The human population relies heavily on petroleum products, including fuel oil for heating homes, gasoline for cars, and plastics for housewares. Oil is extracted primarily from conventional oil reserves by drilling wells, although other reserves are being tapped in different ways.
Types of Oil Reserves
There are two classifications of oil reserves:
- Conventional Oil Reserves include liquid oil trapped in pockets between layers of rock. The movement of the oil into reservoirs is influenced by the geologic qualities of the rock and the presence of groundwater.
- Unconventional Oil Reserves include oil that is contained in alternative geologic structures, such as oil sands or oil shale. Oil sands are a mixture of sand, clay and oil that has been partially degraded by oil-eating bacteria. The resulting oil is very heavy. Oil shales are hydrocarbon deposits that have not yet reached the state of becoming conventional oil. The natural process to convert the rock contents to conventional oil can be simulated by human processes.
The vast majority of oil reserves being tapped today are conventional, because it has not been cost-effective to process unconventional oil reserves. However, with traditional reserves diminishing and oil prices rising, more companies are developing processes to extract petroleum from unconventional oil reserves.
Extracting Oil from Traditional Oil Reserves
After a reserve location has been identified by petroleum engineers, a well is drilled to access the reserve. Depending on the location, the reserve could be near the surface, deep underground, or under water, and each of these require specific techniques and equipment to safely access the reserve. The well bore is created with a drill, and a steel casing is installed inside the well bore to provide greater structural integrity than the bore wall itself would provide.
A conventional well will go through three phases during its lifetime:
- Primary recovery – A new well is usually under pressure from natural gas and subterranean structures. This means that the crude oil will freely flow up and out of the well through the well bore. This pressure can be quite large, and a series of valves and blowout preventers are installed on the top of the well head to control the outflow of the oil. A report by the Institute for Energy states that as much as 5 – 15% of a reservoir’s oil can be extracted through primary recovery.
- Secondary recovery – After the natural pressure within the well has been expended, pressure has to be applied via a different method to force the oil out of the well. The primary method used in secondary recovery is water injection. Since oil is less dense than water, the water sinks below the oil, forcing the oil up through the well bore. Alternatively, a gas, such as natural gas or carbon dioxide, can be injected into the reservoir to increase the pressure on the oil at the well location. Secondary recovery generally results in 30-50% of the oil reservoir being extracted.
- Tertiary recovery – After secondary recovery efficiency tapers off, tertiary recovery methods are used if economically feasible. The remaining oil in the reservoir is often contained in rock pores and in thin sheets. Recovery methods generally attempt to move the small oil particles together so that they can be recovered. Injection of carbon dioxide gas into the well is one method, since the carbon dioxide works to reduce the viscosity of the oil, reduce the effects of capillary action in the rock, and expand the oil volume. Surfactants are often added to the reservoir to decrease the surface tension between the oil and the water in the well.
Oil wells that are no longer being used are generally capped with cement, although it is not necessary to fill the whole bore with cement.
Extracting Oil from Unconventional Oil Reserves
The rising cost of oil allows for the production of petroleum from unconventional reserves to be more cost effective. Unconventional oil reserves are often located in different portions of the world than conventional reserves, allowing for different parties to participate in the oil trade.
Extraction of petroleum from oil sands is very much like a mining operation. Raw oil sands are dug up and mixed with water to form a slurry and the slurry is pumped from the mining location to a production plant. At the production plant, both mechanical and chemical processes are applied to the slurry to remove rocks and sand, water, and any other particulates from the oil. The bitumen from the oil sands then has to be processed into a form that can be used by traditional refineries.
Oil shale can be tapped by heating the rock, causing the kerogen in the rock to break down and be released as a vapor. The vapor can be distilled into shale oil. Shale oil can be used as a fuel oil or as a basis for chemical processes.
Susan Kristoff - Susan Kristoff is mechanical engineer by trade, but has a diverse set of professional and personal interests. The glue that binds all of ...
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