Natural gas is a clean-burning fuel source that powers millions of homes and businesses, yet its journey from deep underground to a residential furnace is a complex, multi-stage logistical feat. The gas traverses vast distances through an intricate network of specialized infrastructure. This process requires understanding a coordinated system of purification, high-pressure transport, inventory management, and low-pressure delivery.
Initial Preparation at the Field
Raw natural gas brought to the surface is a mixture containing much more than just methane. Before it can be transported through long-distance pipelines, this raw gas must be treated to meet strict “pipeline quality” standards. This initial preparation prevents damage to the infrastructure and ensures the gas is safe and efficient for end-use.
The first step involves physical separation where oil, water, and large solid particles are removed from the gas stream near the wellhead. The gas is then routed to a processing plant for further purification. Here, a process called “sweetening” removes corrosive contaminants like hydrogen sulfide and carbon dioxide, which can damage steel pipelines.
Water vapor is also removed through dehydration to prevent the formation of methane hydrates, which are ice-like structures that can clog pipelines. Finally, valuable heavier hydrocarbons, known as Natural Gas Liquids (NGLs), are extracted. NGLs are removed to prevent them from condensing back into liquids within the transmission pipeline and are sold separately.
High-Pressure Transmission Pipelines
Once the gas is purified into a stream of almost entirely methane, it is injected into the high-pressure transmission network. These wide-diameter, coated steel pipelines operate at pressures ranging from 200 to 1,500 pounds per square inch (psi). The high pressure significantly reduces the gas volume, making it economically feasible to transport vast quantities of energy over long distances.
To maintain the gas’s forward momentum, which constantly loses pressure due to friction and elevation changes, compressor stations are strategically placed along the route. These stations, typically spaced 40 to 100 miles apart, use turbines or engines, often fueled by a small portion of the gas, to boost the pressure back up. Their continuous operation propels the gas across the country.
The entire network is monitored and controlled in real-time by Supervisory Control and Data Acquisition (SCADA) systems. SCADA utilizes sensors placed along the pipeline to track pressure, flow rates, and valve positions, allowing operators to make adjustments instantly. This precise management system also controls “line pack,” the physical volume of gas stored within the pipeline by varying the pressure. By increasing the line pack during periods of low demand, operators create a temporary buffer inventory to meet sudden spikes in consumption.
Storage and Inventory Management
Massive fluctuations between summer and winter demand require large-scale, dedicated storage facilities separate from the transmission system. Natural gas is primarily stored underground in geological formations that offer natural containment. The three main types of underground storage are depleted natural gas or oil reservoirs, salt caverns, and aquifers.
Depleted reservoirs, which are old production fields, are the most common storage type and account for the largest total volume capacity. Gas stored in these formations is categorized into “working gas” and “base gas.” Working gas is the volume that can be withdrawn and delivered to market when needed, especially during the winter heating season.
The base gas, or cushion gas, is the permanent volume that must remain in the reservoir to maintain the pressure needed to push the working gas out. Salt caverns offer lower total capacity but possess extremely high injection and withdrawal rates. This makes salt caverns well-suited for meeting short-term, peak-day demand spikes.
Local Distribution to Consumers
The final stage begins when gas exits the high-pressure transmission system and enters the local distribution network. This transfer happens at the “city gate” or town border station, which serves as the custody transfer point where the gas is sold to the local distribution company (LDC). Here, the pressure is drastically reduced from the high transmission level to a much lower pressure suitable for the distribution mains, typically less than 300 psi.
The city gate is also where odorization, a crucial safety measure, is implemented. Because pure methane is naturally colorless and odorless, a chemical compound called mercaptan is injected into the gas stream. This additive gives the gas its distinctive rotten-egg smell, ensuring that a leak is detectable far below the flammability level.
From the city gate, the gas flows through a network of distribution mains beneath streets and neighborhoods. These mains feed into smaller service lines that connect directly to individual homes and businesses. At the point of entry into the building, a final pressure regulator reduces the gas to a very low pressure, often around 0.25 psi, which is the safe operating pressure required for residential appliances.