A few years ago, I managed to get myself on a mailing list from a fellow who fancied himself an expert on energy. Actually, it seemed that no area was beyond his expertise, and the fact that EVERY EMAIL FROM HIM CAME WITH A SUBJECT LINE IN CAPS WITH A LOT OF EXCLAMATION POINTS!!!! really sealed the deal on his bona fides. One of the facts he liked to tout was that natural gas was the perfect fuel. Not only is it clean-burning and relatively cheap, it’s also delivered directly to consumers using a completely self-powered grid. Even under “zombie apocalypse” conditions, he claimed that natural gas would continue to flow.
At the time, it seemed a bit overstated, but I figured that there was at least a nugget of truth to it — enough so that I converted from an electric range and water heater to gas-powered appliances a couple of years ago, and added gas fireplaces for supplemental heat. I just sort of took it for granted that the gas would flow, at least until the recent kerfuffle over the Nordstream pipeline. That’s when I got a look at pictures of the immense turbine compressors needed to run that pipeline, the size and complexity of which seem to put the lie to claims about the self-powered nature of natural gas grids.
Surely a system dependent on such equipment could not be entirely self-powered, right? This question and others swirled doubt in my mind, and so I did what I always do in these cases: I decided to write an article so I could look into the details. Here’s what I found out about how natural gas distribution works, at least in North America.
Gathering In
Assessing the claims of my all-caps interlocutor requires a basic understanding of oil field geology. Something like 12% of natural gas production in the US in 2018 was from so-called associated dissolved sources, in which the natural gas is a byproduct of oil production. Associated gas was once, and often still is, considered a nuisance that was either vented to the atmosphere or flared off at the well. Associated gas is often dissolved into the liquid phase within the crude oil reservoir, in much the same way that carbon dioxide is dissolved into the liquid in a bottle of soda. And just like when you uncap a soda bottle suddenly, the natural gas dissolved into crude oil can bubble out of solution when the reservoir is tapped by drilling. Associated natural gas can also be a gas phase that exists in a crude oil reservoir but is not dissolved into the liquid phase.
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On the other hand, some natural gas reservoirs are non-associated, where the gas is found without any significant crude oil present. Non-associated natural gas is often found where an impervious rock layer forms a cap over a porous rock like sandstone, through which the gas produced by decaying fossil vegetation filters. These reservoirs are often under significant pressure too, making it easy to extract once the reservoir is tapped. There are other types of natural gas reservoirs in the broad non-associated category, like shale gas and coalbed gas.
No matter what the type, exploitation of a natural gas reservoir starts with drilling a well and connecting it to a network of gathering pipelines. These pipes form an immense network that connects thousands of wells to upstream processing and pipeline systems. Depending on the type of reservoir, gathering pipes can conduct either raw natural gas or liquid crude oil. The US on-shore gathering network has about 240,000 miles (386,000 km) of pipe — enough to cover the distance to the Moon.
Cleaning It Up
Raw natural gas usually has much more than just methane in it. Depending on the reservoir type, natural gas can range from a mix of methane, propane, and butane along with other gasses like water vapor, carbon dioxide, and even helium, to an emulsion of crude oil and water foamed up with gas. Most natural gas wells need at least some processing before entering the gathering system, using a device called a gas production unit, or GPU. These machines are essentially adapters between the high-pressure gas in the reservoir and the lower pressures used in the gathering system. GPUs drop the gas pressure with a choke, but to prevent the sudden drop in pressure from cooling the gas mixture to the point where it freezes, the GPU heats the process gas. On the low-pressure side of the GPU, a horizontal separator lets water and liquid hydrocarbons settle out, while the gas phase rises. Some of the gas is tapped off as instrument gas, which is burned to provide the heat for the first stage of the GPU.
Instrument gas is one of the first examples of the self-powered nature of the natural gas grid. Instrument gas is tapped off to power all sorts of equipment along the entire system, parts of which are often very remote and well beyond the practical reach of the electrical grid. In addition to being burned for processes that require heat, instrument gas can be used to run generators that provide backup power for electrical components. The pressure of raw natural gas is also sometimes used to run turbine pumps, and often to pressurize reservoirs and force products to the surface.
Under Pressure
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Water is a constant problem for pipeline operations. Even non-associated deposits classified as “dry gas” reservoirs will usually contain at least some water vapor, which could condense within a pipeline and cause internal corrosion, or potentially freeze and occlude a pipe. Dehydrating natural gas is an important step in getting gas ready for the transmission pipeline system. Natural gas is often dehydrated using a chemical treatment process that exposes the wet gas to triethylene glycol, or TEG. TEG is highly hygroscopic, and is used in tall dehydrator towers that are filled with horizontal trays. TEG enters the top of the tower and flows down each tray, while wet natural gas enters the bottom of the tower. The gas bubbles through the TEG, which absorbs the dissolved water vapor. Dehydrated gas exits the top of the tower, while the wet TEG flows out the bottom to a reformer, which uses instrument gas to power burners that heat the liquid to drive off the water, leaving the TEG ready to use again.
After the natural gas is dried and any contaminating propane or butane is removed, it’s ready to enter the transmission system. The transmission system is the long-haul transportation system for bulk gas, consisting of large-diameter, high-pressure pipelines. In the US, there are both interstate and intrastate pipelines, with a total length of about 3 million miles (4.8 million km) that carry about 2.7 trillion cubic feet (76 billion cubic meters) of gas each year.
Transmission pipelines operate at anywhere from 200 to 1,500 psi (1.3 to 10.3 MPa). To achieve the pressure and flow rate to efficiently transport the gas, and to make up for the pressure losses resulting from customer usage and those incurred by pipeline friction, transmission pipelines use compressor stations along the line. Most transmission compressors are powered by gas turbines, which are powered by the very natural gas that is being shipped. Compressor stations tap natural gas from the high-pressure side to power the gas turbine engine, which in turn powers the compressor — another example of the self-powered nature of the natural gas grid. Of course compressor stations require electricity, too, which is provided by the regular electrical grid, or by backup generators that are powered by natural gas. Compressor stations located beyond the electrical grid will often run completely off gas-powered generators.
To The Customers
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While some natural gas customers, like power plants and large-scale chemical plants, can be serviced directly off the high-pressure distribution system, most end-users are actually serviced by a local distribution company, or LDC. These operators maintain the local network of gas mains, laterals, and metering devices that snake under the streets of most cities. LDCs maintain one or more connections to the natural gas transmission system, and use regulators to lower the gas pressure within their system and flowmeters to measure gas usage. LDCs are also responsible for injecting the methyl mercaptan odorant that gives natural gas its characteristic smell.
By and large, the pressure regulation and metering done by LDCs are mechanical in nature; regulators tend to use diaphragms and springs to reduce the pressure in a main to the very low pressure, often just a fraction of a psi, used by many gas appliances. Metering methods vary, but the meters for residential and commercial customers are often powered by bellows expanding under gas pressure to tally up the flow of gas.
Like transmission pipeline operators, LDCs rely on electricity to power quite a bit of their equipment, including monitoring and control gear. And in most cases, the LDCs are connected to the grid just like everyone else. Like the transmission operators, they’ve also got natural gas-fired backup generators in case of local outages, but almost all of the major distribution functions are powered by the pressure of the natural gas itself.
So, while there’s far more to the story than Mr. All-Caps let on, it looks like he wasn’t far off the truth. The natural gas grid really is largely self-powered and engineered to keep on working no matter what.
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