Acoustic instruments have been used routinely for many years as an aid in analyzing well performance of normal-pressure oil producers.1
Recent developments in equipment and techniques now permit more-accurate calculations of acoustic static bottomhole pressures in oil and gas wells at surface pressures up to 15,000 psi in corrosive (CO2 and H2S) environments. The gas gravity can be determined from acoustic velocity data instead of using a surface gas gravity measurement instrument. Equations and charts are presented herein for determining static bottomhole pressures from acoustic and well data. Also, a special technique is recommended for shutting-in a well which in most cases will yield more-accurate results.
This method has been programmed for an inexpensive, portable notebook-size computer which can be used in the field to easily perform these calculations. Software is also available for the IBM PC.
The liquid level in a well may be determined acoustically by generating a pressure pulse at the surface and recording the ethos from collars, obstructions, and liquid level.
A blank cartridge was the conventional source of pressure pulse until development of the modern gas gun. On wells having less than 100 psi, the gas gun volume chamber is pressurized to approximately 100 psi in excess of well pressure. On wells having pressures in excess of 100 psi, the volume chamber in the gas gun is bled to a pressure less than the well pressure. Then, a valve is rapidly opened (to permit wellhead pressure to expand into the volume chamber and create a rarefaction pressure wave.
A microphone converts the pressure pulses reflected by collars, liquid, and other obstructions (or changes in area) into electrical signals which are amplified, filtered, and recorded on a strip chart (Fig. 1). The liquid level depth can be determined by counting the number of tubing collars to the liquid-level reflection.
Changes in cross-section area are also recorded. When these changes are known, they can be used as depth references to determine liquid-level depth. Also, the distance to the liquid level can be calculated by travel time from the acoustic chart and acoustic-velocity data. Acoustic measurements were generally obtained by "shooting" down the asing/tubing annulus in packerless completions (Fig. 1). However, equipment has been recently developed for shooting down the inside of close makeup (flush) tubing at high pressures.
In most of the deeper, high-pressure gas wells, the casing/tubing annulus is isolated from. the production string by a packer. Thus, a static bottomhole pressure must be obtained by shooting down the inside of the tubing. Weeks2 discusses a high-pressure gas gun (Fig. 2) which will operate up to 15,000 psi and can be operated through needle valves already installed on the well. This gun utilizes gas in the well to generate the initial pressure pulse. Neither an external gas supply nor a "blank" is necessary and the gun is suitable for walls having high concentrations of H2S and CO2.
The static: bottomhole pressure is the sum of surface pressure, gas column pressure, and liquid column pressure.