At present, China is in the initial stage of shale gas development, and in order to reduce the water environment risks caused by the large-scale and sustainable development of shale gas in China and promote its sustainable development, it is particularly important to strengthen the management of shale gas fracturing flowback fluid treatment and disposal. In recent years, some scholars have studied the water environmental impact in shale gas development, and advocated the establishment and improvement of China's shale gas development environmental supervision laws and regulations from the aspects of environmental impact assessment system, environmental deposit system, and environmental risk emergency management system. This paper focuses on the protection of water environment in shale gas development, and puts forward the countermeasures for the management of fracturing flowback fluid in China by reviewing the relevant policies, laws and regulations on the disposal of fracturing flowback fluid in the process of shale gas development in China and the United States.
Schematic diagram of groundwater aquifer and aquifer.
Environmental pollution in the fracturing process of shale gas extraction
Schematic diagram of hydraulic fracturing (A) and flowback fluid Produced water TDS and δ18O over time (B).
Hydraulic fracturing technology is the core technology of shale gas development, and the hydraulic fracturing operation process needs to consume a large amount of water resources in a short time, and the number of drilling wells required in the mining process is large, and the water demand of a single well is large. Some of the injected hydraulic fracturing fluid will remain in the formation, but there will still be 5% to 80% of the injected water that will return to the surface after the fracturing fluid releases pressure, and these refluxed fluids usually contain proppantants, chemical residues, and various substances in formation water and cuttings. Surface water pollution from shale gas development is mainly caused by the complex composition of fracturing flowback fluid. Due to the long-term close contact between the flowback fluid and the shale reservoir, there are also substances other than the fracturing fluid components, such as petroleum hydrocarbon condensate and other organic matter to the nearby water body, which will pollute the surface water. The soluble chemical components of additives in fracturing fluids may also leach into groundwater, increasing the risk of groundwater contamination by dissolved chemicals such as toxic and harmful organics.
A large number of studies have been carried out in the United States on whether shale gas development will cause groundwater pollution, and it is found that the methane content in groundwater within 1km of Marcellus shale gas wells is much higher than that of groundwater outside 1km. U.S. shale gas development in Wyoming is a shallow subsurface, brine (high concentrations of sodium, chlorine, bromide and other inorganic components) and naturally occurring radioactive materials. The content of chemical oxygen demand (COD) and suspended solids (SS) in the water quality index of flowback fluid is often higher than the first-class standard limit in China's GB89781996 "Comprehensive Sewage Discharge Standard", and this kind of wastewater is difficult to treat due to the variety of pollutants and large salinity, and the fracturing fluid additive components are detected in the discharge water if it is not properly treated. Therefore, in order to reduce the risk of environmental pollution and maintain the green and sustainable development of the shale gas industry, it is necessary to effectively control the water environmental impact caused by the fracturing process. The following is a comparison and summary of the regulatory approaches and management bases in the United States and China.
U.S. regulatory approach and management basis
Regulatory authority in the United States is jointly owned by the federal and state governments, and the regulatory authority is tiered. Among the laws and regulations promulgated by the U.S. federal **, the main laws and regulations related to water environmental protection are the Clean Water Act, the Safe Drinking Water Act and the Oil Pollution Act.
Among them, the Clean Water Act uses the National Pollutant Discharge Elimination System permit process to control the possible impact of oil and gas exploitation on the water environment from the perspective of sewage discharge permits and total volume control. In 2009, the Federal Fracturing Liability and Chemical Awareness Act was introduced, which further clarified that the regulatory framework for underground injection control (UIC) includes underground injection materials and support media used in oil and gas development, and required companies to disclose the chemical composition of hydraulic fracturing fluids to monitor and regulate the use of various chemicals in fracturing fluids.
The environmental aspects of shale gas development in the United States are delegated to the states under agreements and plans approved by the relevant federal** agencies. U.S. states may enforce regional, state-specific regulatory standards if state standards are equally protective or more protective to meet local environmental requirements.
