This article is the second in a series taking a close look at alternative fuels production and source materials. In the next few months we will cover propane autogas, ethanol, hydrogen, and electricity, as a vehicle fuel.
This month, we will focus on natural gas.
Natural gas prices are influenced both by fluctuations in petroleum prices and by supply and demand. For example, the U.S. Energy Information Administration (EIA) forecast of U.S. total natural gas consumption averages 76.5 Bcf/d (billion cubic feet per day) in 2016 and 77.7 Bcf/d in 2017, compared with 75.3 Bcf/d in 2015. Alternative methods for producing natural gas, including using organic waste to produce biogas with methane recovery, also are being explored to meet the increases in demand.
Natural Gas Formation and Source Materials
Natural gas is believed to have been formed from organic matter (animals and plant biomass) decomposed by heat and pressure; organic matter decomposed by microorganisms; or oil, coal, and other heavy hydrocarbons that have released methane by heat and seepage.
Over millions of years, dead plants and animals sank to the bottoms of lakes and oceans. These remains were covered by mud, sand, and other debris. Over time, the accumulation of this debris exerted high pressures on the buried organic matter. These pressures created high temperatures. Chemical reactions occurred, and the organic remains converted into natural gas and crude oil. At the same time, mud and sand deposits turned into rocks, many of which were porous. Oil and gas seeped into these porous rocks to be stored under high pressure beneath layers of additional solid rock. Geological movements created underground domes and cavities where oil and natural gas were trapped.
There are usually several layers of rock on top of oil and natural gas deposits. Over long periods of time, the earth’s crust moves and the layers become mixed with each other. A “cap rock” layer that is not porous helps keep the oil and gas underground. Oil and natural gas are usually found in porous rocks called shale, although not all deposits of shale contain oil. Deposits of limestone and sandstone are often found above oil deposits. Underground water sometimes mixes with oil, making refining more challenging. Faults in the Earth’s crust move these various layers of rock in several directions, which concentrates oil and gas in pockets deep underground and sometimes allows them to escape to the surface.
New gas fields are located by many of the same methods used to search for petroleum. Sometimes, there are obvious surface indications, such as gas or oil seeping up through the ground. Otherwise, geologists perform surveys on the ground by airplane and satellite to look for rock formations such as sandstone. These formations may indicate the presence of porous rock overlaid by solid rockthe condition necessary for oil or gas pool. Gas and oil wells are also drilled off-shore from platforms built into the ocean floor. Oil and gas exploration and drilling are highly developed technologies.
Gas and oil often lie in porous rock formations that must be pressurized or stimulated from above to encourage fracturing. Fracturing involves creating and enlarging cracks through which oil and natural gas may flow more freely. Chemicals are pumped into the casing to clean the horizontal hole that gets contaminated by debris mixed in with the gas or oil; aggregate materials called “prop” are injected into the seams to help support and encourage fracturing into the gas or oil zone.
Cross section of natural gas wells. Credit: EIA.
The operation of a gas well is much like the operation of an oil well. Gas is always under pressure that developed during its formation. The gas can easily flow to the surface from natural pressure (often greater than 30,000 pounds per square inch (psi). Natural gas is gathered from multiple wells. Some well fields may contain a few thousand wells. A gathering system typically includes small-scale field compressor sites where the gas is filtered through oil to remove dirt particles, then compressed and forced through a glycol dehydrator to remove water and impurities. (The glycol is then recycled after evaporating the water.) The gas pressure must be sufficient to transport the gas through miles of pipelines.
Natural gas drilling well pad. Credit: Bureau of Land Management.
Renewable Natural Gas
Biogas is a methane-rich gaseous product similar to natural gas that is made from anaerobic digestion (decomposition without oxygen) of biomass, or organic matter. Examples of organic matter include manure, sewage, and municipal waste. Once biogas has been cleaned and decontaminated to a higher standard of purity, it can be called renewable natural gas and be compressed for use as an alternative fuel source for vehicles, just like naturally occurring natural gas.
Natural gas itself was formed over millions of years by the anaerobic decomposition of organic materials and trapped under layers of sediment. The same types of anaerobic bacteria that produce natural gas also produce methane, which is the main component of natural gas. Methane can be domestically produced as biogas through the decomposition of organic matter and harvested from landfill waste, in a process known as methane recovery. This is a promising technology to aid in the production of natural gas for years to come. As long as organic waste matter is available, natural gas can be considered a viable alternative fuel option.
Transport and Distribution
Natural gas is delivered to consumers through 300,000 miles of transmission pipelines, and is available in the lower 48 states, with an additional 1.9 million miles of distribution pipes transporting gas within utility service areas.
This article is based on the Clean Cities Learning Program Petroleum Reduction Technologies curriculum. For more information, visit the Clean Cities Learning Program or contact the NAFTC.