Shale Gas Flowback Research Articles

Electrochemical analysis of ion-exchange membranes fouling during electrodialysis treatment of real shale gas flowback water

Electrodialysis (ED) is considered an effective technology for desalinating shale gas flowback fluid toward environmental risk control and resource recovery. However, membrane fouling remains a significant challenge and the particular fouling behavior of ion-exchange membranes (IEMs) in real scenario remains unclear. In this study, we employed interface characterization techniques including electrochemical impedance spectroscopy (EIS) to investigate the fouling behavior of both anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) during the desalination process. Notably, a positive correlation was observed between the transmembrane electric potential (TMEP) of the target membrane and the extent of membrane fouling during ED processing. EIS data revealed the membrane bulk exhibited the most pronounced increase in modeled electrical resistance, suggesting that internal fouling more significantly inhibited mass transfer compared to external fouling. Under varying current densities, the resistance of the AEM and CEM membrane bulk increased from an initial value of 3.93–16.53 Ω cm2 and from 3.90 to 9.87 Ω cm2, respectively, corresponding to their maximum TMEP increase by factors of 6.0 and 2.0 during the ED treatment. Our results demonstrate that AEM undergone a more severe organic fouling than CEM (scaling) during desalination of real shale gas flowback fluid. Effective strategies to mitigate the fouling issue in IEMs were finally proposed. These findings provide valuable guidance on future process optimizing and fouling mitigation in the ED treatment of real shale gas flowback fluid.

Rapid cultivation of aerobic granular sludge for shale gas flowback water treatment by bioaugmentation with inoculating multifunctional fungal pellets

Aerobic granular sludge (AGS) technology showed great potentials in treatment of hypersaline industrial wastewater such as shale gas flowback water (SGFW), while long start-up period was still a major bottleneck. To both strengthen its granulation and pollutant removal efficiency, herein, one multifunctional fungus Aspergillus sydowii NCSL-XY1 was first isolated with the capacities including formation of fungal pellets, salt tolerance, and efficient degradation of organics in SGFW. Subsequently, its appropriate fungal pellets were used to try to shorten the start-up period of AGS in SGFW. The results showed that inoculating fungal pellets evidently accelerated the formation of AGS from 41 to 16 days. Also, it could enhance the TOC removal from ∼30% to ∼45% in the initial start-up period (Day 2-Day 11). Mechanisms analysis showed that the inoculation of fungal pellets increased EPS secretion and microbial community diversity. When the reactor stabilized, both of the AGS in R1 and R2 had similar D50 (∼1200 μm), SVI30 (∼30 mL/g), and SVI30/SVI5 (∼0.95). Also, the TOC and PAM removal in R1 and R2 were also similar and maintained at ∼71% and ∼20%. Overall, the inoculated fungal pellets played significant roles of both biomass carrier and synergistic degrader in facilitation of AGS start-up, while it had limited functions after the granules maturation. This study provided a universally effective bioaugmentation mode (screening degrading fungus and inoculating its fungal pellets) for AGS cultivation in treatment of refractory industrial wastewater.

Long-term and efficient treatment of shale gas flowback wastewater by the novel double SEP@Fe-Mn/RGO composite membranes method

The long-term and efficient treatment of shale gas wastewater is an inevitable tendency. This study employed the SEP@Fe-Mn/RGO composite membrane series for the membrane separation treatment of shale gas wastewater. Different proportions and different function of SEP@Fe-Mn/RGO composite membranes were obtained through the method of electrostatic self-assembly. This study researched the “Trade-off” issue of membranes separation through the analysis，resulting in membranes that best meets the separation requirements for shale gas wastewater. After two membrane separation, The oil-water separation efficiency (COD 2040 mg/L and TOC 487.14 mg/L) of shale gas wastewater was >99.3 %. The removal rate of the main salt component (Na2SO4) reached 83.56 %, and the removal rate of major heavy metal ions (Fe3+, Cr3+, Ba2+) was above 60 %. Because of the continuous generation of in-situ bubbles by the reaction of MnO2 with H2O2, and Fenton reactions on the membrane, which further accelerated the separation of pollutants. In a variety of harsh water environments, this membrane series remained stable within 12 h. Simple to prepare, cost-effective, low in energy consumption, and environmentally friendly, which makes it promising for wide applications in the field of petroleum water treatment.

