Clean Fracturing Research Articles

Supramolecular self-assembly of robust, ultra-stable, and high-temperature-resistant viscoelastic worm-like micelles

HypothesisWorm-like micelles are susceptible to heating owing to the fast dynamic exchange of molecules between micelles. Inhibition of such exchange could afford robust worm-like micelles, which is expected to largely improve rheology properties at high temperatures. ExperimentsA cationic surfactant docosyl(trimethyl)azanium chloride (DCTAC) and a strongly hydrophobic organic counterion 3-hydroxy naphthalene-2-carboxylate (SHNC) were used for the worm-like micelles fabrication. The microstructure was characterized using cryogenic transmission electron microscopy and small-angle neutron scattering, and the interactions between DCTAC and SHNC were characterized using nuclear magnetic resonance spectroscopy. Rheometer was employed to measure the rheological properties of the solution. FindingsSHNC/DCTAC at the molar ration of 1:2 forms ultra-stable worm-like micelles, whose viscosity remain stable at temperature up to 130 °C. SHNC is found to strongly adsorbs on DCTAC micelle with the orientation on the surface of micelle, keeping the naphthalene backbone entire penetration into the palisade layer while both carboxylic and hydroxyl groups protrude out of the micelle. With temperature increasing, this adsorption further strengthens, resulting in the growth contour length and accompanying the enhancement of rheological properties. One SHNC molecule and two DCTAC molecules are speculated to form a stable complex via multiple interactions including hydrophobic, cationic-π, and π-π interactions, which decreases the dynamic exchange of them between micelles. These findings are helpful to understand surfactant aggregates stability and assist the development of novel stable supramolecular nanostructures. Additionally, the excellent thermal stability of this worm-like micellar fluid makes it a potential high-temperature resistant clean fracturing fluid for deep oil reservoirs.

A Gemini viscoelastic surfactant stabilized by steric hindrance group on its spacer to construct wormlike micelle structure in concentrated brines

To overcome the disordered aggregation of viscoelastic surfactant (VES) molecules in high concentrated brines resulting in phase separation, a steric hindrance group was modified onto the spacer of a Gemini VES for stabilization in brine, and then a novel Gemini novel VES, named VES-BC, with double erucicamide tails, was synthesized successfully. Molecular dynamic simulation showed that the benzoic acid as a steric hindrance group effectively inhibited the disordered aggregation of VES-BC in concentrated brine, and RDF between the oxygen on the spacer and water varied slightly with increase salinity. Comparably, the Gemini cationic VES without steric hindrance group aggregated excessively and disorderly in high salinity environments, and the counterpart RDF varied significantly. VES-BC exhibits superior stabilization in brines, its salinity cloudy point reaches over 10 wt%. The surface tension, DLS, and fluorescent probe were carried out to explain stabilization mechanism of VES-BC in brine. The wormlike micelle (WLM) construction by VES-BC was studied through viscosity measurement, oscillatory measurement and microstructure observation, which proves the WLM networks can be formed with salinity between 3 wt% and 10 wt%, and 6 wt% is optimal. The steric hindrance group imparts VES-BC an insensitivity to salinity realizing the stabilization in high concentrated brines, and the salinity for WLM-driving is comparably high. Thus, VES-BC is an excellent clean thickener under extremely concentrated brine environment, especially for the preparing of clean fracturing fluid to save freshwater resources.

Synergistic effect of dual hydrogen-donor deep eutectic solvent for performance improvement of fracturing-oil expulsion fluids

