating the depth of that point by multiplying the number of 

 tubing-collar reflections by the average length of tubing. The depth 

 to fluid will then be found by the simple proportion 



Depth to fluid 

 Depth to point 



If 

 S, 



Fig. 701.— Typi- 

 cal reflection rec- 

 ord without filter 

 showing shot-point, 

 tubing-collar re- 

 flections, and fluid 

 reflections. 



where Sf is the length of record between the shot and the fluid 



reflection, and Sp is the length between the shot and the point of 



calculation. In like manner use may be made of the reflections from 



liner-top, tubing catcher, or any other partial obstruction whose accurate location or 



depth is known from the well records. Successful work has been done in many wells 



exceeding 8,000 feet in depth. 