Performances and mechanisms of the peroxymonosulfate/ferrate(VI) oxidation process in real shale gas flowback water treatment

Shale gas flowback water (SGFW), which is an inevitable waste product generated after hydraulic fracturing during development, poses a severe threat to the environment and human health. Managing high-salinity wastewater with complex physicochemical compositions is critical for ensuring environmental sustainability of shale gas development. Desalination processes have been recommended to treat SGFW to adhere to the discharge limits. However, organic fouling has become a significant concern in the steady operation of desalination processes, and the effective removal of organic compounds is challenging. This study aimed to develop an effective oxidation method to mitigate membrane fouling in real SGFW treatment process. It adopted the peroxymonosulfate (PMS)/ferrate (Fe(VI)) process, involving both free and non-free radical pathways that can alleviate the negative effects of high-salinity environments on oxidation. The operating parameters were optimized and removal effects were examined, while the synergistic oxidation mechanism and organic conversion of the PMS/Fe(VI) process were also analyzed. The results showed that the PMS/Fe(VI) process exhibited a synergistic effect compared with the PMS and Fe(VI) processes alone, with a total organic carbon (TOC) removal efficiency of 46.8% under optimal reaction conditions in real SGFW. In the Fe(VI)/PMS process, active species such as Fe(V)/Fe(IV), ·OH, and SO4−· were jointly involved in the oxidation of organic matter. Additionally, 99.5% of the total suspended solids and 95.2% of Ba2+ in the SGFW were removed owing to the formation of a coagulant (Fe3+) and SO42− during the reaction. Finally, an ultrafiltration membrane fouling experiment proved that oxidation processes can increase the membrane-specific flux and alleviate fouling resistance. This study can serve as a reference for the design of real SGFW treatment processes and is significant for the environmental management of shale gas development.

Molecular comparison of organic matter removal from shale gas flowback wastewater: Ozonation versus Fenton process

Shale gas extraction process generates a large amount of shale gas flowback wastewater (SGFW) containing refractory organic compounds, which can pose serious environmental threats if not properly treated. However, the extremely complex compositions of organics in SGFW are still unknown and their transformation pathways in O3- and •OH-dominated systems are not well recognized, which restrain the selection of treatment technology and optimization of operational parameters. The removal characteristics and reaction mechanism of dissolved organic matter (DOM) in SGFW treated by ozonation and Fenton processes were comparatively investigated using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results showed that both processes could degrade low-oxygen highly unsaturated and phenolic organics, polyphenolics and polycyclic aromatics, and transform them into aliphatic organics and high-oxygen highly unsaturated and phenolic organics. With increasing action of reactive oxygen species (O3 for ozonation and •OH for Fenton process), the degradation products (mainly aliphatic organics) increased during ozonation. However, in Fenton process, a wider range of DOM was removed without aliphatic organics accumulation. The degradation mechanisms of DOM during ozonation and Fenton processes included oxygen addition reactions (+3O, +H2O2, and +2O) as dominant pathways. However, ozonation showed more violent oxygenation, hydroxylation, and carboxylation, while Fenton process presented more violent chain-breaking reactions. These results revealed the selective oxidation of ozone and nonselective oxidation of •OH during SGFW treatment, and provided theoretical support for selecting SGFW treatment approaches.

The combination of precipitative softening and ozonation as a pretreatment of ultrafiltration in flowback water treatment: performance and fouling analysis

ABSTRACT Ultrafiltration (UF) technology is an efficient shale gas flowback water treatment method. However, severe membrane fouling is the primary restriction on the application of UF technology. Here, we studied the impact of three pretreatments: precipitative softening (PS), precipitative softening, followed by ozonation (PS-O) and ozonation, followed by precipitative softening (O-PS), on pollutants’ removal efficiencies and membrane fouling. The results showed that (1) the hardness, bacteria, scaling trend and compatibility with formation water exceeded the requirements for water reuse; (2) pretreatments effectively increased water flux and prolonged ultrafiltration membrane life, and both of them followed the order of PS-O process > O-PS process > PS process; (3) the fouling mechanism was changed from the complete blocking model to the standard blocking model by the PS process and the addition of ozonation enhanced the correlation of standard blocking model; (4) the quality of fracturing liquid prepared by the effluent treated by the PS-O-UF process was the best and satisfied the requirements of slick water. This paper indicated that the PS-O-UF process was suitable for the treatment of Changning shale gas flowback water for reuse.