The states in the United States have distinctive characteristics in the implementation of environmental supervision, the advantages of great initiative, the state ** and the development area of the environmental regulatory agencies to carry out environmental supervision according to local conditions, put forward specific regulatory policies and regulations suitable for the development of blocks, so as to make up for the lack of supervision of specific links of hydraulic fracturing in federal law, some states have established a more strict and perfect, each applicable regulatory regulations, systems, standards system. For example, 14 states in the United States require shale gas developers to submit and disclose information about the chemical composition of hydraulic fracturing fluids to the public on the ** basis without divulging trade secrets, with Montana and Wyoming requiring shale gas developers to voluntarily disclose the chemical content of the fracturing fluids used.
U.S. states, ** and shale gas developers treat flowback fluids in line with the principle of protecting surface and groundwater resources and even reducing future demand for fresh water, and by following the pollution prevention and control principles of "reduce, reuse, recycle", there are three main ways to treat and manage the flowback fluid: underground injection, utilization and standard discharge.
Under the U.S. Regulatory Framework for Monitoring Underground Injection (UIC), Montana and Wyoming require shale gas developers to voluntarily disclose the amount of fracturing fluid chemicals used.
According to the EPA report, most of the liquids from Class II injection wells are brine brought to the surface during oil and gas production, which is naturally occurring in the formation, is generally high in salinity and may contain naturally occurring radioactive materials, which can harm the environment and public health if discharged into surface water or the surface of the land.
To prevent contamination of land and surface water, Class II injection wells allow for co-layer reinjection. According to the N**igant Consulting report, Class II injection wells have become the preferred treatment of flowback fluids in hydraulic fracturing operations, taking into account environmental factors such as the cost of transporting, disposing of, and off-site treatment of flowback fluids subject to surface wastewater treatment, as well as environmental factors such as geological conditions, as well as treatment regulations adopted by states to prevent brine contamination of surface water and soil.
However, the use of underground injection to treat flowback fluids is not available in all shale gas development areas, as some areas are not injectable, such as Pennsylvania with only five Class II injection wells for underground injection of flowback fluids, while Texas and Colorado are more suitable for Class II injection wells. In New York and Pennsylvania, permits are obtained from the U.S. Environmental Protection Agency and environmental agencies to treat flowback fluids through deep well injection.
Before the flowback solution is used, it needs to be evaluated by the service company and the developer to determine the degree of treatment of the flowback solution and the amount of water required to recharge. In general, the on-site treatment equipment and technical methods are determined according to the flow rate and total volume of the flowback liquid to be treated, the components to be removed and their contents, the goals to be achieved by treatment, and the requirements for reuse and discharge.
Flowback fluids are usually treated to the point where they meet the requirements of hydraulic fracturing or can be used as other industrial water, and do not need to be treated to the point where they can be discharged to surface water or used as drinking water.
Although the reuse of flowback fluid can save water, reduce the need for other treatment methods, and solve the problems related to water withdrawal, in order to improve the performance and cost savings of fracturing fluids configured with flowback fluid, research is being conducted in the United States to find new methods and technologies to make the best utilization of flowback fluid more costly.
In the U.S., when the flowback fluid contains contaminants that require further treatment and cannot be discharged to surface water, service companies will have to transfer the flowback fluid to a wastewater treatment plant. Operators of U.S. industrial wastewater treatment facilities will treat wastewater according to the treatment facility's effluent permit standards before discharging it into a nearby receiving water body.
China's regulatory approach and management basis
Unlike North America, China is currently in the early stage of shale gas development, and most of the exploitation areas are concentrated in ecologically fragile and densely populated sensitive areas, and due to the differences in the legal and regulatory systems of China and the United States, there are also differences with the United States in terms of water environment management related to shale gas exploitation (Table 1).
Table 1 Similarities and differences in water environmental protection management in shale gas development in China and the United States
China's shale gas development environmental regulatory agencies are divided into two levels: the Ministry of Ecology and Environment, the Ministry of Natural Resources, the National Development and Reform Commission, the State Administration of Taxation, and the Ministry of Finance.