Petroleum remains the main source of energy consumption over the past decade. The development of rich unconventional oil reservoirs is a huge challenge. Clean fracturing fluids (CFFs) have gained considerable attention for the unconventional reservoir development, owing to their integrated capabilities of fracturing and oil expulsion. Performance of CFFs is a vital factor in enhancing oil recovery. In this work, a ternary deep eutectic solvent (DES) with dual hydrogen donors (HBDs) was developed as a novel and green medium for improving CFF performance. With the hydrogen-bonding interactions, the ternary DES enable a robust micelle network, thereby strengthening the rheological property, temperature resistance and drag reduction performance of CFFs. The salt resistance performance was improved due to the ion–dipole interaction between HBDs and salt cations, decreasing the interactions between salt cations and surfactants. The DES also increase the imbibition efficiency from 30.7 % in 98 h to 43.7 % in 62 h. Besides, the gel breaking performance and reusability of gel-breaking fluid were enhanced in the presence of DES. The ternary DES showed more effective reinforcement for CFF properties as compared with binary DES, due to the synergistic effect of dual HBDs. This work will provide a promising alternative for intensifying CFF performance.

Environmentally friendly wormlike micelles: Rheological behavior and its application in preparing green fracturing fluid

Ionic liquid surfactant (ILS) may be a good alternative to equilibrate the antagonistic relationship between hydrophobicity and water solubility for anionic surfactant. However, the application of ultra-long chain anionic ILS in the viscoelastic surfactant (VES) fracturing fluid was rarely reported up to now. Herein, an ultralong-chain anionic surfactant (abbreviated ChEr) was synthesized from natural erucic acid. As expected, ChEr retained the desirable characteristic of ILSs (melting temperature <100 ℃) and excellent water-solubility (Krafft temperature < 4 ℃). More importantly, ChEr can form elongated wormlike micelles (WLMs) in the presence of inorganic salt potassium chloride (KCl), giving solutions high viscoelasticity. The WLMs solutions were investigated using steady-state and dynamic rheology. Due to the strong van der Waals interaction induced by ChEr's ultra-long hydrophobic tail, zero-shear viscosity (η0) of solutions showed an extraordinarily significant relationship on concentration with an exponent of 8.2, which was more than double the theoretical expected value. The dynamic rheology results demonstrated that as the ChEr concentration increased, the 1:1 ChEr:KCl solution transformed into a gel-like liquid owing to the dense entanglement of WLMs. The salt concentration also had a strong influence on the wormlike micelles. At a KCl to ChEr molar ratio of 2:1, the fluid exhibited the maximum viscoelasticity. Finally, the fracturing fluid system based on ChEr/KCl displayed not only resistance to high temperature (<120 °C) and cyclic shear, but also excellent proppant-carrying and oil-induced gel-breaking capabilities. The present work reveals the remarkable properties of ultra-long chain ILS in forming WLMs and promotes the application of anionic ILS surfactant in the preparation of clean fracturing fluid.

A novel temperature-resistant fracturing fluid for tight oil reservoirs: CO2-responsive clean fracturing fluid

With the increasing demand for oil and gas resources, deep tight oil and gas reservoirs have gradually become the focus of global oil and gas exploration and development. Hydraulic fracturing technology based on water-base fracturing fluid plays a more and more important role in improving the recovery of tight oil reservoirs. Under the background of global CO2 emission reduction, a CO2-responsive clean fracturing fluid with controllable gelling/gel breaking at high temperatures was constructed in this paper. The system was not only friendly to the reservoir and environment, but also overcame the shortcomings of the traditional clean fracturing fluid with undesirable temperature resistance. The viscosity characteristics and viscoelastic behavior of temperature-resistant clean fracturing fluid were investigated through high-temperature rheology experiments. The interaction between fluid and rock was investigated through the core flow experiment. The results showed that the clean fracturing fluid had the characteristics of good CO2 responsiveness, excellent shear resistance at high temperatures, high drag reduction, satisfactory gel breaking performance, low filtration and low core damage. The complex three-dimensional network structure formed by the intertwined micelles and vesicles improves the temperature resistance of the clean fracturing fluid. It is expected that the fluid can not only be used to improve oil recovery in tight oil reservoirs, but also be used to solve the leakage problem in CO2 storage underground.