Performances and mechanisms of ferrate(VI) oxidation process for shale gas flowback water treatment

The aim of this study was to determine effective and green methods for the removal of organic matter from shale gas flowback water (SGFW). The effects of ferrate (Fe(VI)), initial pH, and reaction time on the removal efficiency of Fe(VI) were investigated. A chemical oxygen demand removal efficiency of 57 % was achieved under the optimal conditions of Fe(VI)= 1500 mg/L, pH= 8.0, T = 25 °C, 60 min. Combined with the results of the chemical probe method, competitive kinetic experiments, electron spin resonance spectra, and radical quenching studies, hydroxyl radicals (∙OH) were demonstrated as the dominant reactive species responsible for oxidation. Furthermore, the fluorescence intensity of soluble microbial by-product-like matter and acid-like components were found to be reduced by 64 % and 43 %, respectively, using fluorescence excitation-emission matrix spectroscopy tests. The results of gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry indicated that electron-rich organic compounds were easily removed by Fe(VI) oxidation. Bacterial culture and coagulation experiments showed that Fe(VI) had both disinfection and coagulation effects in the SGFW. Furthermore, considering H2O2 as a precursor for ∙OH generation, the combined H2O2-Fe(VI) process was investigated in this study, and the results showed that the process has a synergistic effect and can increase the Chemical Oxygen Demand (COD) removal up to 68 % (Fe=1500 mg/L, pH=8.0, H2O2 =0.05 mol/L, T = 25 °C). Finally, an ultrafiltration membrane fouling experiment proved that the oxidation processes can increase the membrane-specific flux and alleviate fouling resistance. This study provides a reference for the design and operation of organic removal process in the SGFW treatment.

Multifunctional carbon aerogel granules designed for column reactor for efficient treatment of shale gas flowback and produced water

Shale gas flowback and produced water (SGFPW) greatly hinders sustainable shale gas development and can cause severe environmental threats without proper treatment. Aerogel is an advanced adsorption material for treating SGFPW with complex components; nevertheless, there is little investigation on the aerogel adsorbent designed for SGFPW treatment on-site. Herein, we report a facile and green method for preparing carbon aerogel granules (CAGs) with porous biochar aerogel (PBA) incorporated on physically cross-linked chitosan matrix for high-performance treatment of SGFPW. CAGs exhibit good elasticity with Young’s modulus of 49 kPa for practical use and ultra-high water adsorption ratio of 1936.7 % for fast liquid-phase mass transfer of pollutants from SGFPW to CAGs. CAGs show high removal rates for various organic pollutants in SGFPW, including ≥ 36.6 % of dissolved organic carbon (DOC), ≥ 72.2 % of UV254, and ≥ 92.2 % of total fluorescent organic compounds within 60 min. Meanwhile, CAGs allow steady adsorption of heavy metals such as Zn (18.8–21.7 %), Co (20.1–23.7 %), Cu (18.4–29.8 %), Pb (32.5–62.1 %), and Al (54.8–58.1 %) in SGFPW. The adsorption of dissolved organic pollutants in SGFPW by CAGs is mainly based on their π-π electron-donor–acceptor (EDA) interaction with the PBA carbon skeleton, while the adsorption of heavy metals based on their complexation with the N and O atoms of PBA. CAGs are suitable for the fixed bed to treat SGFPW, which displays good binary functions of filtration and adsorption, showing high potential for treating SGFPW on-site.

