Natural Gas: Formation, Extraction, and Types
Where is Natural Gas Found?
Oil and natural gas are not found in subsurface layers in the form of lakes or rivers. Instead, they are contained within the pore spaces of certain rocks. The existence of these oil-bearing rock layers is scarce, and locating them is the fundamental task of professional geologists and geophysicists. Natural gas was discovered in the United States in the early seventeenth century, although it was known in other parts of the world many centuries earlier. From analyzing hundreds of oil-producing fields, investigators have concluded that the following conditions are necessary for an accumulation of gas or oil:
- Source Rock: The material from which hydrocarbons are formed.
- Migration: The movement of hydrocarbons from the source rock to the reservoir rock, following routes of porosity and permeability.
- Geological Synchronization: The geological trap must exist when hydrocarbon migration occurs.
Natural gas deposits are often found in the company of other hydrocarbons, which are utilized in the extraction and processing of primary products. Depending on the accompanying product, natural gas is classified as either dry gas or wet gas.
- If the gas in the reservoir contains propane, butane, and heavier fractions in sufficient quantity, it is called wet gas.
- If the gas is mostly methane, it is called dry gas.
This classification is due to the organizational changes that occur in the deposits and other derivatives, resulting in liquids with boiling points similar to gas. This classification is also useful for both industrial and domestic applications. Furthermore, the definition of these gas types can be inferred from the type of well from which it is extracted, as the main components will determine whether it is dry or wet.
Drilling
Once gas fields are located, the product can be extracted and marketed. The same technology used for oil drilling is applied to drill a gas field. However, drilling gas fields presents greater operational risks compared to oil drilling. The low density of gas allows it to mix more easily with drilling mud, reducing the mud’s density. Therefore, greater precautions must be taken during such operations. To determine the amount of gas contained in each reservoir and its characteristics, various tests are performed. These tests provide information about pressure and flow fields over time and help to estimate reserves and depletion rates.
Types of Natural Gas
- Associated Gas: Gas found in a field dominated by liquid hydrocarbons, such as oil or condensate.
- Wet Gas: Gas with a high content of propane to heptane.
- Unassociated Gas (Free Gas): Gas that is the unique product or has a low proportion of liquid hydrocarbons (propane to heptane) at the site.
- Dry Gas: Gas with a low content of propane to heptane.
Other Definitions
- Saturated Site: Petroleum that cannot dissolve additional quantities of natural gas.
- Undersaturated Site: Oil in which additional quantities of natural gas can be dissolved.
- Deposit: A hydrocarbon accumulation within the earth that occurs when underground rocks have the appropriate form and compaction conditions for these chemicals to become trapped. There are deposits of oil, gas, bitumen, or a combination thereof.