Introduction to Oil Shale Technology
Oil shale is found in abundant supplies in many countries throughout the world including USA, Australia, Sweden, France, Germany, China and Russia.
By far the greatest resources of oil shale is found at the Green River Formation which straddles three states (Colorado, Utah, and Wyoming) and is estimated as having 800 billion barrels of recoverable oil- three times that of Saudi Arabia!
This is an article on oil shale and the technology used in mining and processing it into a synthetic crude oil. We begin by having a quick look at the formation of oil shale and an examination of its composition.
Oil shale was formed millions of years ago during the Cambrian to Tertiary periods by the deposit of organic matter such as planktonic algae and plants which, along with silt and sediment built up on sea beds and bottoms of lakes.
During this time the layers built up and were subjected to high pressures and temperatures, much like the process which formed crude oil. However, the oil shale was not subjected to the pressures and temperatures over a long enough period to form a crude oil reservoir.
The oil shale is a sedimentary rock which contains kerogen, a tar-like substance that adheres to the sedimentary host rock minerals such as calcite, quartz, and clays in various sized globules.
Some oil shale deposits can also contain vanadium, zinc, and copper.
Mining of Oil Shale
There are several methods of mining for oil shale, most of which are dependent on the depth of the rock below the surface and are listed below:
- Open Pit Mining
When the oil shale deposit is up to 30m underground, this method can be the cheapest means of extracting oil shale rock.
It begins with the clearing of the area by removal of the top layer of soil, trees and shrubs. Vertical holes are now drilled into the cleared area and explosives loaded and fired to break-up the overburden which is removed by earth-moving machines. This is continued until the top layer of the oil shale is exposed.
Once exposed, further blasting is done to create a wide bench with the wall sloping inward to prevent rock-falls. This is continued and the broken lumps of oil shale rock removed using specialist earthmoving shovels and conveyor equipment. A haul-road in the form of a ramp is made to enable trucks to transport the oil shale rocks to a storage area out with the pit.
Further blasting forming benches, which decrease in width, continues until extraction of the rocks by this method becomes impractical.
- Room and Pillar Mining
This is the most popular underground method of extracting oil shale rocks.
It commences with the sinking of vertical shafts for access and extraction of the rocks to the surface.
Once at the workface, there are two methods employed to extract the rock: the drill and blast method and the use of grinding machines.
The drill and blast method has been used for many years to extract various ores such as coal from underground.
The use of grinding machines has also been previously employed; however modern innovations of these machines are now used, including especially hardened water-cooled cutters. These are capable of removing large portions of oil shale rock, reputedly removing 17000 T in 12 hours, being carried away by the conveyor system to the exit shaft in a continuous operation.
Crushing and Screening of Oil Shale Rocks
The chunks of extracted rock are transported to the crushing plant where they undergo processing into small ½” to 3” lumps suitable for retorting.
Next: Above and Below Ground Retorting of Oil Shale
Above and Below Ground Retorting of Oil Shale
There are two basic methods of extracting oil from the shale:
- Retorting the excavated oil shale
- In-situ Retorting
Retorting Excavated Oil Shale Rock
The gas-fired retort is a vertical vessel into which the crushed lumps of oil shale rock is fed through a non-return gas-tight valve located at the top, from where it falls by gravity through different zones in the retort.
The vessel is heated to about 700⁰F and the rock is fed into it. As the rock passes through the first zone of the retort, it is met by the hot gasses moving upwards from the combustion area. Here the shale is preheated, and some oil mist is formed. As the shale falls down the retort through the next zone, it is subjected to pyrolysis – anaerobic combustion removing the kerogen and leaving a char. This falls down into the combustion zone where along with produced gas and combustion air, temperature rises up to around 900⁰F.
Note, from here on in, the retort is self-sufficient in fuel energy input, which makes the process 90% efficient.
The residue then falls to the bottom of the retort where it is cooled by air and falls onto a rotating grate arrangement where it is removed to the residue storage site.
Meanwhile, the oil gas/mist has been moving up through the retort and exiting from the top of the retort, from where it is fed into the oil-mist separators.
Here the oil drops to the bottom and is piped to the storage vessels, with the gas/oil mix being fed through the condenser. The resultant lighter grade of oil is piped to storage, the processed gas is fed to the retort combustion zone, and the remainder is piped to storage vessels.
In-Situ Processing of Oil Shale
Over the years there have been many techniques used in this method of extraction, most of them failing because of costs and environmental concerns.
Listed below are a few of the techniques of in situ processing:
- Radio-Frequency/critical Fluid Extraction Technology (Raychem) – uses radio frequency to heat oil shale rock.
