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What are the considerations for use of the acoustic liquid level instruments in a hazardous environment?
Acoustic Pulse Generation with Manually Fired Gas Guns
The 5000-PSI gas gun and the Compact Gas Gun are routinely used in hazardous locations. The acoustic pulse is generated by either the release of gas from the gas gun volume chamber into the well or by the release of gas from the well into the gas gun volume chamber. When gas expands, the gas cools. So, a hot gas mixture is not introduced into the well gas. The gas gun pulse generation is intrinsically safe.
The 5000-PSI and Compact Gas Guns utilize a microphone to convert the acoustic signal from downhole into an electrical signal that is fed by coaxial cable to a recording instrument. The microphone is a piezoelectric crystal that has a capacitance of approximately 40 nanofarads. A low voltage (33V) semiconductor varistor across the microphone limits the energy output of the microphone to a very low level. The electrical signal from the microphone is transferred by coaxial cable to the recording instrument.
Some older liquid level guns utilized a black powder blank that generated an acoustic pulse. The detonation of black powder creates a high temperature explosion, which could detonate a hazardous mixture. The blank guns are not recommended for use in hazardous locations. Gas guns are preferred to blank guns for almost all operations.
Acoustic Pulse Generation using Remotely Fired Gas Gun
The remote fire gas gun utilizes a 12-volt solenoid for opening the gas valve. The 12-volt power source is supplied from the battery of the Well Analyzer or Model M strip chart recorder. While this configuration has never been reported to cause an accident or problem, the 12-volt power source is not suitable for use in hazardous conditions. The remote fire gas gun solenoid is operated by 12-volts and uses 1 amp.
Data Acquisition and Recording Unit
The recording instrument has an input circuitry that includes a 2M input resistor. The 2M resistor limits the current from the electronics system to the coaxial cable that attaches to the wellhead microphone. The leakage current would be less than 0.1ua.
A 12-volt 2.5 amp-hour lead cell battery is contained within the electronic instrument to power the electronics. The Well Analyzer contains a battery for the computer. An automatic-reset 1.5 amp fuse can be installed inside the battery pack to limit the current output of the battery. This special battery is available at an additional cost if desired.
On offshore platforms and other hazardous locations, the electronic instrument is commonly located 50 to 100 feet from the wellhead. A “hot” permit may be required in some operations. Long cables are available from Echometer Company.
Pressure Data Acquisition
If the Well Analyzer is used, the casing pressure can be automatically measured with a pressure transducer that is attached to the 5000-PSI Gas Gun or the Compact Gas Gun and thus be located at the wellhead also. The pressure transducer requires a low current power supply of 4 to 9 volts depending upon the particular Well Analyzer. The Well Analyzer supplies this power to the pressure transducer through the same cable that carries the pressure transducer output signal from the pressure transducer to the Well Analyzer. No problems have been reported by use of the pressure gauge at the wellhead, but the operator has the option to manually measure the well pressure with a conventional non-electronic gauge.