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The Oxygen Factor
It was so simple years ago.
Whenever oil producers had natural gas vapors building up in the top of holding tanks, they simply vented or flared; after all, they were in the oil business – not the gas business. Today those options are becoming less and less available because of environmental regulations, while at the same time the market for that “free” gas is expanding.
The EPA has issued a number of regulations in recent years monitoring the emissions from hydrocarbon production and storage facilities. These regulations, and the fines that accompany them, have led to new innovations in the industry. Recovering those vapors and selling them to gas purchasers offers two solutions at once: regulatory compliance and new profit streams.
Vapor recovery units
This was all made possible by the introduction of vapor recovery units (VRUs). Liquid hydrocarbons emit natural gas vapors, especially when confined in a storage unit. A VRU pulls these vapors from the tank and separates the natural gas, which is pumped to a sales point. The result is a new profit center for the oil company and a cleaner environment.
The environmental impact is a major concern. Over 26 billion cubic feet of methane is lost each year from storage tanks. While CO2 emission reduction gets the most attention, natural gas has more than 20 times the impact as a greenhouse gas. No wonder federal and local governments are closing the window on venting.
The economics of VRUs makes its own case. The equipment is modular and requires minimum site preparation. Reduced emissions result in immediate regulatory savings while the sale of natural gas provides a new revenue stream. Most installations recoup their investment in six to 12 months … then come the profits from high-quality natural gas. In fact, recaptured natural gas has a higher BTU content than normal pipeline-quality gas.
Up to 95 percent of the hydrocarbon vapors can be captured.
While oil storage tanks provide the most common opportunity for recovering vapors, some companies pull methane from water-holding tanks using an advanced oil recovery technique called water flooding. Water is injected back into the ground to float the oil to produce more. When the water comes back up after injection it is stored in a holding tank where the gas vapors can also accumulate.
Heat can also be a factor in pushing methane out of liquids, especially in warm-weather areas such as Texas. Regardless of the tanks being used, oil storage or water storage, the methane vapors provide a new revenue source.
The oxygen risk
But as with many solutions, there is a potential risk. In this case the risk comes in the form of oxygen (O2) entering through the VRU.
“O2 is an absolutely horrible thing to have in any gas stream,” says Johnny Jones, an automated systems manager for Croft Automation, which builds measurement and analytical systems for midstream and transportation facilities. “First of all it is corrosive. Secondly, as gas is put under pressure, if there is O2 present it can cause a chemical reaction. Third, you can’t sell gas if it has an O2 level that is above specs; you are forced to flare the entire stream. Once O2 is in the gas there is no economical way to remove it. The whole stream is now useless.”
How does oxygen get into a system? It turns out there are many ways. The leading culprits are equipment failure and operator errors such as leaving a hatch open. “Any time there is work done on the wellhead or pipes you can get a hit of O2,” claims Jones. The nature of VRUs includes a series of pipe connections and fittings. Air can leak in at any of those points. Add to that the possibility of improperly installed or maintained equipment and you have a formula for leaks.
The natural gas analyzer role
Many oil companies have turned to optical analyzers that utilize a technique called “fluorescence quenching.” This technology has the advantage of quick response time and immunity to contaminants in the stream. In some cases it is quick enough to reveal which VRU is leaking if a network of VRUs is being used at the same site. This is done by identifying the O2 spike to a specific VRU as it comes online.
Contaminants in the stream are always a problem for traditional sensors that rely on contact with the actual gas stream. Any contaminant can cause a traditional sensor to give a false reading, and in the case of H2S it can damage or destroy the sensor. Optical analyzers are immune to even high amounts of H2S.
“Another advantage can come from knowing the nitrogen/oxygen mix in a stream,” adds Sam Miller of analyzer manufacturer SpectraSensors. “That way you can tell if the leak is coming from the air or from somewhere else.”
Vapor recovery units are here to stay. An increasing number of oil producers have learned that it is the most efficient method to cut regulatory fines and make money at the same time. Who would have thought oil men would be in the natural gas business?
The fact is oil wells have always produced oil and natural gas. For most of the history of oil production it was assumed the gas had no value. But regulatory forces and higher gas prices have changed all that.
For oil producers, VRUs have created a new chapter in environmental responsibility and revenue streams. But critical to this new technology is a robust analyzer that ensures safety and process control.