Associated gas is mainly generated during the process of oil extraction, for the following reasons:
1. Organic matter evolution of source rocks
Principle of Formation: Both oil and natural gas originate from organic matter (such as plankton, algae, etc.) in underground sedimentary rocks. These organic substances gradually transform into oil and natural gas through geological processes such as deep burial, increased temperature and pressure over a long geological period. In this process, organic substances first decompose into smaller hydrocarbon molecules through complex chemical reactions at suitable temperatures (generally considered to be around 60 to 150 ℃) and pressures. These hydrocarbon molecules include liquid petroleum and gaseous natural gas, mainly methane. Due to the fact that oil and natural gas are generated from the same primitive organic matter in the same geological process, natural gas, also known as associated gas, is often present in oil reservoirs.
Geological conditions influence: Different geological conditions can affect the generation and composition of associated gas. For example, in sedimentary basins with high temperatures, high pressures, and abundant organic matter, more associated gas may be generated. Some large sedimentary basins in the Middle East have superior underground temperature and pressure conditions, abundant organic matter, not only huge oil reserves, but also considerable associated gas production.
2. Physical characteristics of oil reservoirs
The role of pore structure: Petroleum is stored in the pores of underground rocks, which are like tiny containers. Natural gas molecules are smaller than petroleum molecules and have higher fluidity, allowing them to coexist with petroleum in rock pores. When oil is extracted, natural gas that originally existed in rock pores will be carried to the surface along with the oil, forming associated gas. For example, sandstone reservoirs have good porosity and permeability, which can accommodate a large amount of oil and natural gas, making it easy for associated gas to be produced simultaneously with oil.
The impact of pressure changes: Deep underground, oil and natural gas are in high-pressure environments. When oil wells are being drilled, the wellhead pressure decreases, and this pressure change can cause natural gas dissolved in the oil to escape. Just like opening a carbonated beverage, when the pressure drops, the carbon dioxide gas dissolved in the beverage will escape. In oil wells, as oil is extracted to the surface, the pressure gradually decreases, and natural gas that was originally dissolved in the oil is released in the form of associated gas.
Associated gas is natural gas that coexists with oil. In addition to methane, there are also the following common gases:
1. Ethane (C ₂ H ₆)
Content range and properties: The volume fraction of ethane in associated gas is generally around a few percentage points. It is a colorless, odorless gas that is slightly heavier than air. For example, in some rich associated gases, the ethane content may reach 10% to 20%.
Usage: Ethane is mainly used for the production of ethylene and is an important raw material in the petrochemical industry. Ethylene can be obtained by cracking ethane, which is the basic raw material for manufacturing many chemical products such as plastics (such as polyethylene), synthetic fibers, and synthetic rubber.
2. Propane (C ∝ H ₈)
Content range and properties: The content of propane in associated gas is also considerable, usually ranging from a few percent to over ten percent. It is a gaseous alkane that is colorless and odorless at room temperature and pressure, but can liquefy under certain pressure. For example, in liquefied petroleum gas (LPG), propane is one of the main components.
Usage: Propane has a wide range of applications, mainly used as a civilian fuel and industrial fuel. In households, propane is often used as liquefied petroleum gas for cooking, heating, and other purposes; In the industrial field, it can be used as fuel gas for cutting metals, with a high flame temperature that can meet the heat requirements for cutting metals.
3. Butane (C ₄ H ₁₀)
Content range and properties: The volume fraction of butane in associated gas is generally lower than that of methane, ethane, and propane, but it also accounts for a certain proportion. Butane has two isomers, n-butane and isobutane. It is a colorless gas at room temperature and pressure, and can be compressed into a liquid.
Usage: Like propane, butane is also an important component of liquefied petroleum gas and is used as a civilian fuel. In addition, butane can also be used to manufacture chemical products such as rubber and plastics, playing a certain role in the chemical industry.
4. Carbon dioxide (CO ₂)
Content range and properties: The content of carbon dioxide in associated gas varies from trace to high proportion depending on the reservoir. It is a colorless, odorless gas with a density higher than air. In some associated gases, the carbon dioxide content may reach 5%, 10%, or even higher.
Effects and uses: High levels of carbon dioxide can reduce the calorific value of associated gas, but it also has certain uses. For example, carbon dioxide can be used for oil displacement, and injecting it into oil reservoirs can improve oil recovery. At the same time, in some environmentally demanding situations, it is necessary to separate and treat the carbon dioxide in the associated gas to reduce greenhouse gas emissions.
5. Hydrogen sulfide (H ₂ S)
Content range and properties: The content of hydrogen sulfide in associated gas varies greatly, with some associated gas having extremely low hydrogen sulfide content, while in some special oil reservoirs, the hydrogen sulfide content may be higher. Hydrogen sulfide is a highly toxic gas with a foul egg odor, and its presence can cause serious harm to human health and equipment.
Treatment measures and uses: Due to the toxicity and corrosiveness of hydrogen sulfide, desulfurization treatment is usually required in the process of associated gas treatment. After processing, hydrogen sulfide can be converted into products such as sulfur, which is an important chemical raw material used in industrial processes such as sulfuric acid and rubber vulcanization.
6. Pentane (C ₅ H ₁ ₂) and heavier hydrocarbons
Content and properties: In associated gas, the content of pentane and heavier hydrocarbons (such as hexane, heptane, etc.) is relatively low. Their boiling points are higher than the previous gases, and at room temperature and pressure, the pentane is partially liquid. The presence of these heavier hydrocarbon substances will result in associated gas containing a certain amount of oily components.
Usage: These heavier hydrocarbons are excellent chemical raw materials that can be used to produce solvents, lubricants, etc. For example, pentane can be used as a solvent in industries such as oil, rubber, paint, etc; In the process of petroleum refining, they can also be used as components of blended gasoline to improve the quality of gasoline.
7. Nitrogen (N ₂)
Content and properties: The nitrogen content in associated gas is uncertain, and in some oil reservoirs, there may be a small or even high proportion of nitrogen in the associated gas. Nitrogen is a colorless, odorless, and chemically stable gas. When the nitrogen content is high, it will reduce the calorific value of associated gas and affect its performance as fuel.
Processing and application: In the processing of associated gas, if the nitrogen content is too high, separation treatment may be necessary. However, in some cases, such as during gas injection production, an appropriate amount of nitrogen can be used to maintain reservoir pressure and improve oil recovery.
