What is Natural Gas?
Natural Gas is a fossil fuel energy source that was formed millions of years ago from the remains of plant and animals. These materials were buried deep within the earth and exposed to high pressures and temperatures. This environment caused the carbon and hydrogen-rich materials to transform into coal, oil and natural gas. Natural gas is primarily methane (CH4), but when extracted from the ground, it will often contain other gases such as ethane, butane and propane. It will also have trace amounts of undesirable gases such as hydrogen sulfide (H2S).
Where Is Natural Gas Found?
Natural gas is found in a variety of sub-surface geological formations. Conventional natural gas is found in cracks or voids below overlaying rock. Conventional gas can be found within petroleum reservoirs and is a byproduct of the extraction of the crude oil. This type of gas is referred to as ‘associated gas’ or ‘wet gas’. Conventional natural gas can also be found in reservoirs that are not connected with liquid petroleum and is called ‘non-associated gas’ or ‘dry gas’.
Natural gas can also be found within the tiny pores of certain formations of shale, sandstone and other sedimentary rock. This type of gas is referred to as ‘unconventional natural gas’, ‘shale gas’ or ‘tight gas’.
Natural gas deposits occur both on land and deep below the ocean floor. Additionally, it can be found within coal deposits and is referred to as ‘coal bed methane’. Within the United States, the top producing states for natural gas are Texas, Pennsylvania, Louisiana, Oklahoma, and Ohio.
How is Natural Gas Processed?
Natural gas that is used by industry, utilities and consumers is almost entirely methane. However, when the natural gas comes out of the ground, it is far from pure methane. It will have other gases and compounds associated with the methane, as well as water, and in some cases oil. The natural gas must go the through multiple steps to remove impurities as well as make it safe for transport and use.
One part of the processing requires removing the associate hyrdrocarbons called “natural gas liquids” or NGLs from the raw natural gas. This valuable byproduct of natural gas can include ethane, propane, iso-butane and natural gasoline. These NGLs are extracted from the natural gas and separated into their base components through a process called fractionation. The NGLs can be used to enhance oil recovery, be utilized in petrochemical plants or serve as a fuel.
Beyond the removal of NGLs, natural gas must be processed to remove contaminates such as hydrogen sulfide, water, carbon dioxide, and oxygen. Pipelines that transport natural gas have restrictions or tariff limits, on the maximum allowable levels of these impurities. If not managed correctly, these items can cause corrosion in pipelines.
AMI’s Oxygen, H2S and Moisture Analyzers play a critical role in making sure that the natural gas meets the quality standards for transport or sale. This can include monitoring the gas at compressor stations, dehydration units, or sulfur removal points. The AMI Analyzers are also commonly used at custody transfer points to verify the gas meets the tariff limits before it changes ownership.
How does Oxygen get into natural gas?
Conventional natural gas does not contain oxygen. Oxygen that is found in gas comes from the ingress of atmosphere during the gathering and processing of natural gas. Some common sources of oxygen include:
- Pipe, flange or fitting leakage in both vacuum and pressurized systems
- Entry through compressors packing leakages
- Improper operation of vapor recovery units (VRUs)
- Failure or lack of a tank blanketing system
- Flaring without proper systems to prevent air infiltration
- Performing only a purge vs. using a “Block & Bleed” technique on a pig trap during pigging operations
Non-conventional gas sources are also potential opportunities for oxygen to be introduced into the primary gas system. This can include gas from both coal bed methane (CBM) and landfill or biodigester producers. These gas sources require additional pretreatment equipment due to the naturally occurring oxygen within the recovery methods. If these gas conditioning systems are not properly managed, oxygen can be introduced into the primary gas network.
Why are trace O2, H2S and H2O Bad?
Oxygen contamination, even in trace amounts, when combined with other trace contaminates, can pose serious and potentially costly problems in gas plants, transmission lines and gathering systems. Specific areas of concern are:
- Pipelines – sulfuric acid is formed when oxygen is present with trace amounts of hydrogen sulfide, carbon dioxide and water. This condition over time can cause corrosion to the interior of piping systems.
- Dehydration Systems – oxygen will enhance degradation of glycol in dehydration systems and accelerate corrosion of the equipment
- Amine treatment systems – oxygen will react with amines to form heat stable amine salts. The salt formation creates multiple problems for the amine plant. First, they will reduce the overall amine available for acid gas removal, which will drive an increase in chemical usage in the plant. Second, the salts will form acids that corrode and damage systems. Finally, the buildup of salts can be passed downstream and foul valves, piping and heat exchangers.
The presence of oxygen in natural gas systems, even at sub 10 ppm levels, is both a reality and a concern for systems operators. They will often utilize AMI’s 1000RS Trace Portable Oxygen Analyzer to perform spot checking to track down and understand how the oxygen is entering the gas pipeline systems.