Assessing effects of shale gas extraction in water-scarce region

The mining of shale gas through the process of hydraulic fracturing is frequently linked to water quality and quantity concerns, particularly in the arid and semi-arid areas of the western United States. Unlike wells located in more water-abundant regions of the eastern U.S., such as the Marcellus Shale in Pennsylvania and New York, the Wattenberg Shale in northeastern Colorado is located in a region that is not as well endowed with water resources. Located near the Front Range of the Rocky Mountains, this gas field must compete with agricultural and municipal water-users in an already quantity-constrained system, making the relative water demand of extraction an important consideration.
 
In a recent study, researchers from Colorado State University and Noble Energy, a U.S.-based oil and natural gas exploration and production company, sought to determine the total water use from hydraulic fracturing wells in the Wattenberg, and the water use intensity (WUI) of the wells. Water use intensity describes the volume of water input per the service or materials output. In this case, the water use intensity of shale gas extraction is the volume of water divided by the net energy recovery. Using well data from 200 randomly sampled wells operated by Noble Energy in the Wattenberg Field, the team calculated the consumptive water use as well as the energy recovered versus energy used.
 
Writing in the journal Environmental Science & Technology, they report that the Wattenberg wells have a WUI of 1.8 to 2.9 gallons per million British thermal units (Gal/MMBtu), roughly equivalent to an average shale gas well, though significantly higher than the 1.3 Gal/MMBtu in the Marcellus. For comparison, conventional natural gas has a WUI of 1.5 and surface coal is between 1 and 4. More importantly, perhaps, is the total water consumption, for which the median is nearly 3 million gallons per well. Multiplied by hundreds of wells, total shale gas extraction water use in the Wattenberg is in the vicinity of a billion gallons or more, and in a basin that is actively seeking new water sources to meet existing demand.
 
Context might be the most important factor in evaluating the water use of extractive processes. It is difficult to compare the relative impact of total consumption and WUI of different geographies without first knowing how water-abundant they are and how much relative demand already exists. As technology improves efficiency and treatment options, physical water availability should become less of a constraint and effluent will be more available for productive reuse. Of course, the only way to guarantee a decrease in extractive water consumption in a given region is to reduce extraction. However, current energy realities do not provide clear support for near-term reductions and the sunk costs of existing wells make it unlikely that they will be decommissioned.