Study on Double Pressure Funnels and Gas-Liquid Two-Way Mass Transfer after Fracturing in Shale Gas Reservoirs

Well shut-in and drainage after shale gas fracturing are important factors affecting the productivity. Due to the imperfect optimization method of shale gas flowback, there has been no clear explanation for the problems such as “formulation of reasonable well shut-in time” and “less fracturing fluid flowback but high-gas production phenomenon” during shale gas drainage. In this paper, the double pressure funnels (one funnel is formed during fracturing by pressure difference from wellbore to formation, and two funnels are formed during flowback by pressure difference from fracture to formation and from fracture to wellbore) and gas-liquid two-way mass transfer (gas transfer by diffusion and liquid transfer by pressure difference) in shale gas drainage are investigated by calculating the pressure distribution after fracturing shale gas wells. The discrete numerical simulation by using unstructured PEBI grid is conducted, and the result is as follows: when shale gas well is shut-in for 20 days and produce for 1 year, the daily gas production corresponding to fracturing fluid flowback rates of 20%, 10%, and 5% are 47700 m3, 5800 m3, and 72700 m3, respectively. The investigation of double pressure funnels and gas-liquid two-way mass transfer explains clearly the phenomenon “less fracturing fluid flowback but high-gas production.” Meanwhile, the two conditions for optimizing the well shut-in time after fracturing are presented. That is, as for the studied case, the moving speed of the pressure boundary line should be less than 0.1 m/d, and the water-gas ratio near the fracture should be less than 1 / d with time. Consequently, the reasonable well shut-in time is optimized to be 20-25 days. The findings in this work are of benefit to enrich the flowback theory of shale gas after fracturing and provide a theoretical basis for the optimization technology of shale gas drainage after fracturing.

Understanding the corrosion of pipe steel due to the adhesion of sulfate reducing bacteria film combined with the deposit layer

In the production of shale gas, solid particles such as gravel and clay can adhere to the pipe wall surface and generate under-deposit corrosion (UDC). Meanwhile, the adhesion of sulfate reducing bacteria (SRB) film can further aggravate the local corrosion problem, which would easily result in the failure of pipe lines. Therefore, understanding the corrosion of the steels when SRB film and deposit layer are co-existed is of highly significance. In this work, UDC in SRB-containing shale gas flowback water was studied by means of wire beam electrode, scanning electron microscope, energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that SRB can adhere to bare steel to form biofilm and then corrosion occurs. For deposit-covered steel, SRB cannot directly contact the metal matrix due to the barrier effect of the artificial deposit layer, therefore no SRB corrosion was observed. Bare steel and deposit-covered steel can form a galvanic couple, where the deposit-covered steel serves as the anode and the bare steel as the cathode. However, SRB could accelerate the corrosion of bare steel and enhance the cathodic reaction, thus promoting the anodic reaction under the deposit-covered steel.

Safety and Technical Feasibility of Sustainable Reuse of Shale Gas Flowback and Produced Water after Advanced Treatment Aimed at Wheat Irrigation

Treatment and reuse of flowback and produced water (FPW) from shale gas extraction for agricultural irrigation has often been proposed as a sustainable alternative to disposal via deep-well injection. Here, we investigate the effects of FPW on the germination period, macroscopic growth, element enrichment, and grain gene expression of wheat upon dilution and advanced membrane treatment of the liquid stream. Compared to tap water, irrigation with treated FPW shortened the germination time, slightly improved the seed vigor index, and ensured a similar germination rate. On the other hand, the biomass and grain yield of mature wheat irrigated with treated FPW and with FPW diluted to 5% groups decreased compared to tests using tap water. After a whole growth cycle of wheat, higher concentrations of nutrients, such as K, Ca, and Mg were enriched in mature wheat tissue irrigated with treated FPW. However, the Pb and Cr contents of mature wheat grains treated with three types of irrigation waters exceeded the standard to varying degrees. A total of 1973 differentially expressed genes were mainly related to binding, catalytic activity, cellular process, metabolic process, and cell part, more than half of which were upregulated and induced by irrigation with treated FPW. These findings provide critical guidance for the reuse of treated shale gas FPW for agricultural application from the perspective of plant uptake of toxic elements, as well as crop and human health risks.

