Natural Gas Components, Transmission Line Leaks and Karst Topography

Smart Pig’s Question of the Week –

We were recently contacted by a resident of Virginia who wanted to know whether natural gas transmission lines leak, and particularly about how the methane and other constituents of transported gas might behave if a leak occurred in an aquifer associated with karst topography.

To best respond to his concerns, we’ve divided his questions up into smaller bits:

1) Do gas transmission lines leak?

2) How do we find out what else is in gas transmission lines in addition to methane?

3) How will gas behave when it is released by a leaking transmission line into an aquifer, specifically one in a karst landscape?

1. Do gas transmission lines leak?

Yes, gas transmission lines leak, but because of a variety of factors such as size of leak, weather conditions, how the gas might migrate and/or collect, and available ignition sources, leaks may or may not reach the right mixture with air to ignite. The natural gas in these types of pipelines is primarily methane which is lighter than air, so if there is a leak the gas rises in the atmosphere and dissipates, and is not typically a problem for groundwater contamination like other types of liquid pipelines and pipelines that carry liquid gases (ethane, propane, butane, etc.).

Emissions from all the different types of gas pipelines is a problem that has recently been recognized by the EPA and White House in relation to concerns over climate change. In general “leaks” from gas transmission lines are not the largest source of gas coming from such pipelines. Emissions from compressor stations, blow downs at valves, and releases associated with maintenance programs account for more gas released. If you are interested in the emission concerns that are starting to be highlighted nationally regarding climate change at our recent national conference we had representatives from a variety of groups talking about those issues. You can find video of some of those presentations here.

How long an operator can allow a leak to continue depends on its characteristics and whether it falls into a geographic area known as a “high consequence area” – usually a higher population area – where there are additional safety requirements with which operators must comply. See our next installation for a more detailed discussion of leak repair criteria.

2. How do we find out what else might be in natural gas transmission lines in addition to methane?

Interstate natural gas pipelines will typically only be transporting natural gas that is made up of a high percentage of methane. The specific ratio of gases will be set by the gas ‘quality’ tariffs that the company drafts. A maximum BTU/SCF (British Thermal Unit per standard cubic foot) quality tariff usually sets a maximum on the “richness” of heavier than methane gas components such as ethane, propane etc., that can be in the natural gas. The pipeline company wants to set quality standards for what is included in the natural gas so the end users can use it with the minimal amount of processing, and so it does not contain things that could damage the pipeline. As an example, here is a screenshot of the gas quality measurements from a Williams Transco pipeline that goes through Virginia.

Click for larger version

Click for larger version

You will see that at the Fredricksburg station, the “natural gas” was made up of about 95% methane, with a few other components (predominantly ethane and CO2) also included. You can find this info for the Transco pipeline at http://www.1line.williams.com/Transco/index.html  There are buttons on the left hand side of the page that will take you to the “gas quality” section. There you can choose between various tabs to see daily gas quality values and the actual gas quality tariff provisions. Most companies post their tariffs on their websites.

3. How will natural gas behave when it is released by a leaking transmission line into an aquifer, specifically one in a karst landscape?

Karst topography and landforms are created by the action of water on soluble rock types like limestone and gypsum. Karst is typified by sinkholes and caves.  Aquifers in karst topography can transport pollutants much more quickly than other aquifers, simply because there are frequently large cracks and caves through which the water can travel, rather than having to pass through less porous, less permeable rock types. One additional risk incurred by pipelines in these areas is that they must be engineered and maintained accounting for the possibility of rapid changes in the geologic stability of the route, as sinkholes and caverns can collapse quite quickly placing abnormal loads that can cause a pipeline to fail. Another additional risk is that leaked gas could migrate to a pocket in the rock or a cave where it could become trapped and eventually become an explosion hazard if it were to find an ignition source.

The methane being transported in a transmission pipeline will, if released, dissipate as a gas into the atmosphere, making its way through whatever soil or rock type the pipe is buried in. At the low concentrations present in natural gas, the ethane in the gas mixture tends to travel as a gas and not a liquid. If there were to be a transmission line leak, the ethane would release into the atmosphere as a gas, although because it is heavier than air, it will not dissipate as quickly as the methane. If a natural gas line were to leak into groundwater, the small percentage of ethane will tend to be carried with the methane and dissipate as a gas.

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  1. Pingback: Natural Gas Transmission Pipeline Leaks & Repair Criteria | The Smart Pig Blog

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