The tradeoff between water and carbon footprints of Barnett Shale gas

https://doi.org/10.1016/j.jclepro.2018.06.140Get rights and content

Highlights

  • Computed the life cycle water and carbon footprints of shale gas production.

  • Four scenarios for wastewater management were compared.

  • A lower water scarcity footprint comes at a cost of higher global warming potential.

  • Desalination reduces the water scarcity footprint by 28 percent.

  • Desalination and reuse of produced water increases the carbon footprint by 38 percent.

Abstract

Shale gas production is a water and energy-intensive process that has expanded rapidly in the United States in recent years. This study compared the life cycle water consumption and greenhouse gas emissions from hydraulic fracturing in the Barnett region of Texas, located in one of the most drought prone regions of the United States. Four wastewater treatment scenarios were compared for produced water management in the Barnett region. For each scenario, the cradle-to-gate life cycle global warming potential and water scarcity footprint was estimated per mega joule of gas produced. The results show a trade-off between water and carbon impacts, because energy is required for treatment of water. A reduction of 49 percent in total water consumed or a 28 percent reduction in the water scarcity footprint in the shale gas production process can be achieved at a cost of a 38 percent increase in global warming potential, if the wastewater management shifted from business as usual to complete desalination and reuse of produced water. The results are discussed in the context of wastewater management options available in Texas.

Introduction

Energy is required to provide water, and water is often required to provide energy. Currently 15 percent of the total world water withdrawals are used for energy practices, out of which 11 percent is consumed during energy production. By 2035, water withdrawals are predicted to increase by 20 percent and water consumed during energy production is predicted to increase by 85 percent. These trends are due to a shift towards higher efficiency power plants with more advanced cooling systems that reduce withdrawals, but increase consumption per unit of electricity produced, and by the expansion of biofuels production (IEA, 2012).

Variability in water resources due to extreme temperatures and uncertain precipitation conditions is anticipated for most regions of the United States in the future (Roy et al., 2012), increasing the importance of accounting for water use in energy production. As water-intensive energy technologies become more widespread, water treatment and/or reuse may help to reduce the strain on water resources. However, handling and treating wastewater is an energy-intensive process. Hydraulic fracturing (fracking) is an example of the type of water-intensive energy technology that has grown rapidly in recent years. Fracking is the process of injecting pressurized water and chemicals in a subterraneous rock (i.e. shale) to create fractures that release natural gas or oil to the surface (EPA, 2015). A single fracking well in the Barnett shale of Texas is estimated to require an average of 15 million liters of water over its lifetime, and at the current rate, 80 to 95 percent of that will be discharged as wastewater (Clark et al., 2011; EPA, 2015) as up to 20 percent of the produced water is treated and reused depending on the logistics and economics of treating high volumes of water and the proximity of injection wells to the production site (Mantell, 2011).

Because of the rapid growth in fracking, efforts have been made to apply Life Cycle Assessment (LCA) to enhance understanding of the environmental implications of this technology. Two key indicators evaluated in such assessments are the greenhouse gas emissions (GHG) and the water scarcity footprint (WSF) of the shale gas production process. With regard to water consumption, prior analyses have largely focused on the Marcellus shale in Pennsylvania (Jiang et al., 2014, 2011; Laurenzi and Jersey, 2013). Although Grubert et al. (2012) have quantified the freshwater consumption for the entire natural gas extraction process in Texas, they only report the estimated cradle to gate values for water consumption per shale play based on their estimated ultimate recovery. This paper highlights how much water is consumed at every life stage of shale gas extraction process, from cradle to gate in a single well, with respect to water scarcity in the watershed, for four disparate wastewater management scenarios. While this study is specific to shale gas production the United States, this issue is also relevant to other geographical locations such as Chang et al. (2014) studies the potential energy and air pollution implications of shale gas production in China, and Tagliaferri et al. (2015) looks at the impact of shale gas production on watersheds in the UK.

In this study, we estimated the global warming potential of Barnett shale gas production from fracking and the water scarcity footprint based on the inventory of water, energy and materials consumed to produce a mega joule of shale gas, from a well's construction to its closure. This ‘cradle to gate’ study is based on the format developed by Jiang et al. (2011, 2014), but we defined and compared a range of wastewater management scenarios in order to understand the impact of wastewater management option used on the overall carbon and water scarcity footprint of shale gas production, given the current state of technology in the Barnett shale play. We apply a consensus-based midpoint water scarcity method, consistent with the ISO 14046 standard, to determine the water scarcity footprint and the associated water deprivation potential (Boulay et al., 2017). Water degradation and quality aspects of use in shale gas production are outside the scope of this study.

Section snippets

Background

The Barnett shale is a hydrocarbon producing, geological system in north central Texas and southwestern Oklahoma. The Barnett shale play was the first in the United States (U.S.) to have shale gas extracted by hydraulic fracturing in 1983 (Wood et al., 2011). In 2013, the Barnett shale produced 20 percent of the total U.S. shale gas (Nicot et al., 2014). Barnett shale is a high “long term produced water” generating play, which means that due to the presence of water in and around the shale, on

Shale gas production process

We developed a shale gas production process in the SimaPro v8.2.0, LCA software package (PRé-sustainability, 2014), which links the types and quantities of the various inputs and emissions to relevant upstream process data contained in the U.S. Life Cycle Inventory Database (National Renewable Energy Laboratory, 2012) and the Ecoinvent v3 (Weidema et al., 2013) databases. These databases provide data for energy and material flows associated with producing a material, component, or assembly. The

Barnet shale water production and management

After fracturing a well in Barnett, anywhere from 15 to 20 percent of the original volume of the fluid will return to the surface within the first 10 days as flowback water (Mantell, 2011). On average 1.1 million liters of water is produced per fracturing job in the Barnett shale (Clark et al., 2011). Additional water, equivalent to anywhere from 10 percent to almost 300 percent of the injected volume returns to the surface as produced water over the life of the well (Clark et al., 2012). While

Results

Complete underground injection (Scenario 2) has the highest inventory water consumption and WSF per unit of shale gas produced (Fig. 2). Complete desalination and reuse (Scenario 4) has the lowest inventory water consumption and WSF but the highest GWP, due to the additional energy inputs required for transporting and treating the produced water. The scenarios that use more energy intensive processes such as desalination have a higher GWP per unit of gas produced but with the trade-off effect

Discussion

The Barnett shale play has an extensive disposal infrastructure that allows operators to inexpensively dispose of flowback and produced water, thus developing and implementing a cost-competitive sustainable water management program using recycling is challenging (Werline, 2011). Deep well injection of produced water requires relatively little energy for transportation and injection of the water into disposal wells (Mantell, 2011). By contrast, the energy requirements to treat Barnett shale

Conclusion

This study takes an in-depth look at the energy water nexus in the context of hydraulic fracturing in a drought prone and natural gas rich region of Texas. Globally, the oil and gas industry uses far less water than agriculture or power generation, though it can be a significant user of water at the local level. Shale gas production is a water intensive process. As climate change affects the availability of water resources, so do other stressors like urban population growth, economic

Acknowledgements

This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of

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