- In Situ Conversion Process (Shell Oil) – uses electric heaters to heat oil shale rock
- CCR Process (American Shale Oil) – injects superheated steam into oil shale rock.
- Crush Process (Chevron) – injects heated carbon dioxide into fractured oil shale rock.
We shall examine Shell Oil’s In Situ Process, which has been on trial in the Mahogany Research Project in Colorado, in more detail
In Situ Conversion Process (ICP)
An area around the process area is frozen to form a freeze wall, effectively isolating the area from any surrounding groundwater. This is carried out by drilling holes using vertical and horizontal drilling techniques. This allows the insertion of nests of continuous pipes though which a critically chilled fluid such as ammonia dioxide is continually circulated. The surrounding area is frozen to -60⁰F forming a cocoon of ice around the process area. Once the wall is formed, the groundwater is drained from the process area and recovery and heating wells are then drilled into the process zone at 12m apart with electric elements being inserted into the heating wells.
This heats the oil shale to around 700⁰F when, after a period of between two and three years, it causes the kerogen to liquefy and flow into the production wells from where it is pumped to the surface for processing.
Shell has completed the developed and testing of their In Situ Conversion Process, which has produced 1700 barrels of oil (with associated gases) from a 30 X 40 foot area of oil shale deposit.
The recovered shale oil from above ground retorting and in situ retorting now needs to be upgraded before it is sent for refining.
This entails the oil being subjected to hydrogenation along with thermal and chemical processing.
Next: Environmental Effects and Concerns of Oil Shale Processing, and Sketches and Illustrations
Environmental Effects and Concerns of Oil Shale Processing
The development of processing oil from oil shale rock has been dogged by environmental issues, and rightly so, as these process can inflict major damage to the flora and fauna surrounding the area as well as the indigenous recreational and agricultural values. In short processing oil shale would incur ecosystem displacement, groundwater contamination and air pollution, accompanied by large usage of fresh water.
Some of the major environmental issues follow:
- Open Cast and Underground Mining, Crushing, and Screening
Open cast mining uses vast quantities of land and produces mountains of spent oil shale rock after crushing and screening to extract the kerogen impregnated rock.
The excavated open cast area can be backfilled with the spent shale and topsoil added to promote resurgence of flora and fauna in the hope of creating new ecosystems and recreational ground for the indigenous; however there are concerns about cultivation of the reclaimed land, due to contamination.
Note: not all the excavated rock can be returned to the original mines as after processing the volume of rock can be as high as 20% more.
Room and Pillar Underground Mining
Underground mining is still very popular, and as with mining for any ores presents various environmental concerns.
These are chiefly the mountains of residual rocks left after crushing, along with the dust and particles from crushing and screening. These contain traces of arsenic and selenium which can be activated by water. Water use is also excessive and leaching to groundwater is possible.
- Surface Retorting
Retorting produces emissions of carbon dioxide reputedly more than processing conventional crude, oxides of sulfur and nitrogen, water vapor and particulates are also produced. However scrubbers and dust extraction systems are now in place and these emissions are being reduced.
The resulting residual char, removed from the bottom of the retort has to be stored, with some of it going to landfill, bur new laws governing landfill will exclude char, so alternative storage methods such as burial in specially segregated areas using polythene sheeting to prevent leaching should be investigated.
Mining and processing oil shale uses large quantities of water- as much as five barrels of water for every barrel of produced oil. Water should be recycled as much as possible and any leakage into existing groundwater must be addressed.
In Situ Processing of Oil Shale
Until Shell oil’s innovative freeze wall technology, there were fears of groundwater contamination of the process surrounding area by migration of liquefied kerogen.
The Economics of Oil Shale Processing
The present (July 2010) price of crude is $70 a barrel, which will allow even the most expensive processed oil from shale to be accomplished at a profit.
Over the years when the price of a barrel of crude dropped below $30 many shale oil processing plants were abandoned. To date many companies have quoted up to $60 a barrel for processed shale oil. However Shell Oil has confirmed that their In Situ Conversion Processing can produce oil at $20-$30 a barrel, which in these days of rising crude prices should return a healthy profit. Provided, of course, that all environmental issues are addressed.
Oil Shale Processing Technology – Sketches and Flow-Charts
Internet Sites Visited
EIS Information Center – Oil Shale
US EPA – Spent Oil Shale
JPT Online – Oil Shale Extraction Technology has a new Owner
US DOE – Fact Sheet – Oil Shale Conversion Technology (MS Word document)