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.