Oil shale has been used as an energy source for centuries. Extracting oil from oil shale requires conversion of the solid hydrocarbons in the rock to liquid form, so that they can pumped or processed. This is done by heating the rock to a high temperature, and separating and collecting of the resultant liquid. This heating process is called retorting.
Oil shale processing and is generally done in one of two ways: surface retorting and in-situ retorting
Surface processing (also commonly referred to as surface retorting) has traditionally been the more common of the two processes. The process basically entails three steps:
- Mining of the oil shale
Mining of the oil shale can be done using traditional mining methods, either by open pit mining or underground mining (sometimes called room-and-pillar method).
- Thermal processing or retorting above ground, and
- Processing of the shale oil to obtain a refinery feedstock and value-added by-products, disposal of the spent shale
Surface processing has many disadvantages:
- Both mining methods have a large land impact and consume large amounts of water (as the process requires water for operations and also requires pumping out groundwater to prevent flooding of the mines).
- Surface (or open-pit) mining involves a considerable land impact while Room-and-Pillar mining methods are considered inefficient: approximately one-third of the resources are left behind in pillars and/or unmined areas. In fact, the thicker the resource the less efficient the mining process becomes.
- Disposal of the waste shale is a major problem for some processes, requiring large quantities of water.
In-situ (Latin for ”in place”) is the technology for processing oil shale underground.
This process obviates the problems of mining, handling, and disposing of large quantities of material, which occurs for above ground retorting. In-situ retorting also offers the potential of recovering deeply deposited oil shale.
In an in-situ process, the oil shale is slowly heated underground and the resultant liquids and gas are being extracted directly from the reservoir, in a manner not different from pumping crude oil.
Utilizing slow heating methods and lower heating requirenments than those used in surface retorting holds another key advantage: the shale oil produced will be of a superior quality that that produced in above-ground retort facilities, materially reducing the upgrade requirements before delivery to refineries can take place.
In-situ technologies have been successfully demonstrated on a small scale. Various technologies differ by the method used to introduce heat underground, but follow the same basic principle.