From nonionic to zwitterionic: Novel CO2-responsive zwitterionic wormlike micelles based on zwitterionic surfactant and bola-type asymmetric diamine

CO2 enhanced oil recovery (CO2-EOR) is one of the CO2 utilization methods which can also geo-sequestrate greenhouse gases. Due to the weak adsorption on rock surface and excellent viscoelasticity performance, CO2-responsive zwitterionic wormlike micelles (WLMs) has been recognized as a promising method to control CO2 channeling. A novel CO2 responsive zwitterionic WLMs was developed by blending N-octadecyl-N,N-dimethyl-3-ammonio-2-hydroxy-1-propane-sulfonate (OHSB), N,N,2,2-tetramethyl-1,3-propanediamine (TADPA) and sodium salicylate (NaSal). The responsive behaviors induced by varying agents (CO2 and HCl) are systematically studied by rheology tests and cryo-TEM measurements. The molecular structures of products are confirmed by 1H and 13C NMR. Moreover, the responsive mechanism is discussed in detail. The OHSB/NaSal/TADPA system shows morphological transformations between spherical micelles (a good flowing capacity) and WLMs (conducive to plugging) before and after purging CO2. The viscoelasticity and molecular structure of the system with CO2 bubbling is different from that with HCl titration, because TADPA molecules can react with CO2 to form a novel zwitterionic product: NH+(CH3)2-CH2-C(CH3)2-CH2-NHCO2− (TAH+DPACO2−, 78.95 %–83.52 %). This work provides a new strategy for the design of CO2-responsive zwitterionic WLMs and their applications in CO2 utilization schemes, including clean fracturing, mobility control, and CO2 foam flooding, etc.

Preparation of degradable amphoteric surfactant and property evaluation of clean fracturing fluid

AbstractThe main agent of clean fracturing fluid, cationic quaternary ammonium salt surfactant, is difficult to degrade in natural conditions because of its long alkyl chain. In this study, a biodegradable amphoteric surfactant, stearate sulfobetaine (SESB), was synthesized by esterification of long‐chain stearic acid and N,N‐dimethyl ethanolamine followed by quaternization to serve as the main agent of clean fracturing fluid. The structure of the synthetic degradable surfactant was verified by infrared and 1H‐nuclear magnetic resonance spectroscopies. In addition, a series of indoor performance evaluations were carried out, including shear resistance, viscoelasticity, sand carrying capacity, and gel‐breaking performance. A series of experimental evaluations showed that the clean fracturing fluid prepared with 2% SESB and 0.4% EDTA‐4Na had an excellent comprehensive performance. The apparent viscosity was higher than 50 mPa s at 80°C for a shear rate of 170 s−1. It has good heat and shear resistance performance. In addition, the system processed high shear stability and recovery at room temperature. According to the viscoelastic test, the system mainly relies on elasticity to carry sand. The static settling velocity of sand was 0.042 mm/s under high‐temperature conditions of 95°C.

A rigid double‐tailed surfactant preparation method and its application in clean fracturing fluid

AbstractUsing 1,8‐dihydroxynaphthalene, thionyl chloride and long‐chain tertiary amine as the main raw materials, a facile preparation method and formulation of a rigid double‐tailed surfactant for clean fracturing fluid were developed. The temperature resistance test and the determination of hydrogen spectrum reveal that the clean fracturing fluid with the prepared surfactant as thickener has excellent salt and temperature resistance. The shear viscosity of the best product at 120°C and 170 s−1 is not less than 65 mPa s, which meets the requirements of 25 mPa s for the field construction of clean fracturing fluid.

Study of crosslinker size on the rheological properties of borate crosslinked guar gum

Borate crosslinked guar gum gels have played a vital role in stimulating oil and gas wells for many years; however, the high dosage of guar gum in the existing fracturing fluid will increase the cost and cause more damage to the reservoir and ultimately affect the effect of stimulation. In this study, borate esters are modified onto polyethyleneimine (PEI) of different molecular weights, affording organic borate crosslinkers of different sizes. By analyzing the effect of crosslinker size on gel rheology, sand-carrying properties, and microstructure, it is observed that the crosslinking efficiency is most significantly enhanced when the crosslinker size is similar to the diameter of the guar gum molecules. This makes it possible for the gel to maintain good performance at low polymer concentrations and meet the performance requirements of fracturing, which provides new ideas for developing the next generation of economical, clean, and green fracturing fluids.

