Water Supply Network And Shale Gas Development Water System Optimization

Water supply network is an important part of industrial and municipal water system.The design of water supply network considering multiple resources is an effective way to optimize the allocation of water resources and increase the anti-risk ability of water supply system.Both the source water quantity and the water demand of the user side of the conventional water supply network change periodically with time,which has a direct impact on the selection of water treatment units and the structure and operation of the whole water network in the design or operation of water supply network.Therefore,it is necessary to systematically study the design of multi-period water supply network.The large-scale development process of shale gas consumes a lot of fresh water and releases considerable wastewater at the same time.Design of a reasonable shale gas development water system is particularly important for optimizing the allocation of regional water resources,reducing the environmental impact of the development process,and achieving green sustainable shale gas development.For the optimal design of water supply network with multiple water resources,this thesis establishes a superstructure and corresponding mathematical model including conventional water sources and unconventional water sources,various raw water treatment technologies and various types of water users.To consider the choice of different raw water pretreatment techniques in the design and the impact of the municipal water stepwise water price on the system,this thesis adopts the disjunction programming modeling method.Based on the water supply system of a refinery in the coastal area,a case study was conducted.Through the comparative analysis of the optimization results of different scenarios,it was found that the best multi-resource water supply strategy is the combination of surface water and municipal treated wastewater instead of using a single water source,and the total annual cost was 12.07 million CNY,which was 7.31million CNY less than total annual cost of the municipal water supply alone as the water supply source.To solve the problem of water supply network optimization under multi-period water quantity change,this thesis establishes a multi-period water supply network optimization model.The model mainly considers the influence of multi-period water quantity change on raw water pretreatment technology and selection of capacity specification,and the processing unit operating cost penalty factor is used to quantitatively describe the impact of the operating state of the equipment on the operating costs.The comparison results in the two scenarios with or without considering the multi-period variation of water volume verify the validity of the proposed model,for example,the water price was 4 CNY/t,and the operating cost penalty coefficient was 0.3,for the situation without considering multi-period water volume variation,an 800 t/h reverse osmosis desalination unit was selected.However,in the case of multi-period situation,two reverse osmosis desalination units with treatment capacities of 500 t/h and 300 t/h were selected.This paper examines the impact of key parameters(water price,operating cost penalty factor)on system design.For the optimal design of the shale gas development water system,the previous model has neglected or simplified the pollutant balance and constraints,and the optimization results have certain limitations.This thesis proposes the superstructure and corresponding mathematical model of the shale gas development water system with total dissolved solids(TDS)as the key pollutant constraints.The model considers three wastewater treatment pathways including onsite wastewater treatment,commercial wastewater treatment center and reinjection well.The wastewater reuse between wellpads is also considered in the model.A case study was conducted on three wellpads in the Weiyuan shale gas development region in Sichuan,China,and the optimization results were compared and analyzed under various scenarios.The optimization results show that the total cost of the system is 6.990×10~5$,and the corresponding fresh water consumption is 3.132×10~5 m~3,without considering the wastewater reuse between the wellpads and the onsite wastewater desalination.The total cost is 4.702×10~5$,and corresponding fresh water consumption is 2.832×10~5 m~3 when all possible ways are considered.The results show that comprehensive consideration of onsite desalination treatment and wastewater reuse between wellpads can effectively reduce the cost of water management in the shale gas development process in Weiyuan,Sichuan,while achieving water-saving and emission reduction.By comparing with domestic and foreign shale gas development areas,the optimized average single well fresh water usage in Weiyuan,Sichuan is 15,078 m~3,which is similar to the median single well water consumption of 16,100 m~3 in the Eagle Ford,USA,and is much lower than the average single well fresh water consumption of24,415 m~3 in the Sichuan area.