AN ENERGY RETURNED ON ENERGY INVESTMENT STUDY FOR UNCONVENTIONAL MARCELLUS SHALE NATURAL GAS WELLS

The production of natural gas in Pennsylvania has grown rapidly, rising from approximately 0.5 billion cubic feet (bcf) per day in 2005 to 6.1 bcf per day in 2012. This increase is largely due to the extraction of gas from the Marcellus shale via unconventional wells, which combine horizontal drilling and hydraulic fracturing (fracking) to access resources within the shale.

To obtain (i.e., produce) any fuel requires an energy input. We calculated an energy returned on investment (EROI) to estimate the netenergy production from unconventional Marcellus Shale wells. The EROI is a dimensionless ratio: energy output/energy inputs. There are about 25 classes of energy input for gas from a Marcellus well; this study accounted for approximately 80% of those inputs. Energy uses during the following stages were determined: well site preparation and construction, gas gathering and compression, dehydration and processing, booster compression, and transmission

Two types of EROIs were performed: a net energy ratio (NER) model and a net external energy ratio (NEER) model. The NER includes all energy use for all processes in its inputs, including “self-use” (gas from the wellhead that is used to compress and process the gas). In the NEER, only external energy sources are included as inputs (e.g., diesel fuel to power drilling rigs) and does not include self-use. For energy returned in both NER and NEER models, we used high, middle and low (90^th, 50^th, and 10^th percentile, respectively) natural gas production values from industry and government sources. The NER method produced EROI values of 6.8, 6.6, and 5.4. Using the NEER method, resulting values were higher, with EROIs of 47, 38, and 17. These shale gas EROIs are compared to EROIs for other energy sources, including oil (16) and coal (80), as well as other EROI studies for natural gas. These EROIs are also put into the context of the “EROI cliff” or “EROI threshold,” which claims that EROIs less than approximately 10 are too low to meet energy demands of modern societies.