PH-responsive clean fracturing fluid based on pseudo-trimeric surfactants

pH-responsive clean fracturing fluid based on pseudo-trimeric surfactants

Research of a novel fracturing-production integral fluid based on cationic surfactant

Fracturing plays an extremely important role in increasing oil and gas production, however, the fracturing flowback fluid has problems such as large volume, difficult treatment, high processing cost, and environmental pollution. In order to expand the application range and reuse of clean fracturing fluid,a fracturing-production integral fluid based on small molecule surfactants, cetyl trimethyl ammonium chloride (CTAC), salicylic acid (SA) and oleic acid amide methyl hydroxypropyl ammonium chloride (OAMDC) is proposed. At 90 °C the resistance of the fracturing-production integral fluid reached values up to 63.5 mPa·s. The surface and interface tension of the fracturing-production integral fluid broken by crude oil were 28.5 and 8.19 × 10-3 mN/m, respectively. It shows good oil flooding ability (up to 16.35 %), static oil washing rate (up to 21.5 %) and outstanding emulsification effect. Meanwhile, the application scope clean fracturing flowback fluid will be expanded to achieve its efficient and comprehensive application in the field of oilfield chemistry, thereby improving the benefits of oilfield development.

Development of the Gemini Gel-Forming Surfactant with Ultra-High Temperature Resistance to 200 °C.

In order to broaden the application of clean fracturing fluid in ultra-high temperature reservoirs, a surfactant gel for high-temperature-resistant clean fracturing fluid was developed with a gemini cationic surfactant as the main agent in this work. As the fracturing fluid, the rheological property, temperature resistance, gel-breaking property, filtration property, shear recovery performance and core damage property of surfactant gel were systematically studied and evaluated. Results showed the viscosity of the system remained at 25.2 mPa·s for 60 min under a shear rate of 170 s−1 at 200 °C. The observed core permeability damage rate was only 6.23%, indicating low formation damage after fracturing. Due to micelle self-assembly properties in surfactant gel, the fluid has remarkable shear self-repairability. The filtration and core damage experimental results meet the national industry standard for fracturing fluids. The gel system had simple formulation and excellent properties, which was expected to enrich the application of clean fracturing fluid in ultra-high temperature reservoirs.

Polyhydroxy cationic viscoelastic surfactant for clean fracturing fluids: Study on the salt tolerance and the effect of salt on the high temperature stability of wormlike micelles

In this study, we designed and synthesized a polyhydroxy cationic viscoelastic surfactant (SY-TO). Though the FT-IR, 1H NMR and surface tension methods characterize and determine the structures and critical micelle concentration. Through the test of apparent viscosity, viscoelastic, and microstructure observation, we study the salt tolerance of SY-TO. And for the salt tolerance mechanism, we explain it from the molecular structure and hydrophilic headgroup. Additionally, through the high temperature rheological tests, we studied the effects of salt and its concentration on the behavior of wormlike micelles (WLMs) at high temperatures. In particular, the effects of salt and its concentration on temperature-response are analyzed. More importantly, the effects of salt and its concentration on the high temperature stability of WLMs were analyzed from the perspective of molecular thermal motion. Through the research on the salt tolerance mechanism of hydrophilic headgroup and the analysis of the effect of salt on the high temperature stability of WLMs, we hope it will be conducive to promote the preparation and wide application of salt and temperature resistant clean fracturing fluids.

