Acoustic liquid level tests are performed successfully in many different applications throughout the world. Advanced techniques for acoustic liquid level analysis are required for wells where unusual conditions exist such as very shallow liquid levels, annular partial obstructions, flush pipe, short tubing joints, etc. Some wells have liners, upper perforations, paraffin, odd length of tubing joints, poor surface connections and other conditions which result in an acoustic trace that may be very difficult to interpret. Normally, the computer software locates the liquid level and automatically processes collar reflections to accurately count almost all of the collars from the initial blast to the liquid level. This automatic analysis will determine the liquid level depth for 95% of the wells. However, some wells have conditions or anomalies that these procedures will not function as desired. This paper describes special advanced techniques that can be used to determine the liquid level in wells with these unusual conditions.
The most common application of an acoustic liquid level instrument is to measure the distance to the liquid level in the casing annulus of a well. A single test is performed on a well to determine the producing bottomhole pressure. The acoustic signal is digitized and stored in the computer. The computer processes this digitized acoustic data to accent collar reflections. The Total Well Management, TWM, software program automatically counts the number of collar reflections from the surface to the liquid level and determines the liquid level depth. Simultaneously, the casing pressure is acquired. If gas is flowing up the casing annulus, the casing pressure will increase because the casing valves are closed during an acoustic liquid level depth measurement. This buildup in casing pressure is utilized along with well data to determine the casing annulus gas flow rate. The casing annulus gas flow rate is utilized to calculate a gradient of the gaseous liquid column above the pump, if present. Thus, the producing bottomhole pressure is determined from an analysis of the acquired data. The producing bottomhole pressure and reservoir pressure are processed using the Vogel IPR analysis to present the operator with the producing rate efficiency and the maximum production rate of the well.
The acoustic instrument can also be applied to depth measurements inside tubing or other piping. Other applications include determination of the distance to the mud or kill liquid level during drilling and work-overs. The liquid level in a gas lift well can be determined. The bottomhole pressures in wells with extremely high surface pressures can be determined. The acoustic instruments can be used to measure the distance to any change in cross-sectional area inside pipe or in the annulus.
The following sections describe the special techniques for acoustic liquid level determination. In most cases, once an acoustic trace has been obtained and the liquid level signal selected, the number of tubing collar reflections from the surface to the liquid level are counted in order to calculate its depth. The corresponding number of tubing joints, multiplied by the average joint length yields the distance to the liquid level.