The accessible ore of oil sands is located 50 to 200 m from the surface. Due to the difference in depth, recovery of oil sands ore calls for different techniques: open pit mining for shallow resources and steam assisted gravity drainage (SAGD) for deeper resources. These two oil sands processes techniques are described in more detail below.
Open Pit Mining
After identifying the potential source of bitumen, the area is cleaned and overburden is removed to ensure the smooth and continuous recovery of oil sands. Once the land is cleaned, it is ready to mine. The first step is to collect the oil sands ore from the ground and transport it to a crushing facility. Gigantic shovels and trucks that can carry loads up to 400 tonnes are used.
Once the oil sands ore is crushed, hot water is added so the ore can be pumped to the extraction plant. At the extraction plan more hot water is added to this slurry of sand, clay, bitumen and water in a large separation vessel where settling time is provided to allow the slurry components to separate. As the slurry settles, bitumen froth rises to the surface and sediments (rocks, clay, and sand) settle to the bottom. The middle layer is removed for further bitumen recovery.
Bitumen froth contains bitumen, clay and trapped water which is skimmed and sent to froth treatment vessels. The purpose of froth treatment is to reduce the water and solid waste going to the upgrader. It calls for adding diluents (naphtha or paraffin solvents) to reduce the viscosity and heating the froth at higher temperatures. At the end of the process, a mixture of clays, water, sand and traces of bitumen combined with process chemicals, known as tailings is pumped out to setting ponds and recovered bitumen is sent to the upgrader.
The settling ponds or large containment dykes are also known as tailing ponds. A significant portion of water released to tailing ponds can be recycled however the residual tailings mixture remains indefinitely. Any water from tailings cannot be released to the environment without undergoing further treatment.
Recovered bitumen from froth treatment is a very thick, heavy and black viscous substance that has to be upgraded or diluted in order to be pipelined and used as feedstock in refineries. The product of paraffinic froth treatment is much less viscous so it requires considerably less diluent for pipelining.
The sole purpose of upgrading is to transform bitumen into synthetic crude oil which can be refined and marketed as consumer products. Although the overall upgrading process flow diagram varies from company to company, generally the process is broken down into two broad types of upgrading, namely primary and secondary upgrading.
Depending upon the quality and characteristics of bitumen, primary upgrading involves feeding bitumen into cokers where it is heated to high temperature (500°C) or to hydrocrakers where it is reacted with hydrogen under pressure.
The intent of primary upgrading is to break the heavy molecules of bitumen into lighter and less viscous molecules. As a result of this upgrading, less viscous bitumen is obtained along with coke, kerosene oil, naphtha and other byproducts.
Secondary upgrading is meant to further purify and distill the obtained bitumen from primary upgrading so that it could be used as feedstock for oil refineries. The main process of secondary upgrading is to pass the bitumen through a distillation tower. It helps separate the lighter and heavier hydrocarbons and remove unnecessary impurities such as nitrogen, sulphur and trace metals of hydrocarbons.
This lighter form of hydrocarbon obtained from the upgrading process is known as synthetic crude oil and it is ready to be sent to refineries to be made into commercial products such as diesel and gasoline fuels.
Steam Assisted Gravity Drainage (SAGD)
About 80% of oil sands reserves are buried too deep for open pit mining. Bitumen present at considerable depth (more than 130 m) is attained through a technique called steam assisted gravity drainage (SAGD). Currently SAGD is the most popular in-situ (or in-place) technique which is able to economically recover up to 55-60% of bitumen (Guide to SAGD Reservoir Characterization Using Geostatistics , PDF 8.72 MB).
In the SAGD technique, two parallel horizontal wells are drilled in the formation, one slightly higher than the other. The upper well (injection well) works as a kind of steam chamber and is used to continually inject steam into the ground. As temperature rises in the oil sand formation, bitumen becomes more fluid, and due to gravity, flows to the lower well (production well). Finally, the condensed water and crude oil or bitumen is recovered to the surface by pumps and transported to an upgrader facility.
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