A novel property enhancer of clean fracturing fluids: Deep eutectic solvents

Fracturing fluid properties are closely related to the oil recovery of unconventional reservoirs. In the meantime, deep eutectic solvents (DESs) have provided a new method to improve the properties of clean fracturing fluids (CFFs) due to their low-toxicity raw materials and mild synthesis conditions. In this work, DESs were developed as the enhancers to intensify rheological characteristics and spontaneous imbibition performance of CFFs. The adjustment of hydrogen bond donors (HBDs) can effectively regulate the rheological characteristics of CFFs. The DES composed of choline chloride (ChCl) and malonic acid (MA) can significantly increase the viscosity of CFF from 38.1 mPa·s to 60.1 mPa·s, the shear resistance and viscoelastic modulus were both improved. The interactions between HBDs and sodium p-toluenesulfonate (TsONa) anion has an influence on the electrostatic shielding of TsONa anion for OAHSB, which further determines the strength of micelle networks. For the spontaneous imbibition of CFFs, DESs improved the imbibition efficiency from 33.0% in 98 h to 40.1–47.2% in 78–90 h through the reduction of oil–water interfacial tension and oil viscosity, change of oil-contacting surface wettability and the increase of core porosity and permeability. The DESs here provide a novel strategy to enhance CFF properties in an environmentally friendly and efficient way.

The spontaneous imbibition mechanisms for enhanced oil recovery by gel breaking fluid of clean fracturing fluid

The quick flowback treatment after conventional fracturing construction can waste a lot of energy from fracturing fluid. Meanwhile, spontaneous imbibition has great application potential for enhanced oil recovery (EOR). In this work, based on pre-developed silica nanoparticle-reinforced CO2-sensitive fracturing fluid system, the imbibition factors of the gel breaking fluid were studied. In addition, the HNMR technology, AFM, contact angle test, and interfacial tension (IFT) test were used to explore the mechanism of spontaneous imbibition. The results show that imbibition mainly occurs in the small pores, and the adsorption of components can reduce roughness and flow resistance. Meanwhile, the adsorption layer was formed by surfactants and nanoparticles, changing wettability from oil-wet to water-wet, reducing the IFT between oil and water. The synergistic effect of multi-component improves the imbibition effect. We expect that this study can broaden the application of multi-component gel breaking fluid in low permeability reservoirs.

Pore Structure Characteristics and Adsorption and Desorption Capacity of Coal Rock after Exposure to Clean Fracturing Fluid.

In the hydraulic fracturing process, fracturing fluid contacts coal rock and physical and chemical reactions occur, which inevitably damage the pore structure of the coal rock and affect the adsorption and desorption capacity of the coal rock. In this paper, a low-temperature N2 adsorption method and scanning electron microscopy (SEM) were used to characterize coal samples. Using gas adsorption/desorption tests, high-, medium-, and low-rank coal samples before and after the clean fracturing fluid treatment were systematically studied. According to the relationship between coal pore structure parameters and gas adsorption/desorption characteristics, a correlation between the microscopic pore structure and the macroscopic gas adsorption/desorption characteristics of coal was obtained. The results show that the number of closed pores in high-, medium-, and low-rank coal samples increased after the clean fracturing fluid treatment. The micropore volume increased by 0.0009, 0.00143, and 0.0035 mL/g, respectively, and the specific surface area increased by 4.87, 9.06, and 57.60%. The fractal dimension also increased compared with that of raw coal. SEM analysis indicated that the influence degree of clean fracturing fluid treatment on the pore structure of different-rank coal samples was Gengcun low-rank coal > Pingba middle-rank coal > Jiulishan high-rank coal. The experimental results of methane adsorption and desorption showed that the adsorption capacity of the coal samples after clean fracturing fluid treatment was enhanced, which is related to increases in the micropore proportion, micropore volume, and specific surface area of the coal. The desorption capacity of the coal samples was also enhanced. The desorption rate of medium- and high-rank coal samples increased after the clean fracturing fluid treatment but that of low-rank coal samples decreased. The main reason is the increase in the number of micropores in low-rank coal, which enhances the gas adsorption ability and makes gas desorption difficult. Therefore, clean fracturing fluid is suitable for medium- and high-grade metamorphic coalbed methane mines. These research results provide a theoretical basis for the application of clean fracturing fluid in different coalbed methane wells.

