|A comparison of estimates of the energy return on investment (EROI) at the wellhead for conventional crude oil, or for crude product prior to refining for oil shale. Source: Cleveland and O’Connor. Click to enlarge.|
A new report commissioned by Western Resource Advocates, a non-profit environmental law and policy organization, finds that oil shale’s Energy Return on Investment (EROI) is extremely low, falling between 1:1 and 2:1 when self-energy—the energy released by the oil shale conversion process that is used to power that operation—is counted as a cost. An EROI of 1:1 means there is no energy “profit” from the investment of energy. If internal energy is excluded, and only purchased energy is used as input, then the EROI calculated is in the range of 2 to 16.
While one could argue that the char and gas produced and consumed within the shale conversion process has zero opportunity cost—i.e., that energy would not, or could not, be used somewhere else in the economy, so it should not be treated as a “cost”—the authors note, “the internal energy is absolutely necessary to accurately assess greenhouse gas emissions”.
“Oil shale” is shale containing kerogen, a combination of chemical compounds that can be converted into synthetic petroleum. The two main processing options for oil shale are surface retorting and in situ extraction. In surface retorting, the shale is mined and brought to the surface, with the material then heated in a retort to extract the compounds that are processed into synthetic crude oil. In situ methods heat the material underground and pump liquids to the surface, where they then undergo further processing.
|A comparison of estimates of the energy return on investment (EROI) for refined fuel produced from conventional crude oil and from oil shale. Click to enlarge.|
The authors of the report “An Assessment of the Energy Return on Investment (EROI) of Oil Shale,” Dr. Cutler Cleveland and Peter O’Connor at Boston University, reviewed the existing literature on the energy return on investment (EROI) for oil shale to reach their conclusions.
This [result] places the EROI for oil shale considerably below the EROI of about 20:1 for conventional crude oil at the wellhead. This conclusion holds for both the crude product and refined fuel stages of processing. Even in its depleted state—smaller and deeper fields, depleted natural drive mechanisms, etc.—conventional crude oil generates a significantly larger energy surplus than oil shale. This is not a surprising result considering the natural resource exploited in each process. The kerogen in oil shale is solid organic material that has not been subject to the temperature, pressure, and other geologic conditions required to convert it to liquid form. In effect, humans must supply the additional energy required to “upgrade” the oil shale resource to the functional equivalent of conventional crude oil. The extra effort carries a large energy penalty, producing a much lower EROI for oil shale.
—Cleveland and O’Connor
The authors noted that firm conclusions regarding the EROI of oil shale are difficult to establish for a variety of reasons:
- There are very few reliable studies of current oil shale operations;
- Many studies use a poor or undocumented methodology, and report what could be best described as “ballpark” estimates;
- Some studies exclude important categories of energy inputs that generate inflated estimates of the EROI for oil shale (i.e., the system boundary problem); and
- The very small number of operating facilities that can be assessed result in a lack of large “sample size” of operations from which to draw robust conclusions.
The low EROI for oil shale is closely connected to a significant release of greenhouse gases, the authors note, due to the large quantities of energy needed to process oil shale, combined with the thermochemistry of the retorting process.
Oil shale unambiguously emits more greenhouse gases than conventional liquid fuels from crude oil feedstocks by a factor of 1.2 to 1.75.
—Cleveland and O’Connor
Also, between 1 and 3 barrels of water are required for every barrel of oil produced in an oil shale operation.
A fuel with a modest EROI that emitted few greenhouse gases could at least be a candidate for an alternative source of energy. However, a very low EROI combined with a very high carbon intensity should remove an energy system from serious consideration as an alternative to conventional crude oil extraction and refining. Oil shale in the western United States appears to fall into this category.
—Cleveland and O’Connor
Cutler Cleveland and Peter O’Connor (2010) An Assessment of the Energy Return on Investment (EROI) of Oil Shale