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EXPLOSION AND IMPLOSION CONSIDERATIONS
Question: How does an explosion initial acoustic pulse compare to an implosion initial acoustic pulse?
An acoustic pulse can be generated at the surface of a well by rapidly adding gas to the casing annulus or by rapidly removing gas from the casing annulus. A gas gun volume chamber can be charged to a pressure in excess of the casing pressure, and this gas can be rapidly released into the casing annulus to generate an acoustic pulse that is a compression wave. An acoustic pulse will also be generated when a volume chamber is attached to the casing annulus, and pressurized gas from the casing annulus is allowed to rapidly discharge (implode) into the lower-pressure volume chamber which will cause a negative (rarefaction) pulse in the casing annulus gas. The casing pressure should be 100 PSI or more to get satisfactory results in the implosion mode.
At low pressures, an external gas supply must be used to pressurize the gas gun volume chamber to a pressure of approximately 100 to 500 PSI in excess of well pressure. Typically, the gas gun volume chambers are 10 to 20 cubic inches in size. Deeper, low-pressure wells require a larger initial acoustic pulse. Larger volume chambers and higher gas pressures in the volume chambers will result in larger liquid level reflections.
Normally, the casing pressure must be 100 PSI or greater before gas can be released from the casing annulus into a volume chamber to create the initial pressure pulse. The valve should be opened rapidly. This negative or rarefaction pulse will travel through the casing annulus gas and reflect from the collars and liquid level and other anomalies just as the compression pulse that was generated by releasing gas into the casing annulus. At higher pressures, the release of gas from the well into the volume chamber or the implosion mode is much preferred because an external gas supply is not required.
The acoustic pulse speed or velocity generated by a compression wave or a rarefaction (or implosion) wave is the same. The velocity of the acoustic pulse is a function of the composition, temperature and pressure of the casing gas. It is not related to the size of the pulse or whether the pulse is a compression or rarefaction wave.