Among the various laws and regulations promulgated by China on the protection of the water environment, the main ones related to shale gas exploitation are the Law of the People's Republic of China on Environmental Protection, the Law on the Prevention and Control of Water Pollution, the Law on Environmental Impact Assessment, and the Technical Policy on Pollution Prevention and Control in the Oil and Gas Exploitation Industry. In general, the environmental regulations and standards related to the environmental supervision of shale gas development are scattered in the relevant laws and regulations, and there is no specific solution to the special ecological and environmental impact problems generated in the process of shale gas development, such as hydraulic fracturing technology and the flowback fluid with high salinity.
In addition, according to the National Energy Administration's Shale Gas Development Plan (20162020), although China emphasizes the need to significantly increase shale gas production while paying attention to ecological protection, it has not yet planned environmental protection policies for the shale gas industry from the perspective of pollutant discharge permits and total volume control.
The treatment of shale gas fracturing flowback fluid in China mainly adopts the method of "large amount of reuse, small amount of reinjection and discharge". In general, both China and the United States use three methods of reinjection, reuse, and efflux to treat and dispose of fracturing flowback fluid, but there are similarities and differences in the implementation of fracturing flowback fluid due to the different management bases of the two countries (Table 2).
Table 2 Management characteristics of fracturing flowback fluid for shale gas extraction in China and the United States
In terms of underground injection of fracking flowback fluids, the U.S. is regulated under the regulatory framework for monitoring underground injection at the federal regulatory level, while China mainly relies on energy industry standards and oil and gas company standards for reinjection. China requires that the target layer of fracturing flowback fluid reinjection in shale gas development should be a closed formation of geological structure, and should generally be reinjected into active oil and gas reservoirs or depleted abandoned oil and gas reservoirs.
The extraction wastewater for reinjection treatment shall be in accordance with the energy industry standard NB T140023 2015 "Shale Gas Reservoir Renovation Part 3: Fracturing Flowback Fluid** and Treatment Methods", Oil and Gas Industry Standard SY T6596 2016 "Technical Requirements for Gas Field Water Injection" and Oil and Gas Company Enterprise Standard Q SY01004 2016 "Technical Specification for Gas Field Water Reinjection".
In recent years, environmental authorities in some regions have been cautious about the reinjection method and have put forward strict environmental supervision requirements for the reinjection treatment method. For example, the Notice on Further Strengthening the Management of Environmental Impact Assessment in the Oil and Gas Industry issued in December 2019 mentions that if wastewater reinjection is involved, the environmental feasibility of reinjection should be demonstrated, feasible groundwater pollution prevention and control and monitoring measures should be taken, wastewater unrelated to oil and gas exploitation should not be reinjected, and groundwater pollution should not be reindivided; For example, the "Sichuan Shale Gas Exploitation Pollution Prevention and Control Technology Policy" promulgated in February 2018 proposes that "for the reinjection treatment of fracturing flowback fluid, the integrity of the reinjection well, the sealing, isolation and feasibility of the injection layer, and the compatibility of the fracturing flowback fluid and the injection layer should be fully considered to ensure environmental safety." The relevant procedures for the reinjection well must be complete, and the operation monitoring and management system must be sound."
For example, in the "Guiding Opinions on Environmental Protection in Chongqing Shale Gas Exploration and Development Industry (Trial)" promulgated in 2016, it was proposed for the first time that "fracturing flowback fluid is used to configure fracturing fluid, and if the reuse is not exhausted, it should be discharged after the treatment reaches the standard, and it is strictly forbidden to secretly discharge, dilute or re-inject".
The reuse of fracturing flowback fluid in China is mainly in accordance with energy industry standards and local industrial policies, while the reuse of fracturing flowback fluid in the United States is mainly based on commercial market demand.
At present, a number of environmental protection technology policies have been introduced in China to regulate the prevention and control of water pollution in the oil and gas extraction industry. According to a large number of on-site factory application data, in 2015, the National Energy Administration issued N-B T140023 2015 "Shale Gas Reservoir Renovation Part 3: Fracturing Flowback Fluid** and Treatment Methods" stipulates the water quality requirements for wastewater reuse and treatment such as fracturing flowback fluid.
In 2018, the "Sichuan Shale Gas Exploitation Industry Pollution Prevention and Control Technology Policy" was issued, which clearly pointed out that drilling wastewater and fracturing flowback fluid should be prioritized for reuse, and the reuse rate of platform drilling wastewater and platform or regional fracturing flowback fluid should reach more than 85%.
