What is natural gas and how is it extracted?

Iranian Oil Fields

Iranian oil fields include reservoirs, basins, and oil fields located in the territorial and aquatic territories of Iran. Some of these fields are currently active, while others are inactive or under development. The ownership of all Iranian oil and gas fields is in the hands of the National Iranian Oil Company, which currently operates production operations from these fields by four of its subsidiaries: the National Southern Oilfields Company, the Iranian Offshore Oil Company, the Central Iranian Oil Company, and the Arvandan Oil and Gas Company. According to official statistics published by the Iranian Ministry of Oil, the total recoverable reserves of crude oil and gas condensates in Iran are estimated at more than 154 billion barrels. This amount is equivalent to 10 percent of the total crude oil reserves in the world. According to experts from the National Iranian Oil Company, Iran’s oil reservoirs could be exploited by 1453.

Origin of Natural Gas

The remains of plants and animals whose bodies have been pushed to the bottom of ancient lakes and oceans over millions of years gradually decomposed and turned into organic elements, and due to the pressure of the earth’s internal heat, they have been converted into oil and gas and stored in underground reservoirs at a depth of three to four thousand meters and at a pressure of about several hundred atmospheres.

Extraction of Natural Gas:

Natural gas is a mixture of flammable hydrocarbon gases that are naturally found in underground reservoirs. This gas may be dissolved in crude oil (PG) or under the upper caps of crude oil layers or exist independently. Natural gas is mainly a mixture of methane, ethane and a small amount of heavier gases, including propane and butane. Natural gas extracted from reservoirs usually contains impurities such as hydrogen sulfide, carbon dioxide, and sulfur compounds. It should be noted that the amount of these compounds depends on the field from which the natural gas is extracted; for this reason, the amount of methane present in natural gas varies between 84 and 97 percent depending on the gas field.

Raw natural gas is extracted from three types of wells:

1- Oil wells

2- Gas wells

3- Tight wells

Natural gas extracted from oil wells is generally known as “associated gas.” This gas can exist separately from the oil in the formation (free gas) or dissolved in the crude oil (dissolved gas). Natural gas that comes from gas and tight wells, where there is no or very little crude oil, is known as “associated gas.” Gas wells generally produce raw natural gas, while tight wells produce free natural gas along with a condensed semi-liquid hydrocarbon.

Natural Gas Refining:

Natural gas is purified in a refinery due to the presence of fine sand, clay, liquid hydrocarbons, CO2, H2S, and other impurities, thereby preventing these materials from entering the transmission system. The entry of these materials into pipelines and pressure reduction stations causes problems, including blockage of gas pipelines, destruction of regulators and turbine meters, premature failure of filtration systems and filters, corrosion, and toxicity in the gas. The actual process of processing natural gas into dry, pipeline-quality natural gas can be very complex, but it typically involves four main processes to remove various impurities:

Passing the gas through scrubbers to remove solid particles

Removing oil and gas condensate

Removing water

Separating natural gas liquids

Passing the gas through a dehumidification unit to remove moisture

Removing carbon dioxide and sulfur

In addition to the above processing steps, heaters and abraders are usually installed at or near the source. Abraders are primarily used to remove sand and other large impurities. Heaters ensure that the temperature of the gas does not drop too low. Natural gas containing even very small amounts of water will form natural gas hydrates as the temperature drops. These hydrates are solid or semi-solid compounds that resemble ice crystals. When these hydrates form in natural gas, they block the flow of natural gas through valves and gathering systems. To reduce hydrate formation, natural gas-fired thermal units are commonly installed along gathering pipelines where hydrates are likely to form.

Natural gas that is pumped from the ground to the wellhead is quite different from natural gas that consumers use. Although natural gas processing is simpler in many ways than processing and refining crude oil, it is still as necessary to process it before it can be used by consumers as oil. Natural gas that consumers use is composed mostly of methane. Although the gas found at the source, which is mostly methane, does not require much processing and is pure.

Transmission lines and booster stations:

The gas leaving the refinery is transported by pipelines for consumption in cities, industries, power plants, and export to all parts of the country. Depending on the need and design, the diameter of the transmission lines varies from 12 to 56 inches. At certain distances from the transmission lines, there are valve stations, which generally consist of a line cut-off valve on the main route, bypass valves, and a discharge valve. The function of the line cut-off valve, or V.B.L., is to close when a sudden pressure drop is felt on one of the two sides of the valve. In terms of safety and security issues, the presence of these valves at certain intervals causes both valves on both sides of the incident site to be closed in the event of an explosion or pipe rupture, and thus only the gas between the two stations is burned or discharged. Considering that after traveling long distances, due to friction, the gas pressure in the pipe drops, therefore, in order to increase the gas pressure to the desired pressure, pressure boosting stations are used along the transmission lines. These stations include turbines and compressors that increase the pressure of the incoming gas. For example, there are eight pressure boosting stations along the route of the First National Pipeline of Iran. The order to operate the compressors is issued by the Gas Dispatch and Control Center.