Shale gas load recovery modeling and analysis after hydraulic fracturing based on genetic expression programming: A case study of southern Sichuan Basin shale

For shale gas reservoir, fracture network fracturing in horizontal well is the key technology to guarantee its commercial exploitation, and the load recovery is a critical parameter which determines the post-fracturing performance. It has been reported that there is a huge difference in load recovery but the control factors are not well understood. It seriously affects the stimulation effect of fracture network fracturing in shale gas wells. Therefore, it is important to analyze the main control factors affecting the load recovery to optimize the design of fracture network fracturing. Further, the load recovery is affected by many factors such as geological, engineering, and production. However, traditional methods are blind to the accurate analysis of the impact on the load recovery. Notably, machine learning (ML) technology has achieved remarkable success in solving the problems of multi-factor nonlinear fitting and black box prediction. Therefore, the genetic expression programming (GEP) is adopted to express the nonlinear relationship in a clear and precise manner in this paper. The data of 189 wells were collected in southern Sichuan, including geological and engineering factors. A feature comprehensive index calculation method was established, and the relative importance of these features analyzed, and then screened out 18 reconstructed features based on geological and engineering factors that affect flow back. The mutual influence between the features was eliminated through principal component analysis of the reconstructed features. Thus the load recovery calculation model was developed and the influence of main control features (variables) on the flow back was analyzed by using partial dependence plot. Statistical parameters showed that satisfactory performance can be obtained through GEP model (training set R = 0.835, test set R = 0.815). The research results show that the GEP calculation model can quickly and accurately calculate the load recovery, obtain the influence law of main controlling factors of geological engineering on shale gas flow back and improve the control of load recovery. Therefore, the method based on GEP can effectively study the main control factors affecting the flow back of shale gas, and hence it can be used as a fast reliable tool to effectively evaluate the load recovery.

Lithium extraction from shale gas flowback and produced water using H1.33Mn1.67O4 adsorbent

The rapid growth of demand for lithium, especially in lithium batteries calls for an increase in the supply of lithium resources. We synthesized H1.33Mn1.67O4 adsorbent by solid-phase reaction method to recover lithium for the first time from shale gas flowback and produced water (SGFPW) in the Sichuan Basin, China. Na2CO3 precipitation pre-treatment was employed to improve the adsorption capacity and selectivity of lithium. The adsorption capacity of lithium in the pre-treated water (P-SGFPW) was 16.24 mg/g higher than that in SGFPW (13.27 mg/g), and the partition coefficient of Li+ increased from 731.58 to 1073.58 mL/g after precipitation. After four cycles of adsorption and desorption, the adsorption capacity of lithium was stable and showed only a slight decrease equal to 3.53% and 5.35% in P-SGFPW and SGFPW, respectively. H1.33Mn1.67O4 is a promising adsorbent for lithium extraction from SGFPW, and precipitation before adsorption can improve its performance significantly.

Granular activated carbon (GAC) fixed bed adsorption combined with ultrafiltration for shale gas wastewater internal reuse

Membrane processes are widely applied in shale gas flowback and produced water (SGFPW) reuse. However, particulate matters and organic matters aggravate membrane fouling, which is one of the major restrictions on SGFPW reuse. The present study proposed fixed bed adsorption using granular activated carbon (GAC) combined with ultrafiltration (UF) for the first time to investigate the treatment performance and membrane fouling mechanism. The adsorption of GAC for SGFPW was best described by the Temkin isotherm model and the pseudo-second-order kinetic model. GAC fixed bed pretreatment with different empty bed contact times (EBCT) (30, 60 and 90 min) showed the significant removal rate for dissolved organic carbon (DOC) and turbidity, which was 34.7%–42.4% and 98.1%–98.9%, respectively. According to characterization of UF membrane fouling layer, particulate matters and organic matters caused major part of membrane fouling. After being treated by GAC fixed bed, total fouling index (TFI) and hydraulic irreversible fouling index (HIFI) respectively decreased by more than 32.5% and 18.3% respectively, showing the mitigation effect of GAC fixed bed on membrane fouling. According to the XDLVO theory, GAC fixed bed also mitigated membrane fouling by reducing the hydrophobic interactions between the foulants and the UF membrane. The integrated GAC fixed bed-UF process produced high-quality effluents that met the water quality standards of SGFPW internal reuse, which was an effective technology of the SGFPW reuse.

