(317b) Shale Gas Wastewater Management Using Membrane Distillation: An Optimization Based Approach

Authors: 
Tavakkoli, S., University of Pittsburgh
Lokare, O., University of Pittsburgh
Vidic, R. D., University of Pittsburgh
Khanna, V., University of Pittsburgh
Unconventional shale gas industry offers tremendous economic benefits but is also accompanied by a number of environmental challenges. One such critical challenge is sustainable management of high salinity wastewater that returns to the surface after hydraulic fracturing and poses potential negative impacts on the surrounding ecosystems. Currently, about 90% of shale gas wastewater is being recycled for fracking purposes in Marcellus shale play. Recycling shale gas wastewater is a practical short-term solution to minimize total water use in the fracturing process, however, it may not be a viable strategy from a long-term management perspective especially when a well-pad becomes a net water producer. Another common strategy for shale gas wastewater management is direct disposal of the high salinity wastewaters into Class II disposal wells. However, this strategy is not cost effective and has come under scrutiny because of increased seismicity in regions close to class II disposal wells. Shale gas wastewater could also be sent to commercial centralized wastewater treatment (CWT) facilities. However, this strategy is not viable in the long term as the volumetric rate of shale gas wastewater to CWTs is limited to 1%. In order to choose the best strategy for integrated shale gas wastewater management for a specific shale gas extraction site, a holistic systems approach is required that takes into account associate cost of wastewater treatment, transportation, feasible treatment technology, and the potential beneficiary uses for the treated wastewater. A handful of recent studies have focused on shale gas water management using optimization techniques. However, the majority of existing work is focused on short term planning where shale gas wastewater could be recycled for future hydraulic fracturing requirements with little emphasis on long term planning for wastewater management. Furthermore, most conventional desalination technologies are not suitable for handling the high total dissolved solids (TDS) content of shale gas wastewater which could be as high as 350,000 mg/Liter.

We present a comprehensive optimization-based decision-making framework for guiding environmentally and economically conscious management of high salinity wastewater with application to the Marcellus shale play. We evaluate water management alternatives ranging from direct disposal in Class II injection wells to advanced centralized, decentralized, and onsite treatment options. We propose to use membrane distillation (MD) technology which holds great promise for desalination of high salinity wastewaters from shale production. MD is a thermally driven membrane-based process with relatively higher energy requirements compared to conventional desalination techniques such as reverse osmosis (RO) and forward osmosis (FO). However, the application of RO and FO is limited to wastewaters with TDS levels up to 40,000 and 70,000 mg/Liter, respectively. We performed techno-economic assessment (TEA) for shale gas wastewater treatment using MD technology with and without availability of waste heat. The results of TEA show that the major cost driver for a MD plant is the cost of thermal energy required for distillation process. We specifically focus on natural gas compressor stations (NG CS) in the U.S. where two-thirds of the energy input is currently rejected as waste heat. Preliminary results of our work reveal that only 56% of the available waste heat at NG CS in Pennsylvania is sufficient for desalination of shale gas wastewater generated in this state using MD technology. A mixed-integer non-linear programming (MINLP) model will be presented that optimizes the shale gas wastewater management. The model investigates optimal management strategies for wastewater by incorporating detailed treatment cost data obtained from TEA of MD as the treatment technology as well as detailed transportation cost data from shale gas sites to treatment or disposal facilities. We address the problem of finding the optimal design for shale gas wastewater management as well as determining the size and location of treatment facilities in order to minimize the total cost of handling wastewater. The developed optimization framework is applied to a case-study in Greene County in southwest PA where major shale gas development activities takes place. Preliminary results of our optimization model reveal that treating shale gas wastewater at onsite MD units at shale gas extraction sites is the most economically promising wastewater management option. Current work is conducting detailed scenario analysis to investigate how changes in production scenarios and regulatory environments may affect the choice of wastewater management alternatives. The implications of these findings for long-term management of shale gas wastewater will be described.