Computerized acoustic pressure transient data acquisition was first introducedto the industry in 1987. Since this time many advances have been made to boththe software and hardware components of the data acquisition system, Theseadvances have simplified the acquisition of pressure transient data andincreased the accuracy of the results. By using the latest in acoustic builduptechnology, an operator can obtain the data necessary to optimize recovery atan economical cost. The use of acoustic methods to acquire pressure transientdata also eliminates the need to trip the rods and pump out of the tubing inorder to run a wireline onveyed pressure recorder.
This paper describes the advances made in the equipment. These advances providefor increased accuracy in the determination of liquid level depth and surfacepressure measurements regardless of background noise, temperature or otherenvironmental effects. This results in increased accuracy of calculatedbottomhole pressures.
Improvements in acoustic pressure transient data acquisition equipment haveimproved the accuracy of calculated bottombole pressures, Accuracy ofacoustically determined bottombole pressures are on the same magnitude asbourdon tube type downhole pressure gauges, The use of conventional downholegauges requires pulling the rod and pump on a rod pumped well in order to runthe pressure bomb, while the use of acoustic methods simply requires measuringsurface pressure and determining the distance to the liquid level in order tocalculate the bottomhole pressure. Acoustic bottomhole pressure determinationalso has advantages in deviated wellbore and sour environments where it may notbe practical to run gauges into the wellbore. Acoustic determination ofbottomhole pressure provides a cost effective means of obtaining pressuretransient data to calculate reservoir properties such as formationpermeabilities, skin effect and reservoir pressure1,2.
The liquid level in a well may be determined acoustically by generating apressure pulse at the surface and recording the reflections of this pulse fromtubing collars and the liquid level. This method to determine the liquid levelhas been in use since the 1930's when a blank cartridge was the conventionalsource of the acoustic pulse, The modem gas gun was first developed in the1970's and provides a safer, less expensive means to determine liquidlevels.
Both the blank cartridge gun and gas guns use a microphone to convert thereflected pressure pulses into electrical signals these electrical signals areprocessed and recorded on either a paper strip chart by an analog recordingdevice or are stored digitally by computerized equipment. The liquid leveldepth can be determined by counting the number of tubing collar reflections tothe liquid level reflection. The liquid level can also be determined using theacoustic velocity and travel time of the acoustic pulse.
The bottomhole pressure is the sum of the surface pressure, the gas column pressure and the liquid column pressure3,4,5. Theprinciple uncertainty generally arises from errors In the calculation of theliquid column pressure.