An efficient system of aerogel adsorbent combined with membranes for reuse of shale gas wastewater

The internal reuse of shale gas flowback and produced water (SGFPW) can ease the pressure on treatment and management costs and freshwater supply. Internal reuse of SGFPW based on membrane treatments is faced with major challenges of the divalent ion and organic pollutant removal as well as membrane fouling control. To solve these problems, a green and efficient adsorbent of porous biochar aerogel (PBA) was combined with ultrafiltration (UF)-nanofiltration (NF) for SGFPW internal reuse in this study. The removal performance of dissolved organic carbon (DOC, >85.9%) and divalent ions (>70.6%) for three SGFPW showed feasibility of PBA-UF-NF process for internal reuse. Three-dimensional excitation-emission matrix (3D EEM) fluorescence spectrum, organic fractionation and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory were used to identify the organic composition of SGFPW and analyze membrane fouling. PBA tended to adsorb hydrophobic fraction and alleviated UF membrane fouling caused by attractive hydrophobicity. NF membrane fouling was mitigated due to the increased polar repulsion as well as the decreased synergistic fouling between organics and divalent cations through the reduction of DOC by PBA. PBA-UF-NF process had good performance on organic pollutant and divalent ion removal with decreasing membrane fouling, presenting potential to treat SGFPW onsite for internal reuse.

The influence of salinity on the chronic toxicity of shale gas flowback wastewater to freshwater organisms

The potential environmental risk associated with flowback waters generated during hydraulic fracturing of target shale gas formations needs to be assessed to enable management decisions and actions that prevent adverse impacts on aquatic ecosystems. Using direct toxicity assessment (DTA), we determined that the shale gas flowback wastewater (FWW) from two exploration wells (Tanumbirini-1 and Kyalla 117 N2) in the Beetaloo Sub-basin, Northern Territory, Australia were chronically toxic to eight freshwater biota. Salinity in the respective FWWs contributed 16% and 55% of the chronic toxicity at the 50% effect level. The remaining toxicity was attributed to unidentified chemicals and interactive effects from the mixture of identified organics, inorganics and radionuclides. The most sensitive chronic endpoints were the snail (Physa acuta) embryo development (0.08–1.1% EC10), microalga (Chlorella sp. 12) growth rate inhibition (0.23–3.7% EC10) and water flea (Ceriodaphnia cf. dubia) reproduction (0.38–4.9% EC10). No effect and 10% effect concentrations from the DTA were used in a species sensitivity distribution to derive “safe” dilutions of 1 in 300 and 1 in 1140 for the two FWWs. These dilutions would provide site-specific long-term protection to 95% of aquatic biota in the unlikely event of an accidental spill or seepage.

Investigation of Flowback Behaviours in Hydraulically Fractured Shale Gas Well Based on Physical Driven Method

Due to the complex microscope pore structure of shale, large-scale hydraulic fracturing is required to achieve effective development, resulting in a very complicated fracturing fluid flowback characteristics. The flowback volume is time-dependent, whereas other relevant parameters, such as the permeability, porosity, and fracture half-length, are static. Thus, it is very difficult to build an end-to-end model to predict the time-dependent flowback curves using static parameters from a machine learning perspective. In order to simplify the time-dependent flowback curve into simple parameters and serve as the target parameter of big data analysis and flowback influencing factor analysis, this paper abstracted the flowback curve into two characteristic parameters, the daily flowback volume coefficient and the flowback decreasing coefficient, based on the analytical solution of the seepage equation of multistage fractured horizontal Wells. Taking the dynamic flowback data of 214 shale gas horizontal wells in Weiyuan shale gas block as a study case, the characteristic parameters of the flowback curves were obtained by exponential curve fittings. The analysis results showed that there is a positive correlation between the characteristic parameters which present the characteristics of right-skewed distribution. The calculation formula of the characteristic flowback coefficient representing the flowback potential was established. The correlations between characteristic flowback coefficient and geological and engineering parameters of 214 horizontal wells were studied by spearman correlation coefficient analysis method. The results showed that the characteristic flowback coefficient has a negative correlation with the thickness × drilling length of the high-quality reservoir, the fracturing stage interval, the number of fracturing stages, and the brittle minerals content. Through the method established in this paper, the shale gas flowback curve containing complex flow mechanism can be abstracted into simple characteristic parameters and characteristic coefficients, and the relationship between static data and dynamic data is established, which can help to establish a machine learning method for predicting the flowback curve of shale gas horizontal wells.

