Gas Considerations

What are the uses of the pressure regulator?

What are the pros and cons or using a hydrocarbon gas from a nearby well for generating the initial pulse?

What gas should be used for pressure transient testing?

What gas cylinders are available for charging the gas guns?

This document discusses the various gas pulse sources that are available for determining acoustic liquid level depth in a well. An acoustic pulse is generated at the surface of the well that travels through the casing annulus gas and reflects from all anomalies including tubing collars and the liquid level. The reflected acoustic pulses are converted to an electrical signal by a microphone. The electrical signal is either recorded on a strip chart or digitized and stored in a computer for software processing.

Carbon dioxide (CO2) gas, nitrogen (N2) gas, well gas, and sometimes, hydrocarbon gas from a nearby well are used to generate the initial acoustic pulse. Normally, a gas gun volume chamber is charged to a pressure in excess of well pressure. The gas is suddenly released into the well to generate an acoustic pulse. This is an explosion technique that results in a compression wave. Also, an implosion technique can be used where high-pressure gas from the well is rapidly released into a lower-pressure volume chamber. This creates a negative (rarefaction) pressure pulse that travels through the gas and reflects from anomalies, as does the positive or compression pulse. Both compression and rarefaction pulses travel at the same velocity in the same gas.

The remote fire gas gun operates in the explosion mode only. The compact gas gun and the 5,000 PSI gas gun operate in the explosion or implosion mode. The 15,000 PSI gas gun operates in the implosion mode only.

Carbon dioxide gas is the most common type of gas source for most single shot acoustic well tests. Below 88° F, a full CO2 cylinder will contain both CO2 gas and CO2 liquid. The gas pressure above the CO2 liquid is a function of temperature:

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The pressure will be 1072 PSI at 88°F, 650 PSI at 50°F, 300 PSI at 0°F and 100 PSI at -57°F. The CO2 pressure in the cylinder will be above 1050 PSI when the CO2 temperature is above 88°F and the cylinder is full. 

A standard 5 lb. aluminum CO2 cylinder has dimensions of 5.25" O.D. X 15" long. The full cylinder contains 44 cubic feet of CO2 gas at standard conditions. The cylinder has a service pressure of 1800 PSI and a test pressure of 3000 PSI. The volume of the remote fire gas gun chamber is 12.5 cubic inches. A full 5 lb. CO2 cylinder will charge the remote fire gas gun to a pressure of 200 PSI for approximately 500 tests. The cylinder is equipped with a USA industry standard CGA-320 valve with 0.965-14 NGO R.H. female threads. A ¼ " hose and quick connector are supplied with the 5 lb. CO2 cylinder to charge the gas guns.

Nitrogen gas can also be used to charge the gas gun volume chambers. Nitrogen gas is commonly available throughout the world and is normally supplied in cylinders at a pressure of approximately 2200 PSI. Echometer Company offers a portable N22 aluminum cylinder that contains 20.8 cubic feet of N2 gas at a service pressure rating of 2216 PSI. The cylinder length is 17" and the O.D. is 5.25". The cylinder weighs 9 lbs. or 4.1 Kg. This cylinder will charge the remote fire gas gun to 200 PSI for 200 tests. A larger N88 aluminum cylinder weighs 30 lbs. with a length of 33" and an O.D. of 7.25". The N88 cylinder holds 83.3 cubic feet of N2 gas at a service pressure of 2216 PSI. This cylinder will charge the remote fire gas gun to 200 PSI for 800 tests. Also, the N88 cylinder can be used to supply gas for pressure build-up testing where the Well Analyzer is left unattended. The cylinder should be used with a pressure regulator to obtain a reasonable number of shots during the pressure build-up test. The pressure regulator is used to control the maximum outlet gas pressure to 1500 PSI or to the maximum pressure required on the gas gun during the pressure transient test. This regulator conserves N2 gas. This cylinder will supply gas for one pressure transient test containing 120 data points with an initial well surface pressure of 200 PSI and a final well surface pressure of 1,000 PSI. Both cylinders have a service pressure rating of 2216 PSI and a test pressure of 3694 PSI. Both cylinders are equipped with a USA industry standard CGA-580 valve that has 0.965-14 NGO R.H. internal threads.

The CO2, N22 or the N88 cylinders can be used to store high-pressure hydrocarbon gas from a well or a pressurized hydrocarbon gas source if CO2 or N2 is not available. The gas gun volume chamber can be charged with the hydrocarbon gas if the use of hydrocarbon gas is required. Hydrocarbon gas from a nearby well will contain water vapor which will corrode the inside of the cylinder. Do not exceed the pressure rating of the cylinder when filling the cylinder. The maximum pressure in the cylinder should be limited to the pressure rating of the gas gun or to the service pressure of the cylinder or to the pressure rating of the hose and fitting, whichever is less. A ¼" hose and fitting are supplied for the cylinders.

For pressure transient testing, nitrogen gas is recommended. The pressure in the nitrogen cylinder and the gas gun volume chamber must exceed the well pressure in order to obtain an acoustic test. Carbon dioxide gas is not commonly used because the CO2 gas pressure in the cylinder may not exceed the well pressure during the later portion of the pressure transient test if the outside temperature is low.

A 5 lb. CO2 aluminum cylinder with valve, hose and connector are supplied standard. The customer may desire the N22 cylinder instead of the CO2 cylinder at an extra cost. The N88 cylinder can be supplied instead of the 5 lb. CO2 cylinder at an extra cost. The pressure regulator is also available at an extra cost.  Contact Echometer for details.

Note that various combinations of CO2 gas, N2 gas and hydrocarbon gas with various gas guns can be used to generate an acoustic pulse to measure liquid level depth. Always be certain that the pressure within the cylinder is less than the service pressure of the cylinder and is also less than the ¼" hose and fitting ratings as well as the gas gun ratings. The compact gas gun has a maximum pressure rating of 1,500 PSI. The maximum pressure rating of the remote fire gas gun is 2,000 PSI. The maximum pressure rating of the ¼" hose and fitting to charge the compact gas gun or the remote fire gas gun is 2,000 PSI. A quick connect for the 5,000 PSI gas gun has a pressure rating of 3,000 PSI.

Hydrocarbon gas is a hazardous explosive material. If a cylinder is filled with hydrocarbon gas, the cylinder should be considered hazardous. Be extremely careful carrying this cylinder in a car or storing in an enclosed environment. If the container should leak the hydrocarbon gas, an explosive mixture could result which would result in hazardous conditions. Also, if hydrocarbon gas from a well is utilized, the hydrocarbon gas will probably contain water vapor that will corrode the inside of the cylinder. The equipment should be checked and tested more often if used with hydrocarbon gas containing water vapor. Always, operate with safe equipment.

Blank guns that utilize hazardous black powder were formerly used to generate the initial acoustic pulse. Some violent explosions have occurred with the use of black powder blanks that resulted in damage to the well and to the environment. Do not allow air (containing oxygen) to enter the casing annulus during workover or during any operation, and then use black powder blanks to measure liquid level depth. The use of black powder blanks is not recommended except when required.