In the process of building the Weiyuan National Shale Gas Demonstration Zone in Changning, Sichuan, CNPC focused on strengthening the recycling of fracturing flowback fluid to reduce water consumption, and adopted the methods of intra-platform reuse and intra-regional platform reuse to reduce the amount of clean water taken and disposed of by fracturing construction, which is in line with the requirements of "the reuse rate of fracturing flowback fluid should reach more than 85%" in the "Sichuan Shale Gas Exploitation Industry Pollution Prevention and Control Technology Policy" issued in 2018.
There is still a lack of wastewater discharge standards for the shale gas extraction industry in China. In China, the effluent after treatment refers to the ability of the flowback liquid to meet the requirements of the corresponding water quality standards and indicators after being treated by the sewage treatment plant. The reuse rate of fracturing flowback fluid usually does not reach 100%, especially when the discharge period is not the period of fracturing operation at the well site, and the storage pool cannot meet the long-term storage of a large amount of flowback fluid for reuse, which will cause a large amount of fracturing flowback fluid to be discharged.
From the perspective of environmental management and policy orientation, reinjection is not conducive to environmental management, only temporarily controls pollution, and does not fundamentally eliminate pollution from the source.
At present, the discharge requirements of the flowback fluid in some areas of China meet the discharge limit of the first-class standard of GB89781996 "Comprehensive Sewage Discharge Standard", in which the type of sewage industry to which the fracturing flowback fluid belongs is not specified, nor is the limit of the sewage discharge standard for the shale gas extraction industry given.
With the improvement of national requirements for ecological and environmental protection, more and more provinces have restricted the content of salt or chloride, and local policies such as Guizhou, Yunnan, and Sichuan have required centralized disposal and advanced treatment of fracturing flowback fluid to meet emission standards.
In addition, a pollutant discharge permit from the competent environmental authority must be obtained before the fracturing flowback fluid is discharged, and the crystalline salt produced by the fracturing flowback fluid desalination treatment process must be handed over to an enterprise with hazardous waste disposal qualifications for disposal.
In 2018, Sichuan Province issued the "Sichuan Provincial Shale Gas Exploitation Industry Pollution Prevention and Control Technology Policy", which pointed out that if there are conditions for centralized treatment in the region, it can be included in the centralized sewage treatment plant for treatment; If there is no centralized treatment condition in the region, it can be discharged after reaching the relevant standards, and the management of chloride can refer to the relevant provisions in GB50842005 "Farmland Irrigation Water Quality Standard".
At present, there is no accurate wastewater discharge standard for the shale gas extraction industry, and there are no exact regulations on the chloride content limit in the efflux of fracturing flowback fluid in various standards and normative documents at all levels.
Guangdong Huanmei
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Characteristics of shale gas drilling wastewater
The quality of the raw water of the fracturing flowback fluid.
The flowback fluid has the characteristics of large yield change, complex composition, instability, refractory degradation, high COD, high TDS, and high hardness.
Fracturing flowback fluid efflux treatment technology
The efflux treatment technology must include the removal of SS, oils, COD, ammonia nitrogen, hardness and TDS, and the difficulty and focus is to separate the high TDS from the water to meet the efflux standard.
According to the external drainage standard, the efflux treatment includes air flotation, catalytic oxidation, softening and precipitation, ultrafiltration, concentration, evaporation crystallization and advanced treatment, which can not only meet the efflux standard, but also put the NaCl in the flowback solution as a product. The isolated NaCl can be used as a chemical raw material and animal husbandry salt, which achieves the purpose of valuable material recycling. The advanced treatment device in the standard efflux process is mainly increased for the use of different membrane concentration processes or different evaporation conditions to cause non-compliance, but different advanced treatment devices need to be set up according to different water quality. If an electrodialysis device is used, a fresh water COD removal device needs to be added; If the evaporation crystallization evaporates under alkaline conditions, and the ammonia nitrogen enters the condensate, a deammonia nitrogen removal device needs to be added. Due to the high standard of fracturing flowback fluid, the treatment process is more complex and the process is longer, so it is advisable to collect the flowback liquid and build a sewage plant for centralized treatment.
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