Multifunctional Carbon Aerogel Granules Designed for Column Reactor for Efficient Treatment of Shale Gas Flowback and Produced Water

Multifunctional Carbon Aerogel Granules Designed for Column Reactor for Efficient Treatment of Shale Gas Flowback and Produced Water

Efficient removal of organic contaminants in real shale gas flowback water using Fenton oxidation assisted by UV irradiation: Feasibility study and process optimization

The aim of this study was to identify an effective method for removing organic pollutants from real shale gas flowback water (SGFW). The results showed that ozonation could only achieve limited total organic carbon (TOC) removal when the initial pH was set to 3 or 7. Compared with the O3/H2O2 and Fenton processes, better performance was observed with the ultra violet (UV)-Fenton process in SGFW. The effects of H2O2/COD, initial pH, H2O2/Fe2+, and reaction time on the removal efficiency via the UV-Fenton process were investigated through single-factor experiments. The optimal experimental conditions were obtained using a quadratic polynomial prediction model (RAdj2 = 0.9295) from the central composite design of response surface methodology. The TOC removal efficiency could reach as high as 70.02% even in real SGFW under optimal conditions (H2O2/chemical oxygen demand ratio 11.54, H2O2/Fe2+ ratio 130.20, pH 3.72, temperature 25 °C, and reaction time 60 min). Moreover, the TOC removal kinetics could be better explained by pseudo-second-order kinetics (R2=0.9609). In UV-assisted Fenton oxidation, the cycle of Fe3+ species to Fe2+ was accelerated to regenerate more ∙OH radicals, thereby leading to higher TOC removal efficiency. Moreover, gas chromatography-mass spectrometry analysis revealed that the UV-Fenton process could mineralize most organic pollutants in the SGFW. Dibutyl phthalate was selected as the model organic matter to explore the possible degradation mechanism. This study provides a reference for the design and operation of organic removal units in the SGFW treatment process.

Storage strategy for shale gas flowback water based on non-bactericide microorganism control

Shale gas is a promising unconventional natural gas in the world, however the produced flowback water have severe challenges to surrounding water resource. Conventional reuse technology uses bactericide to control corrosive microorganism, which might bring uncontrolled drug resistance and other secondary pollution. In this study, storage strategy of flowback water was designed as a pre-control stage to decline corrosive microorganism. Dissolved oxygen and temperature were chosen as two key parameters based on microbial physiological and biochemical characteristics. Results showed that under the cross effect of temperature and dissolved oxygen, 15 °C and anaerobic condition had the optimal microorganism control effectiveness. Microorganism amount and live/dead cell ratio decreased by 63.7% and 68.74% respectively compared raw water. COD removal efficiency reduced to only 20%, indicating that the microorganism activity was extremely inhibited. However, microorganism in flowback water was more sensitive to dissolved oxygen compared to temperature. Redundancy analysis confirmed that dissolved oxygen contribution was as high as 91.5% while temperature was not significant (p > 0.05), the contribution rate was only 8.5%. Thermococcus, Archaeoglobus, Thermovirga, Thermotoga and Moorella were the dominated thermophilic, anaerobic and sulfate reduction or metal corrosion microorganism in flowback water, so all these identified microorganisms were control targets. Importantly, all the target microorganisms detected in flowback water were declined after different storage strategies. This study provides an effective storage strategy for flowback water to inhibit the microbial amount and activity without biocides addition, which could help promote the green exploitation of shale gas.