Using excess natural gas for reverse osmosis-based flowback water treatment in US shale fields Aritra Kar , Vaibhav Bahadur * Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, USA article info Article history: Received 21 June 2019 Received in revised form 8 February 2020 Accepted 11 February 2020 Available online 12 February 2020 Keywords: Reverse osmosis Natural gas Flowback water Shale oil Flaring Thermal desalination abstract This work addresses three significant issues associated with hydraulic fracturing in US shale fields: flaring/venting of excess natural gas, disposal of flowback water and freshwater procurement. Flaring/ venting of excess gas is a significant contributor to global emissions. This work presents a novel utili- zation concept, wherein excess gas is used onsite to power reverse osmosis (RO)-based treatment of flowback water to supply freshwater for oilfield operations. This study details technical and techno- economic analyses of the above concept. An analytical model is extended and improved to quantify RO-based freshwater production for flowback water of different salinities. The technical performance of RO systems is analyzed and compared with two competing gas utilization technologies (thermal desa- lination, atmospheric water harvesting). The use of these technologies in the top eight US shale fields is analyzed, and a techno-economic analysis of RO-based water treatment is conducted. Results indicate that this concept will significantly benefit the Eagle Ford and Niobrara shales. It can meet 200% of water requirements and reduce wastewater disposal by 60% in the Eagle Ford. Furthermore, such RO-based projects can have favorable payback periods of as low as one year. Importantly, this waste-to-value concept has worldwide relevance since the underlying issues are present globally. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Hydraulic fracturing has enabled large scale exploitation of shale reserves and positioned the United States (US) as the top oil producing nation worldwide [1]. While shale oil has been a global game changer in the energy landscape, there are significant nega- tive outcomes associated with hydraulic fracturing. Issues like the risk of earthquakes and groundwater contamination are well- publicized. This work addresses three other issues associated with hydraulic fracturing: flaring/venting of excess natural gas, sourcing water for hydraulic fracturing, and disposal of flowback water . Flaring of associated natural gas (co-produced with oil) is commonly employed worldwide in regions lacking gas collection, processing and transportation infrastructure. Estimates show that 140 billion cubic meters of natural gas was flared worldwide in 2015 [2], which is equivalent to 4% of the global production and 20% of domestic gas consumption in the US. Flaring in the US has increased by 4 times since 2000 [1], and the US is presently the 4th largest flaring country. The surge in flaring in the US can be attributed to the widespread use of hydraulic fracturing to produce oil from shale formations. Amongst US shale fields, the Bakken in North Dakota (primarily) and the Eagle Ford in Texas are respon- sible for 40% and 15% respectively of total flaring. Locally, the flaring percentages are significantly higher [3]. Another important consideration is the venting of natural gas during completion of gas wells. Vented gas emissions from shale wells are estimated to be two orders of magnitude higher than the emissions associated with conventional wells [4]. This is significant since methane is a much more potent greenhouse gas than CO 2 (product of flaring). The second issue addressed by this work is the water require- ment associated with hydraulic fracturing. The fresh water requirement per well ranges from 7.5 to 34 million liters [5], with an average of 9.5 million liters [7]; this is enough to fill four Olympic sized swimming pools [6]. Sourcing water is challenging since many shale fields are located in acute water stress regions [8]. Fifty percent of US Shale wells lie in extreme stress regions, where freshwater procurement and transportation costs can reach 3.2 cents/liter [9]. Fresh water is a critical bottleneck for shale oil production in many regions [2]. The use of brackish groundwater is a possible alternative to freshwater use; however, groundwater * Corresponding author. E-mail address: vb@austin.utexas.edu (V. Bahadur). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy https://doi.org/10.1016/j.energy.2020.117145 0360-5442/© 2020 Elsevier Ltd. All rights reserved. Energy 196 (2020) 117145