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Debunking the myth on methane leakage in the oil and gas industry. Gas gathering reciprocating compressors are a major source of methane.
Much was made in the news about the greenhouse gas emissions (GHG) caused by last year’s Nordstream pipeline act of sabotage. Although these greenhouse gas emissions were certainly highly undesirable, the total amount of natural gas released was actually less than what is released on a monthly basis by some of the less concerned oil-producing nations.
The Intergovernmental Panel on Climate Change has found that even if the world cuts emissions by the rate governments have promised, the goal to limit global warming to 1.5°C (2.7°F) around the year 2050 will still ‘more likely than not’ be missed.
In recent years, methane emissions have come into focus. Natural gas has a global warming potential that is about 20 to 35 times higher (depending on who you ask) than that of carbon dioxide, so there is justifiable reason to insist on the reduction of natural gas emissions, especially since they are a waste of a useful fuel source. Methane produces a stronger greenhouse effect than the same amount of CO2, but methane lasts in the atmosphere for only about one decade. This explains two things: 1) the wide range of multipliers for methane’s global warming potential and 2) the attractiveness of methane reduction, since (unlike CO2 reduction) the effects of methane reduction will be felt much faster.
Natural gas’s principal constituent is methane which is considered a strong GHG. According to the U.S. Environmental Protection Agency, only about 30% of methane leakage in the U.S. is caused by the oil and gas industry. The rest is attributed to agriculture, landfill, and other mostly biological sources.
There are many avoidable, unavoidable, and accidental emissions points for methane in the oil and gas production and value stream, and there is significant misunderstanding about them among the public. Natural gas emissions from oil or gas production can be divided into accidental leakage, incomplete combustion (unburned hydrocarbons), and/or intentional blowdowns/releases. These emissions account for approximately 3-4% of total U.S. greenhouse gas emissions on an equivalent carbon dioxide basis but are potentially much higher in other oil-producing countries. To provide some clarity, we list the natural gas point leakage sources and discuss the potential for their reduction or elimination.
By far the largest segment of methane leakage in the oil and gas industry occurs in the upstream production process, which accounts for at least half methane emissions (according to some publications the number is substantially higher). The big three sources here are production venting, leakage, and gas gathering compression. While leakage and venting are mostly accidental or safety related and, thus, can be reduced through improved operations practices, leakage from gas gathering compression is more complex.
Gas gathering reciprocating compressors are a major source of methane emissions since it is difficult to eliminate a gas leakage path from the process gas cylinder to the piston rod. Abradable packing seals made from polymers are commonly used to minimize leakage. However, these tend to wear over time so that leakage can become excessive unless they are frequently changed. Several studies indicate that nearly a third of upstream methane emissions come from leakage through reciprocating compressor rings and packings. The majority of this leakage, as well as unburned hydrocarbon production, is associated with smaller upstream machines in gas gathering duty which are often operated in unmanned compressor stations with little supervision or control. More frequent maintenance and better condition monitoring could reduce this very significant source of methane emissions. But these requirements would add cost and could reduce availability. A viable alternative in many cases is to convert to centralized gas gathering compression stations using larger centrifugal compressors rather than many small units.
In midstream gas transportation and down-stream gas processing applications (including LNG production), most methane emissions are related to blowdown venting, valve leakage, and a smaller percentage to compressor leakage. Blowdown and venting may be reduced by better safety technology, reduced maintenance intervention, and blowdown avoidance devices. However, most of these events are compressor station safety driven and cannot be completely avoided. Reciprocating compressors in the midstream segment also leak but tend to be larger and better maintained than in the upstream segment.
Finally, centrifugal compressors are common in pipeline and refinery compressors. Their leakage is primarily caused when outdated wet oil seals are used. Modern dry gas seals on centrifugal compressors drastically reduce methane leakage. Because of this, few centrifugal compressor manufacturers still offer compressors with wet gas seals. Dry gas seal systems could certainly be improved since they are still somewhat sensitive to contamination and sometimes fail.
As noted, GHGs (due to facility blowdowns) are a significant contributor to the total industry GHGs. Blowdown situations may be caused by maintenance requirements related to safety or due to false sensor readings. Some of these could be eliminated using improved alarm/shutdown sensor systems, increased sensor redundancy, more frequent inspections, and generally through higher machine reliability. Also, few compressor installations utilize continuous measurement leak detection. But the technology, especially optical sensor based, is technically feasible and commercially available to a limited extent. Pressurized hold is a potential alternative to full station blowdowns for extended shut-down periods. But the equipment must be designed to stay in pressurized hold for extended periods of time and must be able to start from a pressurized hold. These decisions require a good understanding of plant safety and maintenance practices and improved operator personnel education. Finally, blowing down into low pressure gas storage tanks, or utilizing a thermal oxidizer (flare) rather than a cold stack for non-safety related shutdowns, should be considered to reduce methane emissions.
It’s also important to note the often-advertised conversion of natural gas to blue hydrogen does not significantly help to reduce methane emissions. In almost all cases, the blue hydrogen is produced at the back end of the oil and gas value stream, i.e., all methane losses are already incurred in the process by the time of the gas conversion.
Technologies exist to reduce methane emissions of centrifugal compressors to zero. Modern centrifugal compressors with dry gas seals avoid uncontrolled methane emissions, so the methane leakage can be gathered and recompressed back into the pipeline. The technologies exist and are commercially available. Furthermore, dry gas seals have been developed that avoid any methane leakage, but they would leak nitrogen. Some jurisdictions see nitrogen as a regulated leakage too, even though the atmosphere consists of 78% nitrogen, and nitrogen is inert under atmospheric conditions. While reciprocating compressor manufacturers are also developing technologies to prevent methane emissions, these may be more difficult and costly to implement.
Emissions from the energy industry are being regulated; since methane is valuable, the loss avoidance of methane in production, transport, and combustion also has positive side effects on the industry. One of the dangers is that relatively small contributors (such as the leakage from dry gas seals or the practically negligible unburned hydrocarbon emissions from modern gas turbines) are pursued with the same vigor as large contributors, thus slowing the effort. However, methane emissions are not the exclusive problem of the energy industry. The majority of methane emissions are produced by landfills and the agricultural industry.
Machinery emissions of methane are often a function of the amount of maintenance or safety interventions when methane must be blown down. Thus, the effort of the industry to use digital solutions to reduce shutdowns will also reduce methane emissions. Technologies that allow keeping units pressurized during shutdowns that are not maintenance-related are already in place.
This Myth Buster column went over the potential for methane leakage reductions from the natural gas energy infrastructure. The incentives are obvious. Leaked gas is revenue lost and green-house gas produced. But most importantly, to avoid future government environmental regulatory intervention, the industry should be proactive in voluntarily implementing as many of the above methane leakage reduction technologies as is technically feasible and commercially viable.
Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.
Rainer Kurz is the Manager of Gas Compressor Engineering at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil and Gas Applications Committee.