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Oil shale is a sedimentary rock containing clays impregnated with a solid hydrocarbon known as kerogen that can be processed to produce a fuel oil similar to crude. Shale oil is an unconventional source of oil whereas crude oil is a conventional source.
Oil shale is found in about forty countries worldwide, with the USA having the biggest unconventional reserves- four times that of Saudi Arabia’s conventional crude reserves.
Other countries having reserves of oil shale are Russia, Brazil, China, Canada, and Estonia. Some of these countries are still mining and processing the kerogen bearing rocks despite the many environmental effects of shale oil processing that have led to numerous production plants being shut down. The oil can be processed from the kerogen by pyrolysis either above ground in a retort or through in-situ underground techniques.
The following sections examine oil and gas extraction from unconventional sources, in this case oil from oil shale sedimentary rock. The first section looks at the mining of the sedimentary rock.
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Mining of Oil Shale Sedimentary Rock
The oil shale rock can be extracted by opencast or underground mining methods, with opencast being the currently popular method.
Opencast mining is usually limited to extracting the rock at depths up to 30 m by exposing the rock strata through scooping away the covering layers of ground and then blasting the rock.
Underground mining is normally carried out using chamber and pillar mining. This entails normal underground extraction methods, but instead of using conventional pit-props of wood or steel, pillars of oil shale rocks are left in place to support the mine roof, forming numerous chambers, and hence the name.
The kerogen-bearing rock is then sent to a processing plant where it is crushed and screened to give optimum size of feedstock for the retorting process.
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Processing Oil from Kerogen
This can be carried out either in-situ where the oil is extracted from the rock while in the ground, or ex-situ using a retort. Both methods have been tried and tested over the last fifty years, with some being successful but normally falling foul of environmental laws or financial constraints.
We shall examine the pyrolysis method using a typical Kiverter Retort, of the type that has been producing shale oil in Estonia for many years. The retort consists of a vertical steel vessel that is supplied with crushed rock from a hopper via a gastight valve through the top of the retort. The vessel also has provision for steam injection, recycled gas, unburnt gasses recirculation, and a fossil fuel burner.
The burner heats the retort to around 300⁰C, then the crushed rock is fed in. Falling down through the heated vessel, the solid kerogen is converted to gas. Steam is injected as the gas rises upwards to be gathered at the top of the retort, and the gas is piped to a distilling unit where it is cooled and distilled to shale oil.
This oil is now enriched with hydrogen to produce syncrude, which is used directly in industry or may be further processed by conventional crude refining methods.
Meanwhile, the rocks that contained the kerogen have been turned to char and have gathered at the bottom of the retort. Recycled gas from the distilling plant is passed through this layer which provides fuel to continue the pyrolysis heating process, bringing the temperature inside the retort to around 500⁰C, with the char being extracted through a water seal at the bottom of the retort.
The retorting process also produces pitch, sulphur, ammonia, and aromatic compounds, which can be further processed and sold to the chemical and building industries.
A sketch of a typical Kiverter Retort is shown below.