Pressure Reducing Stations:

Feeding lines and city networks, transmission lines ultimately reach the pressure reducing station located at the city gate called S.G.C. At S.G.C. stations, the inlet pressure is reduced from 1000 psi to 250 psi. It should be noted that the inlet pressure to pressure reducing stations under normal conditions generally varies between 700 and 1000 psi. This pressure reduction is carried out by regulators. The gas exiting the S.G.C. stations enters the feeding lines at a pressure equivalent to 250 psi. The gas pressure in the feeding lines must be reduced at another stage to enter the city network; for this purpose, the gas is directed to stations called S.B.T. The gas pressure entering the S.B.T is 250 psi and the gas pressure leaving it is 60 psi. Gas within the city network is distributed to subscribers throughout the city (alleys and streets in circulation) at a pressure of 60 psi. A device called Service.T is used to branch to homes. Before entering the consumption location, the gas pressure of the city network is again reduced by the regulator to a relative (0.25 psi) and the gas is consumed by gas-burning appliances at this pressure. In all the above operations, provisions and equipment have been incorporated to ensure the safety of employees and consumers. If safety standards are not observed, each of the above operations can be very dangerous, but if the design, installation, maintenance, and operation process is carried out correctly and regulations are followed in all final stages, the potential risks will be greatly reduced. Natural gas plays an effective role in providing energy for domestic and commercial uses, industries, and power plants.

Gas transmission and distribution by pipeline network:

The transportation of natural gas from production units in each area is the responsibility of the pipeline industry. The natural gas transmission pipeline is made of strong and thick pipes that can withstand high pressure. During the transmission stage, the gas rises from an altitude of approximately 300 meters (Bid Boland) to an altitude of 3000 meters above sea level (Zagros Mountains) and also, due to the long distance, there is a pressure drop in the pipes and fittings, so it is necessary to keep its pressure constant.

Pressure Booster Units:

The pressure of natural gas in the transmission line is boosted to minimize the cost and size of the pipe required to carry it. As the gas flows through the pipeline, friction reduces its pressure and flow rate. Therefore, by using pressure booster units installed at certain intervals, the gas pressure in the pipeline is boosted and the pressure in the installed pipeline is maintained. In order to achieve the above goals, the pressure in the gas transmission line must be kept constant between 700-1000 pounds per square inch. For this purpose, in places where the gas pressure reaches about 520 pounds per square inch, its pressure is increased to about 1000 pounds per square inch by large compressors. Keeping the gas pressure high at this level causes: a) a decrease in the diameter of the pipe and ultimately a reduction in the cost of transmission, while also allowing gas to be stored for emergencies. b) it prevents the gas from reaching the dew point, because reaching the dew point of the gas in addition to causing severe corrosion in the system causes disruption in the gas pressure control systems.

Gas pressure reduction stations for consumer use:

A gas pressure reduction station is designed and built in various ways according to the input and output pressure capacity, type of consumer, number and stages of pressure breakdown and its lines, but despite this diversity, the general equipment used in their construction is as follows, and the use of some of them may change as necessary or their number:

1- Filters

2- Heaters

3- Regulators

4- Meters

5- Safety valves

6- Pressure cut-off valves (pressure breakers)

7- Pressure gauges and thermometers and electrical insulators

8- Valves and flanges

9- Odorizers

If we want to provide a division according to the type of consumer, we have:

a) Stations that feed gas distribution networks (urban stations).

B) Stations that supply the internal network of industries and large commercial and residential units (industrial commercial stations).

These stations are designed and commissioned by engineering affairs. In stations whose capacity is more than five thousand cubic meters per hour and the continuity of gas supply is vital, the number of station lines will be more than one line, which usually in such cases the capacity of each line is a percentage of the total capacity of the station.

Safety and privacy of transmission lines:

The control system of the stations is such that in the event of an accident, the inlet and outlet valves are automatically closed. Also, at certain intervals on the lines, automatic valves sensitive to pressure differences and recently to earthquakes have been installed. Each gas pipeline has a thickness and privacy appropriate to the gas pressure, environmental conditions and population conditions.

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