Oil sand is often referred to as non-conventional oil or crude or bitumen, in order to distinguish the bitumen and the synthetic oil extracted from oil sands from the free flowing hydrocarbon mixture known as crude oil traditionally produced from oil wells.
Oil sands, tar sands or extra heavy oil is a type of bitumen deposit. The sands are naturally occurring mixture of sand, clay, water and an extremely dense and viscous form of petroleum called bitumen. They are found in large quantities particularly in Canada and Venezuela.
The main drivers of technology innovations in oil sands production are a desire to reduce the cost and environmental impact.
These can be broadly divided into two categories:
- Surface Mining Operations
- In situ processes
In a surface mining operation, oil sand is scooped up by huge shovels, trucked to a crusher, then slurried with warm water and piped to an extraction plant. There, hot water(40 – 50 deg C) is added, air and process aids are injected into the stream, & the mixture is fed into a conical primary separation vessel. Sand settles to the bottom of the vessel and the overflow is a froth that contains 60% Bitume, 30% Water and 10% Clay.
Naptha is typically mixed with the froth to lower the viscosity of the bitumen, making it easier to separate it from the water and clay by centrifuging. Finally, the bitumen and naptha are separated by distillation. The naphtha is recycled and the bitumen, whose specific gravity is 10API is upgraded by delayed or fluid coking to obtain a synthetic crude of 25-30API for refining
Tailings Treatment: Surface mining operations produce tailings that are a mixture of water, clay, sand, residual bitumen and napthenic acids. The problem is that the clay stays suspended, settling to a max of only 30-40% solids content after 3-5years. For this, gypsum is added at times to the tailings to accelerate the release of water. Another way out is CO2 injection. The injected CO2 forms carbonic acid, which changes the pH and coagulates the clay, thereby increasing the settling rate of the tailings.
IN SITU PROCESSES
1) SAGD (Steam Assisted Gravity Drainage)
This is a popular process for in situ mining. A horizontal well is drilled into the oil formation, and a second, producer well is drilled parallel to it at a lower level. Steam is injected into the upper well to liberate the bitumen, which is punped out from the producer well.
2) THAI (Toe to Heel Air Injection)
A horizomntal producer well with a slotted liner is drilled at the base end (toe) of the producer well. Steam is injected fro 2-3 months to raise the reservoir temperature to 100 deg C. Finally, air is injected at 450-550 psi, initiating a combustion front that moves along the axis of the producer well, causing oil to flow into the well.
Compared with SAGD, there are substantial savings in capital costs and water and energy.
Benefits of in situ mining are that the produced bitumen is ‘clean’, theer are no tailings and the water use is much lower than those operations based on surface mining.
- Cheaper than the conventional oil.
- Easier to produce.
- Environmental Issues.
- Land : Large areas of land are converted into useless lakes.
- Water: This uses large volume of water and create huge tailings ponds that contain toxic residues.
- Air: Emissions of carbon dioxide and hydrogen sulphide is a major issue.
- Energy Usage
- Oil from sand is very difficult to transport. It has be mixed with naptha and other things to lower the viscosity.
Some Interesting facts about oil sand:
- World’s largest deposit of oil and sands occur in two countries: Canada and Venezuela, both of which have reserves approximately equal to the world’s total reserves of conventional crude oil.
- Venezuela prefers to call its oil sands “extra heavy oil” instead of Bitumen. Bitumen and extra-heavy oil are closely related types of petroleum, differing only in the degree by which they have been degraded from the original crude by bacteria or erosion.
- Venezuelan deposits are less degraded than the Canadian deposits and are at a higher Temperature (above 50 degree C vs. freezing for northern Canada), making them easier to extract by conventional technique.
- First Oil Sand mining project began in 1967.
- The oil from sands is as cheap as $27 per barrel as against the $70 per barrel some time back.
By Associate Writer - Ms. Nidhi Garg