Comparative experimental study on the effects of water- and foam-based fracturing fluids on multiscale flow in coalbed methane

To quantitatively evaluate their influence on coalbed methane (CBM) flow, water- and foam-based fracturing fluids were prepared. The effects of these two fracturing fluids on the flow of methane in coal were compared and analyzed using isothermal adsorption, diffusion capacity, and seepage tests. The results showed that an increase in the concentration of the fracturing fluid can exacerbate damage to methane flow. After immersed with water-based gel, gas diffusivity in the coal sample dropped to less than one fifth of the original level. However, when immersed with foam-based fracturing fluid, the diffusion coefficient of coal samples exceeded 3 × 10−13 m2/s. The residual permeability of the coal samples measured at a high pressure was significantly greater than that measured at a low pressure. Compare with the foam, the gel will cause a more serious adverse impact on the permeability of coal samples. Foam fracturing fluids with low viscosity and higher volume fraction of the gas phase will cause lesser damage to methane adsorption, desorption, or percolation. These results can help in the development of a coal seam clean fracturing technology.

Interface properties evolution and imbibition mechanism of gel breaking fluid of clean fracturing fluid

Capillary forces are usually considered to be the main driver of imbibition behavior in low-permeability reservoirs. Recent studies show that the presence of mass active substances in the clean fracturcing fluid can reduce the oil–water interfacial tension as well as reduce the hydrophilicity of the pore wall, which would greatly weaken the capillary force driving effect. However, the final oil recovery still increases, suggesting the presence of other effects to promote fluid imbibition. Herein, the evolution law of gel breaking fluid of clean fracturing fluid at oil/water/solid interface is investigated, and the main controlling factors of imbibition behavior are analyzed. Osmotic pressure is proven to be a major imbibition driving force. The change of interface properties due to adsorption of active substances reduces the driving effect of capillary force during the imbibition and intensifies the osmotic pressure of micro-nano pores. For reservoirs rich in mineral content, the contribution of osmotic pressure to imbibition recovery is increasing with time, and even becomes the main driving force in the later stage. In addition, the driving effect of osmotic pressure is positively related to the clay minerals and salinity of reservoir. When the clay mineral content is 18.6%, the contribution of osmotic pressure to ultimate imbibition recovery can reach 33.2%. This study proposes new insights into the imbibition mechanism, providing guidance to the practical applications of long-term stable imbibition and improved oil production.

Experimental study and analysis on physicochemical properties of coal treated with clean fracturing fluid for coal seam water injection

In the process of coal seam water injection, the components of different fracturing fluids have different effects on coal. To analyze the effect of water injection fracturing fluid on the physical and chemical properties of coal, we studied the relationship between guar-based fracturing fluid and cationic clean fracturing fluid (C-VES). Microscopic experiments show that guar-based fracturing fluid has a certain viscosity effect on coal, while C-VES can adsorb and capture the primary coal dust in coal. FTIR analysis shows that C-VES has a great change on the content of coal functional groups, and guar-based fracturing fluid has little effect on coal. Industrial analysis shows that C-VES can better reduce the ash content of coal and has a certain impact on the thermal efficiency of coal than guar based fracturing fluid. Combustion characteristic experiment shows that guar-based fracturing fluid does not affect the combustion effect of coal, and C-VES can expand the exothermic temperature range of coal but has little effect on the main combustion of coal. Molecular dynamics simulation results show that C-VES has the best wetting effect on coal. This research provides theoretical guidance for dust prevention and improving the use and conversion efficiency of coal.

Discussion on Fracturing Stimulation and Reconstruction Technology in Oil Field

Oil field fracturing technology is based on fracturing principle and plays an important role in oil field production. However, there may be safety problems, technical problems and environmental problems when applying oil field fracturing stimulation technology. Therefore, it is necessary to make clear the breakthrough point of oilfield fracturing technology, and transform the oilfield fracturing stimulation technology through horizontal well staged fracturing technology, repeated fracturing technology and clean water fracturing technology to improve the application quality of oilfield fracturing technology.