A while ago I posted “Shale Gas EROI: Preliminary Estimate Suggests 70 or Greater.” I am happy to report that this analysis has been expanded, updated, and subjected to a rigorous scientific peer review. It is now in the form of an article that I wrote with the help of a colleague, Jackie Melillo, which is now in press (1).
The expanded analysis focuses on the Marcellus shale, and estimates that the EROI of horizontal gas wells in this region is in the range of 64:1 to 112:1, with a mean estimate of 85:1. The EROI value is sensitive to a number of variables. The most important of these is the total production of gas from a well. In our analysis, Jackie and I estimated that a typical horizontal gas well in the Marcellus shale region will produce 3 billion cubic feet of natural gas over its lifetime. Recent actual production data suggests that Marcellus wells are on track to produce at least this much. These data will be discussed in a piece I will post shortly.
An EROI in the range of 85:1 for natural gas is surprising in light of other studies that indicate a much lower EROI. For example, a recent article (2) depicts the EROI for electricity produced from combustion of natural gas as 7:1.
How could EROI values for natural gas differ so much? Although all EROI studies attempt to determine the ratio of the energy output (numerator) to the energy input (denominator), a key difference exists between natural gas and other fuels. Approximately 8 percent of natural gas is burned, mostly at large regional compression stations (such as the one pictured), to provide the energy to process and compress the gas in order to get it to market. How this “self-use” quantity is counted makes a big difference in the EROI calculation. Two EROI calculation methods have been used with natural gas, the net energy ratio (NER) and the net external energy ratio (NEER). The NER has as its numerator the net output of refined energy to society, and as its denominator the sum of all energy consumed in the energy production and refining process. In contrast, the NEER’s denominator includes only those inputs that are consumed from the existing industrial energy system, and excludes self-use (i.e., natural gas used to process and compress the remainder of gas).
If the NER approach is used with natural gas, the 8 percent that represents self-use is included in the denominator, and so the EROI can never be higher than about 12:1. On the other hand, using the NEER approach, the self-use quantity is subtracted from the numerator, and only the energy actually consumed that could have been used elsewhere in society, such as diesel fuel and electricity, is included in the denominator. For natural gas, with its large self-use component, the NEER approach leads to a higher estimate of the EROI.
The NER may be a more comprehensive measure of the total energy return from a production pathway, and likely correlates closely with environmental impacts, such as greenhouse emissions, of a pathway. Conversely, the NEER is a more useful measure of the contribution of an energy source to the energy supply of society because it counts only the inputs that must be produced and delivered externally through the existing energy supply system. In my preliminary study, and in the article that will soon be published, the NEER approach is used.
(1) Aucott, Michael and Jacqueline Melillo, 2013, A Preliminary Energy Return on Investment Analysis of Natural Gas from the Marcellus Shale, Journal of Industrial Ecology, in press.
(2) Inman, Mason, 2013, The True Cost of Fossil Fuels, Scientific American, April, 2013, Vol. 308, No. 4, pp. 58-61.