Sonic surveys can also be conducted down: an annulus (casing) with coiled, endless, or seamless tubing installed; inside coiled tubing; inside casing only; even inside drill pipe. Acoustic velocity methods are then employed for depth calculations. This requires an accurate knowledge of the paper chart speed (cm/s) or travel time (s), and the acoustic velocity (speed-of-sound) in a natural gas (v, m/s). Computerized #AWS units have these calculations coded internally.
For paper charts use a ruler to measure the distance from the first kick to the fluid level kick (cm). Divide this by the chart speed (cm/s) to yield sonic travel time (t, s). Depth (d) is then determined by the formula: d = (t•v)÷2.
If you use an analogue Echometer™ their school notes include paper graphs with acoustic velocities for natural gas relative densities. Alternatively, download their free AWP2000 software from www.echometer.com.
You can back-calculate the acoustic velocity (v) if you can identify kicks that correspond to items of known depth such as a tubing anchor, end-of-tubing or perforations. Use the formula: v = (d•2)÷t.
In simple terms the acoustic velocity formula is: v = SqRoot gcZnRT. Please refer to SPE 2579.