Natural-gas condensate

Natural-gas condensate is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. It condenses out of the raw gas if the pressure is reduced to below the hydrocarbon dew point pressure of the raw gas.

The natural gas condensate is also referred to as simply condensate, or gas condensate, or sometimes natural gasoline because it contains hydrocarbons within the gasoline boiling range. Raw natural gas may come from any one of three types of gas wells:^[1][2]

• Crude oil wells—Raw natural gas that comes from crude oil wells is called associated gas. This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil. Condensate produced from oil wells is often referred to as lease condensate.^[3]

• Dry gas wells—These wells typically produce only raw natural gas that does not contain any hydrocarbon liquids. Such gas is called non-associated gas. Condensate from dry gas is extracted at gas processing plants and, hence, is often referred to as plant condensate.^[3]

• Condensate wells—These wells produce raw natural gas along with natural gas liquid. Such gas is also called associated gas and often referred to as wet gas.

Composition

There are many condensate sources worldwide and each has its own unique gas condensate composition. However, in general, gas condensate has a specific gravity ranging from 0.5 to 0.8, and is composed of hydrocarbons such as propane, butane, pentane, hexane, etc. Natural gas compounds with more carbon atoms (e.g. pentane, or blends of butane, pentane and other hydrocarbons with additional carbon atoms) exist as liquids at ambient temperatures.^[4] Additionally, condensate may contain additional impurities such as:^[5][6][7][8]

• Hydrogen sulfide (H
  2S)
• Thiols traditionally also called mercaptans (denoted as RSH, where R is an organic group such as methyl, ethyl, etc.)
• Carbon dioxide (CO₂)
• Straight-chain alkanes having from 2 to 12 carbon atoms (denoted as C
  2 to C
  12)
• Cyclohexane and perhaps other naphthenes
• Aromatics (benzene, toluene, xylenes, and ethylbenzene)

Separating the condensate from the raw natural gas

Schematic flow diagram of the separation of condensate from raw natural gas

There are literally hundreds of different equipment configurations for the processing required to separate natural gas condensate from a raw natural gas. The schematic flow diagram to the right depicts just one of the possible configurations.^[9]

The raw natural gas feedstock from a gas well or a group of wells is cooled to lower the gas temperature to below its hydrocarbon dew point at the feedstock pressure and that condenses a large part of the gas condensate hydrocarbons. The feedstock mixture of gas, liquid condensate and water is then routed to a high pressure separator vessel where the water and the raw natural gas are separated and removed. The raw natural gas from the high pressure separator is sent to the main gas compressor.

The gas condensate from the high pressure separator flows through a throttling control valve to a low pressure separator. The reduction in pressure across the control valve causes the condensate to undergo a partial vaporization referred to as a flash vaporization. The raw natural gas from the low pressure separator is sent to a "booster" compressor which raises the gas pressure and sends it through a cooler and on to the main gas compressor. The main gas compressor raises the pressure of the gases from the high and low pressure separators to whatever pressure is required for the pipeline transportation of the gas to the raw natural gas processing plant. The main gas compressor discharge pressure will depend upon the distance to the raw natural gas processing plant and it may require that a multi-stage compressor be used.

At the raw natural gas processing plant, the gas will be dehydrated and acid gases and other impurities will be removed from the gas. Then, the ethane (C
2), propane (C
3), butanes (C
4), and pentanes (C
5)—plus higher molecular weight hydrocarbons referred to as C₅₊—will also be removed and recovered as byproducts.

The water removed from both the high and low pressure separators may need to be processed to remove hydrogen sulfide (H
2S) before the water can be disposed of underground or reused in some fashion.

Some of the raw natural gas may be re-injected into the producing formation to help maintain the reservoir pressure, or for storage pending later installation of a pipeline.

Drip gas

Drip gas, so named because it can be drawn off the bottom of small chambers (called drips) sometimes installed in pipelines from gas wells, is another name for natural-gas condensate, a naturally occurring form of gasoline obtained as a byproduct of natural gas extraction. It is also known as "condensate", "natural gasoline", "casing head gas", "raw gas", "white gas" and "liquid gold".^[10][11] Drip gas is defined in the United States Code of Federal Regulations as consisting of butane, pentane, and hexane hydrocarbons. Within set ranges of distillation, drip gas may be extracted and used to denature fuel alcohol.^[12] Drip gas is also used as a cleaner and solvent as well as a lantern and stove fuel.

Use as a diluent in heavy oil production

Because condensate is typically liquid in ambient conditions and also has very low viscosity, condensate is often used to dilute highly viscous heavier oils that cannot otherwise be efficiently transported via pipelines. In particular, condensate is frequently mixed with bitumen from oil sands to create dilbit. The increased use of condensate as diluent has significantly increased its price in certain regions.^[13]

Historical use in vehicles

Some very early internal combustion engines—such as the first types made by Karl Benz, and early Wright brothers aircraft engines—used natural gasoline, which could be either drip gas or a similar range of hydrocarbons distilled from crude oil. Natural gasoline has an octane rating of about 30 to 50, sufficient for the low-compression engines of the early 20th century. By 1930, improved engines and higher compression ratios required higher-octane, refined gasolines to produce power without knocking or detonation.

Beginning in the Great Depression, drip gas was used as a replacement for commercial gasoline by people in oil-producing areas. "In the days of simple engines in automobiles and farm tractors it was not uncommon for anyone having access to a condensate well to fill his tank with 'drip,'" according to the Oklahoma Historical Society. Sometimes it worked fine. "At other times it might cause thundering backfires and clouds of foul-smelling smoke."^[14]

Woody Guthrie's autobiographical novel Seeds of Man begins with Woody and his uncle Jeff tapping a natural gas pipeline for drip gas. The gas also has a mention in Badlands, the Terrence Malick movie.^[15]

It was sold commercially at gas stations and hardware stores in North America until the early 1950s. The white gas sold today is a similar product but is produced at refineries with the benzene removed.^[16]

In 1975, the New Mexico State Police's drip gas detail – three men in pickup trucks – began patrolling oil and gas fields, catching thieves and recovering barrels of stolen gas. The detail stopped its work in 1987.^[17]

The use of drip gas in cars and trucks is now illegal in many states. It is also harmful to modern engines due to its low octane rating, high heat of combustion and lack of additives. It has a distinctive smell when used as a fuel, which allowed police to catch people using drip gas illegally.^[18][19]

References

External links

• Processing raw natural gas
• Preparing raw natural gas for sales
• Natural Gas Processing (part of the US EPA's AP-42 publication and includes a schematic diagram)