Gathering
Natural gas gathering and processing infrastructure is a crucial part of the natural gas industry. It is responsible for receiving raw gas from producers at the wellhead, processing it to meet the specifications of pipeline-quality gas, and delivering it into the pipeline grid. This process begins in the field, where raw natural gas is produced and treated through separation and dehydration. It is then compressed and transported through a network of small-diameter pipelines to more extensive gathering lines, which move the gas to a processing plant. These gathering lines are generally less than eight inches in diameter, located in rural producing areas, and operate under low pressure. While most states do not regulate these lines, they play a vital role in transporting natural gas from production sites to processing plants.
The raw gas stream that is transported through the gathering lines consists mainly of methane but also contains other hydrocarbons such as ethane, propane, and butane, collectively known as Natural Gas Liquids (NGLs). Additionally, the gas stream can contain carbon dioxide, nitrogen, helium, hydrogen sulphide, and water. To prepare the gas for further transportation, it is commonly dehydrated near the point of production or during the gas separation or sweetening process at a gas processing facility.
The gas is then transported from the production area to a processing plant to remove NGLs and non-hydrocarbon constituents, such as carbon dioxide, nitrogen, and water. It raises the quality of the gas to pipeline standards. Once the gas has been processed in a gas plant, it is compressed and transported in much larger pipelines, known as transmission lines. These pipelines can be up to 48 inches or more in diameter and operate at higher pressures. When these transmission lines cross state lines, they are regulated by the Federal Energy Regulatory Commission (FERC).
The interstate pipelines transport the residue gas to market hubs, local distribution companies (LDCs), commercial users, chemical plants, or underground storage reservoirs. It allows industrial, commercial, and residential users easy access to natural gas. The natural gas gathering and processing infrastructure is critical in ensuring that natural gas is safely and efficiently transported from production sites to end-users, making it an essential component of the natural gas industry.
Gas Measurement
Measuring the volume of natural gas at different stages of the midstream processing chain is essential for its efficient transportation and commercialization. Orifice meters are the most commonly used meters for measuring gas. These meters work by measuring the differential pressure across an orifice plate. This pressure drop, along with other characteristics of the gas flow, is used to determine the volume of the gas flow. Orifice meters consist of several components, including a meter tube, orifice plate, orifice holder, pressure taps, and a recording device, such as an electronic flow computer or a chart recorder.
Orifice meters are known as "inferential meters" because they infer the volume of gas flow by measuring certain characteristics. However, natural gas measurement is not an exact science, and it is always associated with some degree of uncertainty. Although volume calculations are based on scientific principles, it is impossible to know the exact quantity of natural gas flowing through a meter. Therefore, natural gas measurement is inherently imperfect, and accurate measurement is critical to ensure the integrity of the natural gas supply chain.
Processing
Natural gas is a vital resource used worldwide for power generation, heating, and other industrial processes. However, the raw gas stream produced during extraction often contains impurities and liquid hydrocarbons that need to be removed through processing before the gas can be used or transported.
Gas processing involves two main operations: extracting natural gas liquids (NGLs) from the gas stream and fractionating the NGLs into separate forms. The extracted NGLs include ethane (C2), propane (C3), butane (C4), and pentanes (C5). They can be used as feedstock or end products in various industries such as refineries, petrochemical plants, the agriculture industry, and NGL distributors.
When processing natural gas for delivery to interstate transmission pipelines, various technologies are used depending on the gas stream's chemical content, the hydrocarbons' location, and other factors. It is essential to remove excess water vapor from the gas stream to prevent the formation of hydrates and freezing in interstate pipeline transmission systems. The two most important extraction processes used in gas processing are the absorption and cryogenic expander processes, which account for approximately 90% of natural gas processing. The absorption process involves using solvents such as amines to selectively remove impurities from the gas stream. In contrast, the cryogenic expander process uses a refrigeration cycle to separate the NGLs from the gas stream.
If the raw gas stream contains hydrogen sulphide or carbon dioxide, the gas plant must sweeten the gas by removing these contaminants. Sweetening can be done using chemical reactions, physical solutions, or adsorption processes. In chemical processes, amine solutions are commonly used to react with acid gas compounds, forming other compounds that can be safely removed. Adsorption processes involve passing the gas or liquid through a solid material treated to selectively extract carbon dioxide, hydrogen sulphide, or other contaminants.
The sour gas effluent from sweetening units must be further treated for disposal or sulfur recovery. The sulphur recovery process involves converting hydrogen sulphide into elemental sulphur through the Claus process, an essential step in reducing harmful emissions and meeting environmental regulations.
Gas processing facilities are designed to recover Natural Gas Liquids (NGLs) and deliver pipeline-quality residue gas after sweetening the gas. This process involves three main steps: removing impurities, removing water, and separating NGLs from the gas stream.
The raw gas undergoes a typical process which includes dehydration, mechanical refrigeration, turbo-cryogenic expansion, recompression, and distillation columns. The aim is to remove all NGLs from the raw gas stream, thus separating them into a pressurized NGL product stream (known as Y Grade). This Y Grade can be transported via pipeline, truck, or rail to fractionation facilities for further processing. The process also ensures that pipeline-quality gas is delivered to interstate pipelines at the tailgate of the plant.
